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How to use excess solar electricity for water heating

Posted by Stuart Houghton on 28 August 2012 at 2:02 pm

I am asked again and again how to maximise the use of the generated electricity of my customers' solar PV systems.

Apart from using your large appliances when the sun is shining bright, the largest benefit would be using the excess electricity to power your immersion heater. Unfortunately it is harder than it sounds to make real savings.

As long as you currently use another source of energy to heat your water, and you have an immersion heater installed, all of the following are possible ways to control the immersion heater. 

Totally manual 

The sun is shining and your inverter or remote display is indicating that your PV system is generating over 3 kW and you feel that you are not drawing much more to power the rest of your house. Just turn on the immersion heater. Remember to turn it off when the sun goes down.

It's cheap and if managed correctly will save you money.

You forget you have switched it on and then it rains for a week and you have purchased many kWhs of full priced electricity!

There are not many days a year you will be able to switch it on. You can turn it on when you system is only generating 1kW but then you will be buying 2 kW of electricity from the grid.

Manually controlled 

Your PV system is connected to its own consumer unit (fuse board ) and you install an energy monitor to your house consumer unit supply and your PV supply consumer unit. You watch both remote monitors and subtract the house reading from the PV reading. When there is sufficient spare capacity, which is up to you to decide, you turn on the immersion heater.

Its cheap and, if you only turn the immersion heater on when there is sufficient power available to totally power it, you will make big savings.


You forget you have switched it on and then it rains for a week and you have purchased many kWhs of full priced electricity!

How many days a year will you be able to switch it on. You can turn it on when you system is only generating 1kW but then you will be buying 2 kW from the grid.

Inverter controlled switching

You could have a system installed that monitors your PV output and when it is above a certain threshold it will switch on your immersion heater.

To make the system work more efficiently you restrict the consumption of your 3kW immersion heater to 700 watts via a 110v transformer or have your existing 3 kW element replaced with a 1kW one.

This system is relatively cheap to buy and install. Provided that it is installed with a transformer or your immersion heater element is replaced it will switch on when there is sufficient energy to power the immersion heater.

It does not take into consideration how much energy the rest of your house is consuming. If you turn on a kettle or your washing machine when the PV system is over the switch-on threshold for the immersion heater you will pay full price for the electricity to boil the kettle. Of course, this depends on how much the PV system is generating above the switch-on threshold for the immersion heater.

If you chose to replace the immersion heater with a 1kW version It can be extremely difficult to replace which is why this is a job for a plumber. We would also recommend a dual input element so if your main heating source fails you will not have to wait hours for a hot tank of water.

If your immersion heater is permanently wired through a transformer and your main heating source fails your will have to wait a long time for a tank of hot water unless it is wired via a bypass switch which will add to the cost.

Semi-automatic switching

There are a few systems that monitor your house consumption and PV production using sensors that connect to a main unit via wireless technology. The main unit allows you to choose at what point to switch on the immersion heater, dependent on the ratio of how electricity you are using to how much your solar PV system is producing.

You could set it up so that is turns on the immersion heater when the house is consuming less than 600 watts and the pv is producing more than 3000 watts. In this scenario if the PV was producing 3100 watts and the house consuming 500 watts then 2600 watts would be supplied to the immersion heater from the pv and 400 watts from the grid.


As this system monitors both consumption and production automatically you will save more money than with the other methods. No changes will be needed to your existing immersion heater.

Unless the system is setup to only switch on the immersion heater when there is sufficient excess power, then you will always be drawing some from the grid and it is difficult to know at what point it becomes economic to do so.

For instance if the system was set up to switch on the immersion heater when the house was drawing less than 600 watts and the solar PV was producing more than 3,000 watts. Your house would have to be consuming 500 watts and your solar PV producing 3,500 watts for there to be 3,000 watts spare to totally power a 3kW immersion heater. This system is not cheap and could cost in the region of over £600 to supply and install.

Totally automatic system  

There is a system manufactured by Coolpower called Emma, this is by far the most sophisticated system available to control your immersion heater.

This system monitors your house consumption and PV production using sensors that connect to a main unit. It then calculates the available amount of spare power which is supplied to the immersion heater. For example if the house was consuming 500 watts and the pv was producing 1000 watts it would supply only 500 watts to the immersion heater.


This system is total automatic and as it only supplies what is excess to your requirements it will use all of your available spare electricity until your immersion heater reaches its cut off temperature. No changes will be needed to your existing immersion heater.


This system is very costly to install with likely costs to be around £1,400 inc installation.

The main benefit to all of the above ways of controlling your immersion heater is the use of the electricity that would be otherwise be lost to the grid. Remember every degree increase in the temperature of your immersion heater tank by your immersion heater is a degree less that your gas or oil fired boiler has to heat.

From August there are a number of new competitors to Emma entering the market. YouGen will publish a review as soon as we have tested them all.  

More information about maximising the use of PV-generated electricity

How to store your solar generated electricty for use in the evening

How easy is it to use renewable energy 'off-grid'?

Making the most of your solar generated electricity

Solar PV information page

About the author: Stuart Houghton is director of Abacus 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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22 comments - read them below or add one

Roy Mawford

Roy MawfordComment left on: 27 April 2015 at 11:46 am

We are looking for some help, please. We are very pleased with the 3.5kW PV Roof Mounted Solar System we had installed in March 2014. We now want to explore whether it would be cost-effective to install a mechanism that enables us to use the electricity we are generating to heat our hot water.

We have LPG powered hot water and central heating. This utilises a conventional hot water boiler, which also has an immersion heater that we currently only use as an emergency back-up. So, we currently heat our water almost all the time by LPG.  The immersion heater is controlled by a simple on/off switch, which is off almost all of the time.

We use our generated electricity to run appliances (washing machine, dishwasher, etc) in the daytime. We also use daytime electricity from late spring to early autumn to heat our swimming pool, via a heat exchanger. So, we hope that we are making good use of our generated electricity, and reducing the amount of other electricity we have to take (and pay for) from the grid.

Is it likely to be cost-effective to heat our water, for some of the time, using our generated electricity?  If so, what would be the best mechanism to use?

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Ian Nicholson

Ian NicholsonComment left on: 13 October 2014 at 5:22 pm

The last 2 days has been very poor, fog then rain all day so PV generation low. I have non-optimised immersun installation, that cost less than £300 all up. Being an engineer I installed and commissioned it myself. It is direct to the exiting immersion heater and that only heats top third of my 140Litre tank. The return on investment will take around 4 years, because having mains gas at 4.5p/kw/h and that means a lot need to be diverted to reach £300. But it dose provide a very useful boost to 65C from the 40C that the gas provides and with good water managment the baulk of our hot water needs are meet. Even for a family of 5 adults, it the green engery that counts for me not the money, I have seen over 5Kw in to the tank, a lot of hot water.

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paul53Comment left on: 7 September 2014 at 10:41 pm

think the  immersion will be ok  but lucky  to  run the aga  too, make  sure the  switch you  buy has facility to  send to  2 loads the solarimmersion does but  im waiting until winter to see if there's enough energy to power a  500 or 200 watt heater after the waters  hot

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WellhouseComment left on: 4 September 2014 at 10:12 pm

Hi all - very new to this but wondered if I could get some advice.


We've a new built rural detached house in North Scotland with high-ish levels of insulation. Heating is via wet underfloor drawn from a 1000 litre akvaterm thermal store / tank which also provides DHW.

This tank is currently heated by a 3kW immersion heater running on Economy 7 night rate electric. (there's a second unused immersion heater "pocket" in the tank

In addition we have an electric aga (the night storage 30A variant) also heated in the same way / E7

We were going to fit (and have already bought) a boiler stove (evolution 8) but the background heat from the AGA, the insulation and gains from glazing are such that we'd never actaully light the stove because we're already overheating most of the year.

Therefore we're looking to put in Solar PV but wonder whether we can drive both the immersion heater and the AGA with daytime generated electric?

It seems that we've inadvertently got two big "rechargeable (heat) batteries" but dont know how to re-configure to accept daytime charging?

Would welcome any comments and if need be, be happy to pay for detailed asisstance  / design etc..


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Guy Bolton

Guy BoltonComment left on: 5 December 2013 at 12:10 pm

Still a bit confused!

I am planning to have installed a 4KW PV solar system with an SMA branded inverter.  We currently have an oil fired combi-boiler.  We don’t have an immersion heater or hot water tank of any description. 

Putting cost considerations aside for a minute.  I understand we could fit a hot water tank with an electrical element at the base to use the excess solar generated electricity via an “intelligent” switch; with a coil from the boiler for top-up on a timer.

My installer is planning to use this “intelligent control system” GEM – made by Apollo

Q1.  I think this gets ride of the problem of not importing electricity from the grid to feed the immersion element (as we want to use the oil boiler to do this).  Is this so?

Q2.  There is much talk of using different KwH – elements when employing solar PV excess output.  Was this because you risked importing excess electricity to power a 3Kw element before more intelligent switching devices became available?  Or is there something intrinsically wrong with using a 3Kw element and dribbling small amounts of power through it?  In short; is it better to go for a 3KW element with the intelligent switch or is it a “good idea” to go for a smaller element?

Q3.  There is much talk of 110V transformers.  Why are they needed?  Would I need one in addition to the above?

Q4.  Does anyone know of sensibly priced hot water tank with the electrical immersion element placed in the right place – at the bottom.

Hope this makes sense!  I am a Pharmacist – not a plumber or electrical engineer.

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jccj18Comment left on: 21 March 2013 at 1:59 pm

ive just bought a Solarimmersion Mark III - was only £258 with delivery and I used a code - JC1803 - which gave me a tenner off. so bascially free delivery. 

Have monitored it last few days and seems to be working great! anyone else got one of these?


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Paul leigh

Paul leighComment left on: 11 November 2012 at 3:43 pm

I  have fitted myself 3.6 Kw pv system

I am a plumber and the simple way (and cheap) to warm up your central heating system before the gas central heating, ( combi ) boiler  kicks in is.

I have connected 2 x 600 watt electric towel rail elements. at diffrent parts of the house heating system drops via a 15 x 22 x15 mm tee and a short length of 22mm pipe and 22mm x half inch inc female  (brass or copper to screw the element into.)

with the element inside the 22mm pipe I switch on via a switch in the kitchen, when the sun is shining to use the spare electric to warm the water  you could fit a timer if required .

Cost under £30 each element add as many as you want . 



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AnonymousComment left on: 4 November 2012 at 7:36 pm

Please visit  The immerSUN is what you are looking for, practically "plug and play", totally modulating and "DOES NOT COST AN ARM AND A LEG" to install (payback between 2 years for electric and oil and 5 years for gas!

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Solstice Energy Ltd

Solstice Energy LtdComment left on: 10 October 2012 at 2:48 pm


 You're off the gas network, so PV - immersion heating is a very good use of that energy. It's the many other buildings with gas boilers where the sums don't add up.

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Solstice Energy Ltd

Solstice Energy LtdComment left on: 28 September 2012 at 1:49 pm

Fred 1, I agree;

We've had a look at the financial benefits of directing PV energy to an immersion heater, and the conclusion is that as the cost of gas is less than the current 4.5p/kWh price paid for exported electricity, there is no point in heating a cylinder of water with an immersion heater if you have a reasonably efficient boiler (or even better, solar thermal and/or heat pump).

There is a 'but' though - Until such time as the electricity companies roll out smart meters which can read export values as well as import, you will be 'deemed' to export 50% of your power. So for that period, it seems at first glance to make sense to use that power for the immersion heater - you save money on your gas bill, but still receive the same (50% of generation) export payment for the PV's.
But let's imagine you are fitted with a smart meter in 3 years time (1095 days). In that time you might save 15p / average day* on your gas bill, so you will save 1095 x £0.15 = £164. This still does not cover the £300+ cost of a PV - immersion switch.

So unless you don't have a gas supply we're not convinced by PV immersion switches, and still think that the best use of PV power is to try your best to offset other electrical appliances during the daytime.

The carbon footprint of the house might be reduced by decreasing gas dependence a little bit, but exported electricity is never wasted.

* = according to our calcs - available on request. 

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Fred1Comment left on: 18 September 2012 at 5:57 pm

Looks good technically Stuart,  useful information. From their website it seems to capture nearly all the electricity which would otherwise be exported.

I am in most days and heat water by mains gas, so I would be backing out gas at say 4.5p/Kwh using this system.

Say with my system I could " not export " 1,000 Kw hrs in a year and save gas, that would be £45 per year saving if I have I calculated correctly.

What happens when I gat a smart meter ???, I would no longer be using "deemed export"  and with this system would I lose lose my 3p/kw export payment???

Would I then be saving 4.5 pence on the gas but losing the 3p export payment, if so, I would only save say £15 per year using this system.


Any idea of the cost of the system and what it would cost to get a MCS installer out to fit it ??/







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Current Energy Solutions Ltd

Current Energy Solutions LtdComment left on: 18 September 2012 at 10:02 am

A quick update, a new product has just come to market which cost far less than the EMMA unit but has all the same benefits.  

Please see here

We have fitted several of these units over the past few weeks and our customers are delighted with the results.

Stuart Houghton

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richmcComment left on: 6 September 2012 at 3:37 pm

There are now may enthusiaste building and using inteligent immertion controlers, try looking at-


I've been using one myself for some weeks, cost me under £90 in parts and supplies all my hot water free never goes into import and reduces wastful export.

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PeterDComment left on: 3 September 2012 at 4:33 pm

Hi Ingleside,

"I hired a 110V transformer so that whenever the immersion heater came on it would use the minimum of imported electricity, but it tripped out the 16A breaker when it came on, so I gave that up. No idea why it happened....."

Tripping is quite normal and frequent when switching a large inductive load such as a motor or large transformer.  The problem is the current surge which trips out standard 16A Type B MCBs.  I solved the problem by replacing the MCB with a type C which is meant for that sort of load.  I also upped the rating to 20A but only do that if you know that the immersion heater supply cable and the overall Consumer Unit load will allow it.  The steady current at 115V is about 6.5A and the power is ~700 watts.  That suits very well my 2.16kW peak system.  Admittedly it will probably take a copy of years to recoup the cost of the 110V transformer (From Screwfix).


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Ian Smith

Ian SmithComment left on: 3 September 2012 at 9:28 am

It might be prudent for anyone assessing the economic payback times for investments such as the EMMA system to assume that the current arrangements for paying for exports from small FIT generators (that deem the export amount to be 50% of that amount generated for the purposes of calculating the export payment) may cease when smart meters are fitted as these will be able to measure the actual exports.  This will mean that one's export payments will reduce if you divert the power - which is not the case at present.  If the costs are going to be in the hundreds of pounds for such systems, the proposed roll out programme for these new meters will fall well within such payback times.

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InglesideComment left on: 1 September 2012 at 5:00 pm

I hired a 110V transformer so that whenever the immersion heater came on it would use the minimum of imported electricity, but it tripped out the 16A breaker when it came on, so I gave that up. No idea why it happened; I just changed to another plan. I've kept life simple. We use a timer on our immersion heater to benefit from the Economy 7 tariff. I've simply set it to run from 11:45 - 12:15 GMT. Our 4kWp of PV will be generating at that time, so I will get reduced price electricity at worst, and free electricity at best.

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Robert Sebley

Robert SebleyComment left on: 1 September 2012 at 10:45 am

I have been pondering the challenge of automatically switching on my immersion heater when the sun is bright around mid day, and think I have finally managed it.  I am an electrical engineer, and that helps, so with a few purchases from Maplin (relay, relay base, small solar panel etc..) and using an old plastic electric kettle to house the components (and provide a neon indicator); for around £30 it now switches on when the solar panels are generating around 3kW.  Still have some fine tuning to do and I hope to add a control at a later date to adjust the switch on/off points.

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Robert Sebley

Robert SebleyComment left on: 1 September 2012 at 10:22 am

I didn't take part in the FIT supplier survey, however I was interested in the results.  I have been with BG from the beginning (nearly 2 years now) and they have always sent my FIT cheque within the promised 4 weeks, however your survey may have had an impact with them because the last payment only took 8 days!

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PeterFTComment left on: 31 August 2012 at 1:37 pm

Excellent article clearly explaining the pros and cons of different options. I am looking forward to reading about alternatives to EMMA as  I have a 4kW that I want to maximise the benefit from. Has the option of having an export meter intalled ever been reviewed / assessed?

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davejones79Comment left on: 30 August 2012 at 9:11 am

It's a shame there are fewer and fewer Gas tariffs available with No Standing Charge (NSC), due to the 'simplification' of tariffs. By the looks of it we'd be able to reduce our Spring/Summer gas usage to near-negligible using one of the new devices, as we only use gas for central heating (which is off at that time of year) and hot water.

With an NSC tariff, if you can limit gas usage to less than the Tier 1 threshold per quarter you will be paying less than for the same amount of gas on an equivalent tariff with a standing charge. Without an NSC tariff though the minimum Gas bill will be Days x Standing Charge (£0.22+VAT/day on EDF Fixed s@ver v2 in SWEB region, or ~£21 a quarter).

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Rudge Energy

Rudge EnergyComment left on: 28 August 2012 at 8:07 pm

That just about covers it Stuart. We've been testing different systems since last early last year after I put together the unit described on an earlier blog, and since there has been a whole marketplace of devices that will carry out this function in some way. 

They range from around £150 right up to the EMMA which as you say is the old time honoured unit to carry out this function, but on a domestic level is cumbersome and expensive at £1500 +

I am running some tests right now on a range of units we have purchased or borrowed from manufacturers and the results will be exclusively published on YouGen in the next few weeks, as indicated above.

Chris Rudge  

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shricthismComment left on: 28 August 2012 at 2:41 pm

Thank you for this very informative article, I look forward to your review on the new products. Now there are so many people with Solar panels there will surely be more focus on utilising the generated electricity most efficiently. I realy don't like selling electricity for 3p and buying it for 13p!

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