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Auto control enables use of solar PV for immersion heater

Posted by Chris Rudge on 16 September 2011 at 10:38 am

Chris's previous article on how to use excess solar generated electricity in the home generated so much interest that he has written a follow up that goes into more detail about about the control he has built to use solar-generated electricity to heat water.

One of the side issues of the Feed-in Tariff with Solar PV is that besides the wonderful 43.3p your system will earn from for each unit generated, you are able to use as much as you can at your premises without penalty. However, most dwellings will only use around a third of all the power generated.

Most houses are big power users after dark, with lights, TV, etc, but PV generated is not much benefit in this case! Even if you are at home all day and running the washing machine, lawn mower, computer, etc this will only scratch the surface of peak generated power of a standard 4kWp system in the summer.

At customer’s request, we put together a test unit to use solar generated electricity to heat water by powering their immersion heater a while back. It’s been running faultlessly on test in their house for some six months or so with great effect. They have not needed to use gas to heat their water all summer, which beats getting a solar thermal system!

The unit is fairly straightforward, and although it has a little drawback, which will be described later, it does the job for very little outlay. There are intelligent units on the market which do the job far more elegantly, such as the EMMA and the Power Router (which we install), but as you can imagine, the purchase cost of such units are into £1000’s rather than this little control circuit which comes in at under £100 in components, or £180 to buy complete from us.

Before going any further, I strongly insist this is only assembled and installed by qualified personnel who have the knowledge and capability of recognising safe working with electricity and protection of components and wiring.

The Circuit

The live output from your inverter is diverted through a Current Sense unit, which is set for a switch point optimum for this application. This Current Sense unit has a built in low current relay, which is used to switch a suitable contactor. I would suggest a 10Amp contactor at the very lowest for the sort of use we are looking at.

You will see from the description below, together with reference to the circuit diagram, the ideal location for the completed unit is adjacent to your consumer unit (fuse box). As both the inverter and your immersion heater will be connected directly to this point, it really does make sense.

You can refer to the circuit/ layout diagram for the following:

1)The mains 230v AC output from the inverter needs to have the live conductor diverted through the current sense unit, but otherwise remains connected as normal.

2) The Current Sense built in relay terminal is supplied by a spare 6A MCB in the consumer unit.

This relay will switch the contactor. As well as the 6A MCB, it is advisable to provide a 1A

In line fuse to protect the relay and contactor coil adequately.

3) The main relay on the contactor will switch the live output from the water / immersion heater in the Consumer unit. To do this will involve diverting the existing immersion heater wiring to route through this unit. Due to regular switching this circuit will now be cycling through an inductive load, it is recommended you change the 16A MCB to a ‘C’ type 16A MCB to avoid nuisance tripping.

At the hot water tank, you will need to disconnect the immersion heater wiring from the wall switch and fit with a 110v yellow plug. Change the immersion switch to a standard switched socket. Plug a 3kVa 110v builder’s site transformer into the socket, and the newly plugged immersion heater cable into the transformer. That’s it!


Normally, your immersion heater runs at 3kW. To make the most of the free to use electricity available, we need to make the heater use less power. By far the easiest way to do this using standard and cheap to buy parts is by a 110v site transformer, available from screwfix, etc, at around £50 each. Once the transformer is in place, this will limit the power consumption of the immersion heater to around 700W. For a standard 200ltr water tank, we have found the immersion running for most of the day will heat the whole tank to around 50 degrees. This obviously assumes the day is bright and sunny all day of course!

On a unit supplied by us we would set the current switch to around 800W, so will only switch your immersion heater on when the inverter is generating more than 800W, ensuring immersion heater usage is powered by the PV system.

It must be mentioned, that although the Current Switch has a built in hysteresis to avoid relay chatter when passing current is marginally over or under the set point, on a day with patchy cloud you will experience the contactor clicking in and out regularly. This can’t really be avoided on a simple circuit as this, but has not proved to be an issue in the 6 months of operation in our test unit.

In essence, the controller is very basic and I will always welcome suggestions to improve the design.

There are now a range of products on the market which enable you to divert your home-generated power to an immersion. Click here to find out more.

 Photo by Doug88888

Related information:
Blogs on energy storage
Solar PV Guide
Find a Solar PV installer

About the author: Chris Rudge is a qualified electrician who specialises in renewable energy.

If you have a question about anything in the above blog, please ask it in the comments section below.

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64 comments - read them below or add one

Rudge Energy

Rudge EnergyComment left on: 7 October 2015 at 1:45 pm

It has come to light that the Lamonde Automation ACS200 current sense unit is prone to failing due to the back EMF from the contactor. 

The result of this will start chattering the contactor, causing excess heating of the contactor.

The best way to remedy this issue will be to replace the contactor with a solaid state relay. Suggested RS pert number is 720-4088, though I anticipate any 6-10A solid state relay will work well, though make sure the trigger voltage is 230v AC.


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NBcassieComment left on: 22 March 2015 at 10:38 pm

Advice needed here. We live on a narrowboat and have a clarifier (emersion tank) twin coil (1 from engine and 1 from fire) for our hot water with a backup lpg boiler.

We also have 1kw solar panels and 1kw wind turbine (both 12v).

Could we use some of the excess power we receive from solar/wind to power a 12v heat element within on calorifier? 


This would save on engine run and gas.

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Rudge Energy

Rudge EnergyComment left on: 4 August 2014 at 10:49 pm

Hi NicNac.. The idea may sound good, but unfortunately just when solar PV is generating its lowest, you will be needing the boiler much more. Over the months of December and January solar PV generates only 10% of what it will do later in the year in June/ July, not enough to run an electric boiler.

In addition, electric boilers, though cheap to install, do cost quite a lot to run if the power has to be purchased off the grid. Somewhere in the region of 15p a kW. Compared to mains gas at around 4p a Kw.. Currently mains gas (if you have it available) is the best option for heating in cost. 


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NacNacComment left on: 1 August 2014 at 7:44 pm


this may be a silly question. Instead of changing some equipments to be able to use the PV electricity what about changing the gas boiler by an electric boiler.

So the PV and/or any battery storage will provide electricity for one device, the electric boiler which will do what the gas boiler did, ie heating the water and provide heat for the central heating.

Is that too simple of thought ?

You can imagine the advantage in winter...  everything in the house will run by solar electricity.  I heard that modern solar PV on a south facing detached 4 bedrooms house could generate about 15000kWh per year. This should cover most of the need in electricity including when using an electric boiler.

What do the experts think ?

Thanks in anticipation

Best regards


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UitlanderComment left on: 20 July 2014 at 4:19 pm

I looked around the market and had a SolarCache+ installed in March. This is a wireless version, which was retrofitted to my existing installation by the original installer. The install took a few hours and involved one box going into the cupboard my immersion tank lives in, and another smaller control unit being added to next to the PV generation meter by my fuse box in the kitchen. Very neat and tidy bit of kit.

So far very happy with it, my boiler has been turned off since March. I've had a plentiful supply of hot water, with much reduced gas consumption. It's good to be using up some of the energy generated while I'm at work.

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FarrellMackennonComment left on: 28 January 2014 at 7:59 am

These little systems are very useful and can save a householder a ton of money as they don’t need to invest in a new central heating hot water system. The way they work is very simple but highly effective and there are good points and bad points as there are with all systems. The warranty on these is quite limited for starters and designs are still relatively new so improvements are sure to be made in the next few years.

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metalmagpieComment left on: 4 August 2013 at 9:04 pm

I am about to install a DHW tank to preheat the water to my combi boiler during export periods from my CHP. The storage tank I have has a 1500W heater. My though is that I could use a high power diode to supply the heater with half wave electric. It should provide a reasonably priced way of reducing the heat and without the winding losses of a 240/100 transformer. What do you think?

Have you thought of using a main frame uninterupted power supply to garner excess electric during the day. These thinks automatically sync with the mains, when on, and discharge through the same lead when the mains is interrupted (and vice versa on return of the main supply.

One would need to isolate the mains once the solar output had ceased and re-connect once output was restored. Again, what do you think?


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27martinComment left on: 30 April 2013 at 4:10 pm

Thanks will add but just need someone to fit one for me and the systme to use the solar to heat it. Looked at the new systme on market thats store and manages your electice but with a pirce tag over £7-9000 it would take me 20+years to get my money back, so as my main bill is gas anyways to reduce this would saves me hundreds a year.

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Cathy Debenham

Cathy DebenhamComment left on: 29 April 2013 at 8:20 am

Hi Martin

You need a hot water cylinder and immersion heater to benefit from these devices (there are now a range on the market). One of our energy experts is in the process of writing a blog about the economics of changing heating systems to capture more use from solar in response to a similar question from another reader with a combi boiler. So keep an eye on the blog, follow me on twitter (@YouGenUK) or sign up for our newsletter to see it when it's posted.

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27martinComment left on: 28 April 2013 at 7:48 pm

Hi just been searching for this topic and came across this site. I have 4kw system for solar electric fitted on my house and looking to utilise the electric I over produce. 3p per kw back to grid works out as not much to bother about iver the year compared to reducing my monthly gas usage for hot water or heating . I have a combi boiler and central heating system only. Where can I get one if these systems fitted to help with my heating or hot water.

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Charlotte Heath

Charlotte HeathComment left on: 18 April 2013 at 10:38 am

The SOLiC 200 can be seen at work in this short film - please feel free to share with anyone who might be interested.

More information is also available on the website

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fredfred2Comment left on: 8 March 2013 at 7:45 pm

I've since found out that the solar immersion heater controller AF2.1 is actually called the solic 200, which is why I couldn't find it anywhere. Also you can now buy the Immersun directly from the people who make it which is handy.

Anyway, here's my own opinion on the various solar immersion heater controllers that I've come across and fitted in the last 6 months.


SG Wooldridge system







I think that Wattson make some kind of device and so do another small outfit that has radio controlled power sockets. Maybe someone else will enlighten me, as I can't remember the name. I also think Dimplex just launched a similar product at the Ecobuild show.

The proportional control devices all basically work in the same way. They sense the difference between solar generator and the house draw and dump any remaining power into the hot water tank.

Differences bewteen the models I've seen or installed are basically build quality, features and guarantees. But like someone else mentioned guarantees are only worth the paper they're written on after 1 year.

They all cost about £380 trade (+ VAT), although I'm sure that'll come down in time when some Chinaman starts making them for a pound a piece. Then they really will be good value for money.

They all work (to a point).

Some claim to be more efficient (capturing more of the generated power than the others- difficult to prove), some give you internet or iphone access to their functions, and some just do the basics of diverting power. From my experience, I have found that people stop inspecting their inverters, generation meters and solar immersion controller boxes after a month of operation, so the extras aren't really neccessary. But I guess it is comforting to know exactly how much power you've saved over a year. A scalding hot tank of water at the end of a sunny day generally tells you your system is working.

I don't know anyone who has used the additional functions available in the Immersun, Gem, EMMA or Solacache, so maybe these extras are an additional thing to cause things to go wrong one day.

The Wooldridge system is a very basic unit, in a somewhat amateurish or hobby type enclosure, but it does work well.

The Immersun, which is very presentable, with a network interface and the most talked about also seems to work well, but has been seen to cause interference on power monitors like the Wattson.

The Solic is very basic and is built like a brick which also works well.

The Gem is beautifully put together, seems to work fine but can be tricky to set up.

The solacache is an engineering overkill, with lots of extras- I think it was designed by a professor in rocket science, and also seems to work, but is pricey.

The EMMA is one you don't come accross too often because it is the most expensive unit out there. I'm not sure that it is as efficient as the others, as it uses two sensors, but can do lots of other stuff. It is very tricky to setup.

The solardivert is a very simple switchover unit that I haven't installed for a while since the others have appeared on the market. It uses an adjustable threshold rather than calculates the difference.

The solarswitch is very similar to the solardivert.

All the proportional controllers are affected by grid interference or variable mains power factor, occasionally switching on power to the immersion heater when they shouldn't do. I guess they'll never be perfect because of the way they sense the power in the meter tails. Washing machine motors and heat pumps appear to be a common cause. The wooldridge box seems to be more immune than the other nice looking units for some reason.

The switched control units are completely immune to grid interference, as is the Solic when running in one particular mode. I've heard that a couple of other systems might be copying the dual mode operation for installations that suffer from high grid interference or dodgy power factor.

The proportional units make a humming noise when running. The switching units clunk when they switch on and off.

I've also noticed that the digital displays on most of the units are not accurate- just like most inverters. They indicate 500kWh of energy has been diverted, but if there is a measuring device in line with the immersion heater (like a spare electric meter), the readings are wildly out. A few other people on the forum have noticed this too. A unit's display might say that it is presently diverting 1kW, but when you measure the power using an accurate meter, it reads only 750Wrms. A huge difference. Inverters are the same, with my 'thing' telling me that since brand new (reset when installed), it generated over 3000kWh. My calibrated, accurate and approved generation meter tells me that I've generated only 2100kWh. That's more than an error- it's completely wrong. But I guess it gives users an idea of what power has gone through their system. I trust the official meter.

Some systems can cause the lights to flicker which has been a problem with some of the early designs, but most manufacturers seem to have sorted that out now.

I have been told that units fitted with fans are storing up trouble for later. As once the vents clog up with dust or the fan stops working, they will probably overheat or something, even if fitted with overheat protection. Same goes for inverters in lofts with spiders’ webs.

There are also a couple of kits that enthusiasts can make, but I don't know much about them.

Do I have a favourite? Not really- they are all much the same. A bit like builder's tea and bickies. It always tastes good at teatime.

Hope this has been useful to someone!


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thusharComment left on: 17 July 2012 at 7:30 pm

We just got the solar immersion switch Mark II from installed. It works brilliantly and uses just one sensor to monitor and calculate the net difference between the usage and production. It monitors both house load and the PV production and the single sensor system makes it much easier to install. I used the switch together with the power reducer from to halve the wattage of the existing 3 kW immersion to a 1.5 kW immersion. 

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jbellaComment left on: 1 July 2012 at 2:21 am

Hi Fred,

I do not know exactly what it is but what you are describing looks like a proportional energy controller. There is a lot of discussion about them on Electrician Forum:

but neither of them seems be commercially available yet.

I would definitely recommend to submit your post/photo on Electrician forum since I believe some of those guys could offer better explanation.

Personally I am interested in this kind of controller too.


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fredfred2Comment left on: 28 June 2012 at 1:52 pm

I'm new to this forum, but have been looking to find out who makes this black box. The owners of the house weren't in when we were doing some wiring, but this thing controls a standard 3kW immersion heater directly. The panels are only 2kW capacity, so my guess is that some electronics must turn down the mains to the heater element. There is nothing between the box and the immersion heater, which is rated at 3kW.

How can you run a 3kW heater with a 2kW solar panel, and still not draw grid power? Beats me.

But anyone know who makes this little unit? I'd like to buy one...



pic of the thing, from my phone- sorry about the quality...

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jbellaComment left on: 10 June 2012 at 5:48 pm

Hi all,

I believe this might be a home energy controller (solar version) the most of us were looking and waiting for:           (please be aware that this address might be just temporary for the time being)

This unit has got two current monitoring clamps with wireless transmitters (one way) for home electricity consumption and generation, smart energy controller unit and three (up to twelve) remotely controlled (two way) wireless power sockets.
Unit controls up to three items wirelessly and switches them according to excess solar being generated. The unit measures the power consumption of each plug and only switches the plug on if there is enough excess  power.

Good news is that this unit is reasonably priced too:

You can find more reading about it here:

Although I do not have yet my immersion heater I've just bought this controller and I can report on it in couple of weeks.


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dave_herts_ukComment left on: 17 May 2012 at 10:57 pm

Chris, I am very interested in your Imersion Heater Relay, but have a couple of questions:

1. I am concerned that its seems to only monitor the energy generated by the PV array and not the energy being consumed.

Therefore if, for example the PV array is generating 1.1kW, the Immersion Heater Relay will swich on, regardless of the energy consumption in the rest of the house, so I could end up importing electricity from the grid.

This doesn't make sense as my usual method of heating hot water is via a condensing gas boiler which is a lot cheaper than grid electricity.

Am I correct or am I missing something?

2.  How did you get on evaluating the ‘Adjustable 3800W AC 0-220V Voltage Regulator’ from ebay?  Is this a suitable alternative to a 1kW immersion heater?



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Keith_WalkerComment left on: 7 May 2012 at 9:51 pm

Hi All,

I've finally set up the website showing my attempt at automatically using the spare solar power to heat my hot water. The circuit is rough but it seems to work and will hopefully be improved. Its currently driving a 2kW heater and when the weather improves I switch the output to the immersion heater.

Cheers, Keith

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tedmComment left on: 27 April 2012 at 12:59 pm

Some great ideas here.  I have a 3Kw system installed at a second house and it would be fab to utilise the power generated while the house is unoccupied.

As I have a laptop that continuously monitors the PV output and uploads it to the internet I was thinking of getting some software that would use a wireless X10 signal to switch on a socket (that could be either a heater or the hot water) when the PV output goes above 1Kw.

I'll be watching this with great interest.

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rogercatComment left on: 4 April 2012 at 10:58 pm

Well I'm now running with my final 'two stage' design... look at using heated towel rails in the bottom of the tank either via the 28mm fill or in my lucky case removal of the 15mm drain plug allowed me to put in a 600W electric towel rail heater element.    

As my invertor has a string of LEDs that light up based on power output and a light sensor opposite the 1KW LED fires up my site transformer immersion (720W), then should more sun come out at around 1.7KW another light sensor helps fire up the 600W heated towel rail.

On a super sunny day the immersion heater will trip at say 80 degrees but I'd keep running the towel rail as it'll never get super hot down there. 

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Rudge Energy

Rudge EnergyComment left on: 2 April 2012 at 8:20 pm

I've just ordered a couple of these devices David. Thanks for the 'heads up' on these. Commercial devices are usually more expensive. We will give them a test run on our rig to see if they are robust enough and will report back here.


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David Strong

David StrongComment left on: 1 April 2012 at 12:27 pm

I'm considering using Chrish Rudge's device to control of my immersion heater. Rather than using a builders step-down transformer has anyone tried using a voltage regulator -a 3.8kW unit is available on EBay see and if so, does it work ok, or do they quickly burn-out? Thanks David

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rogercatComment left on: 12 March 2012 at 10:54 pm

I've now put in my solution for now, couldn't find a wireless energy monitor....  however noticed my Invertor has 8 LEDS that go up in a bar depending on PV generation.  So I got of ebay a 12V solar light switch for @ £4.50 and blue taced it around the 1KW led and connected it to a 12V relay (usually closed) connected to the 240V of my 110V site transformer.  On a reasonably sunny day it heats water at top of tank from 60 degrees to 80 degrees.   My only mistake was the 3KW vairable dimmer switch I'd mentioned  it burnt out at a 3KW test, so sticking to meaty, builders site transformer unless anyone has a 5KW+ dimmer ?.

Chris - I would be interested in your latest design but what I really need  is two current sensors and a variable load output connected to 3KW immersion as such ANY PV surplus is pushed to hot water.

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SteveGComment left on: 8 March 2012 at 9:06 pm

 Re: Using an Energy Monitor as a switch source

I too thought this could be an easy way to switch my 1kW immersion but alas things are never that simple! I've seen two monitors with an alarm

1.The Efergy Elite

It emits a pulsed beep/flashing LEDs for 30 secs then goes to sleep for 5mins until it wakes up and if the power is still over the threshold it beeps and flashes again and then repeats every 5 mins. Also the alarm settings are in 1kW steps. So not terribly useful.

Some clever guys have attempted to hack it to access the serial data which I suppose is the more elegant way to do it.

 2. The OWL CM160

Don't know anything about how the alarm functions on this and therefore would be grateful if anybody has any info rather than me shelling out for another surplus meter!

 Plus any other ideas on this gratefully received.

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jones1467Comment left on: 7 March 2012 at 1:59 pm

Hi, i've just signed up to your site and have been looking to do this for some time, I have a 1.5kw solar install through a Fronius inverter. I really like the idea of using the transformer to drop the immersion value to approx 800watts. I see by you drawings that you are going through a set of normally open contacts in the current sensor for feeding the immersion contactor. I have been looking for a suitable contactor myself and would be interested to find out which type you use. I've been looking at a 20amp 2 pole contactor from good ole TLC but were not able to find a 10amp as you had suggested. I imagine this not to be a problem but would welcome your advice. Many thanks, Russ

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Fred1Comment left on: 5 March 2012 at 9:49 am

Please advise the total installed cost, including VAT if applicable, for your system.




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Cathy Debenham

Cathy DebenhamComment left on: 5 March 2012 at 9:17 am

James, they are consulting on raising the export rate. It may be worth encouraging as many people as possible to respond to the consultation on this issue.

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rogercatComment left on: 5 March 2012 at 9:17 am


Could you please confirm the energy monitor you used was an   What I'd like to do is have a single transmitter sending to two receivers, one in airing cupboard set to 1KW alarm / relay out and one in house for my use.  I've played with two energy monitors so far and one beepd for 10 secs on over limit and then stopped, the other was continuous but and on/off beep so would need an extra circuit to trigger relay continuously.

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James@JojuComment left on: 4 March 2012 at 7:10 pm


that's exactly what I did, back in May last year. I used the Effergy unit, set to 1kW, but reduced it to 500W by putting the cable through the clamp twice. ( I had used a similar trick with the Current Cost clamp to roughly measure actual consumption - putting the clamp around both the incoming and inverter cables.)

Like others I was in two minds about green this was, and live in hope that we might get paid a proper rate for export (at least smart metering is coming sooner than you might think...) 

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rogercatComment left on: 4 March 2012 at 4:34 pm

Trick here might be mods to a £25 wireless energy monitor.... they support an alarm function usually preset at something like 7KW.    So reduce this alarm to say 1.3KW and then feed the alarm output from the buzzer into a relay connected to 1KW immersion.    

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Keith_WalkerComment left on: 28 February 2012 at 4:26 pm

Hi All,

 I had the British Gas rent-a-roof scheme installed in October.

Like many on here, I'm wanting to use the wasted power. In the summer to heat water and in the winter to run a heater (either convector, oil filled or storage)

I wanted to only use what was available and after many months I think I'm there.

 My children suggested that I patent the idea. I now discover that EMMA is beaten me to it, but at a huge cost.

 I'm planning to set up a web site showing the circuit, my design reasons and problems and suggest improvements. I'm not an electronic expert, but dabbled many years ago. Hopefully the circuit can be refined by visitors and be available to all.

Anyway, my circuit is a 3KW dimmer automatically controlled by the difference between the power being used and the power being generated.

I just need some sun for final testing before it goes in a box and it's fully live.

 The control circuit has been running 24/7 now for the last couple of months and is working very well. .

The 240V 3KW dimmer may need some more testing and tweaking.

Cheers, Keith



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JohnShrewsComment left on: 26 February 2012 at 9:42 pm

Chris, Thank you for sharing your device on here. I am going to try it. I am quite fortunate as my two pv arrays feed in via a henley block which also feeds the main consumer unit. I am going to fit a mini consumer unit also connected by tails to the henley block to feed the immersion heater. I am therefore going to insert in series a second current sense unit (normally clossed) on the tails to the consummer unit. I willl set this to open at 500 watts. I will set the one on the tails from the PV arrays to close at 1000 watts. The output from the contactor will be fed through a variac transformer which I aleady have and will set this down to about 100 volts so that 500 watts is disippated in the immersion heater. Therefore if appliances like the kettle are switched on in the house the immersion heater will drop out of circuit. Also the first current sense unit which I ordered from Lamode were the fixed core type but I have sent these back as I thought the side adjusted pot was totally impractical to adjust and its proximity to the live terminal was crazy! I see the split core version has the pot top mounted and not so close to the terminals.

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rogercatComment left on: 20 February 2012 at 7:00 am

Here's my cunning plan....  my Invertor has around 8 LEDS in a row that light up as the power level increases....  I intend to 'glue' a solar sensor connected to a relay over the diode that comes on at 1KW being generated.  Hence once 1KW being generated put on immersion - any tips with such a circuit maybe pop into Maplins.  

As for the site transformer I've seen these adjustable 3KW rated dimmer switches on ebay for @£8 and could mean upping the voltage output in peak summertime.

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grantComment left on: 3 February 2012 at 1:52 pm

thushar - I read on Chris's website that they only charge £50 for a 1kw immersion and that ebay item is £59 + £5 delivery

 CHRIS - was about to order this from you but just read there is a Mark2 version on the way that will take into account the actual AVAILABLE surplus PV. Any estimate as to how far off you are with the mark 2? I am keen to get something installed before the summer.

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thusharComment left on: 1 February 2012 at 12:23 am

Just found this 1 kW immersion heater on ebay - suitable for the circuit described here without a site transformer. Hope it will be useful to one who wants to implement this project - as 1 kW immersion is a rare item and really hard to find.

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Rudge Energy

Rudge EnergyComment left on: 29 January 2012 at 10:57 pm


This ability is being worked on now, as will refine the existing deign without having to resort to a £1200 unit from elsewhere.  

This existing design we tried to keep very simple, thus cheap, but peoples demands are now increasing, so we are working on a mark2 version now. 

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Rudge Energy

Rudge EnergyComment left on: 29 January 2012 at 10:55 pm

Hi Gary K

We originally tried a light sensitive switch. Kits are available in places like Maplins. But the problem persisted that the switch sometimes fired up the relay when the inverter was not generating enough power, and vice versa.

The socket in the airing cupboard can be easily carried out using the existing design. Like your idea, another cable has to be run into the airing cupboard.

I tried to describe the circuit as a 'fits all' option. Everyones installation is different. Some people have the inverter in the loft, others in the garage, we have been to install this unit on installations where the PV inverter is in a completely separate building.

If the separate PV module is aligned exactly the same as the main array, this will probably work though if care is taken in correct positioning and switching design. 

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Gary K

Gary KComment left on: 29 January 2012 at 9:51 pm

If you have to put a standard switched socket by the immersion, why not design a unit with a relay/contacter that is switched by the power from a small solar panel-say 20w that is mounted at the same orientation as the main PV system?

This could be placed between the switch and 3 pin plug like a rotary timer.

would this route not be simpler than changing MCB's and rerouting wiring?



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pigasusComment left on: 28 January 2012 at 8:42 am

Chris, will your Immersion Heater Relay only feed the hot water after all other demands for electricity are met?  Or is this ability still to come? Thanks.

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FrancisMcNComment left on: 27 January 2012 at 7:25 pm

Nigeldodd you get that 3p per unit exported regardless of how much you actually use as there is no metering of exported power in most houses.  According to an EST report last year,  most households only use a quarter of their PV generated energy but you are deemed to have exported a half.  Using some of that energy to save on gas for water heating reduces your fuel bills and the resulting carbon emissions.

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nigeldoddComment left on: 27 January 2012 at 6:48 pm

The payment I get for an exported Kwh of energy is approx 3p.

This is about the same as it costs to heat by gas. So I do not benefit greatly by finessing my system to use solar pv electricity to heat domestic hot water. I might just as well export it and use gas to heat.

On the other hand,  to run my server and other electrical equipment that needs low entropy energy, I need electricity. 

It is less easy to use gas to run computers than to use electricity to heat water, but there is no economic advantage in the latter. 

So I shall use solar pv electricity to run my server but not heat my water.

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PeterDComment left on: 16 January 2012 at 8:56 am

I like the simple idea of using a 115v site transformer.  I notice that specs for such transformers include duty cycles.  Can anyone tell me what those figures would mean in terms of feeding a 3kW immersion heater perhaps for some hours continuously?  Could a 3kVA site transformer feed simultaneously another immersion or other small wattage (eg 800W) heaters.

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Rudge Energy

Rudge EnergyComment left on: 26 December 2011 at 7:21 pm

Hi everyone who has been checking the response to this blog, we have now at last started production of the Immersion Heater Relay, which is available right now!

There has been almost a year, well, a Spring, Summer, Autumn and a bit of a Winter of testing on site, and we know that from late Spring to early Autumn, if your not excessive electricity users in the home, our unit will provide a daily supply of hot water.

As mentioned on this and other YouGen blogs, you will need at least a 3kWp PV system to regularly achieve this, but over 2.5kWp will give a benefit.

See the webpage on  for more info

Hope Christmas was a good one for you all :)

Chris Rudge 

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westfielderComment left on: 12 December 2011 at 1:21 pm

Thanks for your comments Chris, I agree with you entirely - my solution would be a hobbyist way of doing it and definitely not for everyone. I also agree that we would have to look at the other items that are on in the house. But we're getting good at that now!

Nick :-)



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sjlyoungComment left on: 11 December 2011 at 10:20 am

I thought about the benefits of doing something like this myself, I even looked at whether batteries could be used to charge in the daylight and pump power back to the inverter overnight, and yes they are out there and available - but after thinking about it, I reached an entirely different conclusion.

If you are on the FIT scheme with 4Kw or less, I believe this is all fairly pointless, it will add complexity to your set up and cost you time and money, why do I think this? On FIT your payments are based on quarterly readings of your supply and export meters, so the net difference between these is your exported amount. This takes no account of day or night, or when you are a net generator or consumer on an hour by hour basis. Because of this, I came to the conclusion that the grid is already functioning as the equivalent of a giant battery for me, so my solution is that if I believe I am going to be a net generator over a particular quarter, all I need to do is leave my existing immersion switched on, and (as suggested) ensure the immersion thermostat is set to a slightly higher temperature than the cylinder thermostat for the gas heating.

So my next question was "how green is this?", at first I thought perhaps I am just being selfish and thinking of the money, but if you think about it, if thousands of us are pumping back into the grid in daylight hours, that will allow the generating companies to run less generation capacity and save carbon emissions in that way.

Perhaps there are still concerns, but my conclusion is that the most I think I would do is perhaps just fit a suitable timer switch to my existing electric immersion so it operates in daylight hours only automatically.

I'd welcome anyone else's comments on this, perhaps I have missed something here?


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Dave Barratt

Dave BarrattComment left on: 8 December 2011 at 1:03 pm

Changed out immersion heater to  a 1 Kw.

Replaced contactor with solid state relay

set point on current sensor set at 1Kw plus any stand by use

Working brill

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Rudge Energy

Rudge EnergyComment left on: 6 December 2011 at 5:55 pm

Hi John

Your immersion heater will be powered by 230v AC mains, thus the modules will not be able to operate them at all on their own.

PV modules will need to be connected to a grid inverter, which is the device which turns the DC current from the modules into AC mains.

However, the 2 modules you have would hardly ever generate enough power to effectively operate an immersion heater, as this would ideally need a 2 KwP system or more (8 of your modules)

I would suggest the simplest method would be to connect one of our immersion heater relays to your existing PV system. It is connected in line adjacent to the fuse box, so will not interfere with the PV system and its warranty.

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john moore

john mooreComment left on: 6 December 2011 at 3:01 pm

Dear Chris,

Re-hot water via solar panels.

I have recently purchased a roof installation fitted profesionally and do not wish to tamper or modify this system!

However I have 2 additional panels.

Nominal Power 240w

Optimum Operating Current 30.2v    7.69A

Open circuit voltage 37.3v

Short circuit voltage 8.46A

Max system voltage 1000v

The rear of the house is facing East. My hot water tank is 36" by 18" with 27" by 3kw immersion heater thermostaticaly set to 60 degs coupled to a timer switch.

My question is: can the two panels be wired in to the existing terminals on the thermostat.


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tin tin 1024

tin tin 1024Comment left on: 21 November 2011 at 5:21 pm

If you what to use your excess electricity and save on your electric have a look at this site  about Intelligent Immersion.
Re: Immersion Heater - PV electricity 

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Rudge Energy

Rudge EnergyComment left on: 1 November 2011 at 11:53 pm

Hi Westfielder

We also tried a Maplin light sensitive switch, and found with some tinkering it could be made to work. However, this is not something to simply easily set up in most situations and will remain the reserve of the experimental hobbyist.

In all the other points you have raised, the main item to pay attention is the fact that with this device, we assume no other big current consuming devices are being used in the house at the same time. Clearly, if the house is being used during the day with washing machines running, etc, the current sense threshold needs to be set at a higher point.


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westfielderComment left on: 27 October 2011 at 7:15 pm

Thinking laterally...

1.If the immersion thermostat is higher than the gas, then if the immersion runs for an hour on a timer during the middle of the day free hot water abounds. Then the gas is set to heat at 5pm. If the immersion has been on then the gas won't kick in anyway because of the lower set thermostat. Money saved

2.If I can do this and only power the immersion when the sun is shining even better. A 2.7 kw heater doesn't need long to heat a tank. And once temp is reached, a well lagged tank won't use much energy if the immersion is left on.

3.So if I set the time switch to 2 hours and then switch when the sun is shining that should cover it. (and just avoid using m/w, w/m and dishwasher during those 2 hours) - No sun then gas is used as before.

A simpler/cheaper method to switch the immersion - many people (Maplin) sell light sensing kits for about £7 that are adjustable for the switching level of light and have a minimum time delay before each change of state. If one of these were stuck on the roof (or in direct light path), then a low voltage low current feed to a contacter to be installed inline with the timeswitch in the tank cupboard. (Not much work for a sparky because all the low votage/current stuff can be done by yourself.

Have I missed something obvious?

If not - I think I will go ahead and install such a system and update this blog


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Dr Chris

Dr Chris Comment left on: 27 October 2011 at 11:21 am

Chris Rudge asked for suggested improvements to his admirable switching system. Here's a suggestion (by a non-electrician!).

I'm not sure of the advantage of knocking down the current using a 110v site transformer (£50 plus plugs and wiring) when for the same money you could buy (or have made) a lower wattage immersion heater, which would then use the existing (230-240v) wiring. There may be some point I'm missing here, but it seems to me that you'd have less clutter in the airing cupboard and a new immersion heater element. Those who want could still run the immersion heater overnight on economy 7 (you'd have to run it for longer, of course).

In these days of programmable timers, it's hard to see why the standard immersion heater has to be rated at 3kW anyway.


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chris money

chris moneyComment left on: 25 October 2011 at 11:15 am

I have a 3.8 kw solarpv sysyem in place; working well; I would like to use the daytime electricity to heat hot water; however, we have a condensing combi boiler, for hot water and ch, but no hot water tank.

There is space in the bathroom for a tank, but what would we need to do to get a supply of hot water to the shower/bath et.c?

I have read about the auto control, but am a bit concerned about the initial cost of installing a tank, and the neccessary wiring et.c.

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PatrickComment left on: 23 October 2011 at 3:40 pm

Hi all,

is it possible to set up a similar system for a wind turbine? I'm running an Eoltec Scirocco 6kW turbine off grid with a battery bank and Standby diesel generater. Is there a way that I can "dump" excess power into an immersion when my batteries reach float voltage? If so, how do I control the load to match generation?

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drleebaldwinleeComment left on: 18 October 2011 at 1:50 pm

I am a little surprised that there is not a commercially available current sensor that is sensitive to direction of flow.  My meter (a current sensor) flashes rEd and switches on a bright red LED when we are exporting current.  Could not this technology be used as a sensor to drive an immersion heater switch relay? 

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JHAMERComment left on: 16 October 2011 at 4:44 pm

This looks like a great idea.  Looking at the diagram, the sensor needs to go on the live coming from the inverter.  I have just had a system installed and there is only a single 2.5mm cable running from the inverter to the fuse box (I was surprised that it is such a small cable for a 4kW system).  The switch sensor looks like it needs to clip onto a relatively large cable to me.  Do you know if the system would work with the whole 2.5mm cable running through it or is there a way to work around this?  Thanks.


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webbyComment left on: 9 October 2011 at 11:41 am

This is a great idea, but it means you need to be able to stop your existing boiler re-heating the hot water after early morning showers/baths. This in turn means you need to set your timer so that the hot water and central heating go off together (or get a new timer that splits them!).

One great solution it seems to me would be to somehow use the excess PV electricity to heat/warm the hot water in the central heating system. There seems to be real gap in the market or am I missing something?

I'm not looking to heat my house via PV, but have some background warmth during the day so the gas boiler has less to do when it comes on when everyone gets home from school / work. Air source heat pumps require a whole new system so are too expensive, don't work in older properties and also not renewable.


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sashtonComment left on: 28 September 2011 at 1:58 am

I like your basic approach to the issue and feel it has real merit. I approached the problem from a different angle and analyse my export by pulse counting my A100C export meter. There are many ways of doing this. I went for an Arduino based reader but I have seen others based on Vellman kits used on other websites. Currently I control loads using X-10 but am considering starting again based on a Sunny Backup system M. The first question I need answering on that count is whether I'm going to have problems with it using my Fronius PV inverter instead of it's recommedation (of course) of an SMA one.

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InglesideComment left on: 27 September 2011 at 10:34 pm

If I've read this correctly, once this circuit has been installed the immersion can't be used on conventional mains? We already have SHW, so we'd need to turn this off during the summer, otherwise we'd boil the water! And 50C won't be adequate all the time, or attained during the winter. Or can we switch the immersion heater back to mains when necessary? As an aside, with 2 adults in the household, both at home all day (and no children) we're exporting about 60%.

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Cathy Debenham

Cathy DebenhamComment left on: 26 September 2011 at 9:16 pm

Hi Rob While I understand your concern, I don't think that it's likely to happen, as research the Energy Saving Trust is doing indicates that the average installation exports considerably more than the 50% deemed amount. I have a meter that records how much I export - and it's getting on for three quarters for a 2kW system. And I work from home, and actively try to use power hungry appliances when the sun's out. Also, given the old and decrepid nature of the grid, it makes much more sense for microgenerators to use as much of the energy they produce on site as they can - that's where it's most efficient.

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AshtreexComment left on: 26 September 2011 at 3:50 pm

Even simpler, we had a timer switch fitted to our 2kw backup immersion heater, and set it for 90 mins in the middle of the day. It comes on come shine or rain. Tank is very well insulated. No gas for water heating needed and water is actually hotter than with the gas boiler. I monitor electricity and gas use weekly and we'll probably switch back to gas come winter.

No complications, no electrician needed, no implications for the FIT and I bet  80-90% of the electricity is free. 

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RobertPalgraveComment left on: 26 September 2011 at 3:07 pm

There is another side to this which I will air and hopefully avoid being seen as too judgemental! I think there is a need for caution with what's being proposed.


The unused energy from a typical grid-connected microgeneration system is exported to the grid. If you don't have an export meter, you are benefiting at the rate of 1.55p per generated unit - this year at least - on the basis that half your generation is deemed to be exported at a rate of 3.1p per unit.


If people build add-on control systems that detect when power is being exported or is above a certain level and make use of it locally, for example as above in a water heat store, this is distorting the basis of the Feed in Tariff.  If a lot of this goes on, the original estimates of how much would be exported from microgeneration schemes across the country will have to be re-assessed. DECC and The Treasury could quite reasonably take the view that the tariff structure needs to be reviewed and possibly adjusted. For example they might conclude that instead of 50% of generated energy being exported from solar PV, the actual figure is only, say 25%. Their response could then be to remove the deeming component for exported units and force everyone to get an export meter so DECC/OFGEM only pay for what is delivered.


If that happens, I will suffer, as will thousands of users who don't want to install an export meter or complex heating systems to absorb 'spare power'.


There is also a risk that this will generate a backlash from those who already believe the FiT is over-generous. And lead to DECC applying degression faster in later years than otherwise.


Or they might say 4kWp is rather a generous threshold for the first break point in the generation tariff and reduce it. Using the argument that generators at that size are milking the tariff by soaking up the excess generation that is being paid for at the highest rate.


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Chris Rudge

Chris Rudge from Rudge EnergyComment left on: 19 September 2011 at 10:45 pm

Yes Pigasus, it is possible to ask your tank maker to incorporate a 2 1/4 bps immersion heater fitting. Then rather than installing a 3kW immersion and a site transformer, simply install a 1kw immersion from the start.

For your proposed application it would be better to use the device to sense the exported power from the premises, than generated power from the inverter. We are working on this at the moment. The main problem to overcome is the current sense unit does not know which way the current is flowing, either into the house off the grid or from the PV to the grid is all the same to this little unit!

Watch this space.


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pigasusComment left on: 19 September 2011 at 10:17 pm

I like the idea of diverting solar pv to heat domestic hot water, but I have a couple of questions.  I'm planning a new build with the heating and hot water to be provided by a wood pellat boiler.  Is there a way that I can make sure that the electricity provided by the solar pv is used first to provide electricity for other than hot water with only the excess being diverted to produce hot water?  I'm also planning to provide domestic hot water via a thermal store.  Is it possible to have an immersion heater in such a store?  Thanks.

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