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Using solar PV to heat your domestic hot water

Posted by Paul Hutchens on 26 May 2011 at 10:25 am

Q: Having recently installed Solar PV, I will soon be replacing our old boiler and hot water cylinder. The heating and water run on gas. Does it now make sense to use electricity to heat the hot water for the house?

A: This is a really interesting conundrum - well I think it is - but maybe I am a dull solar enthusiast! Nevertheless I will try to purvey my passion in my answer.

The thing is that mains gas is much cheaper and has a lower carbon footprint than electricity. In addition to that, we in the UK have spent the last 40 years transforming our energy usage away from other fuel sources to gas if we can – remember the 'dash for gas' in the seventies – because we have had the luxury of cheap gas from the North Sea.

The bad news is that this is now running out and by about 2015 we will be importing around 90% of our gas from Russia and the Middle East.

So guess what will happen now - that’s right prices are rising and will continue to rise. So maybe gas will not continue to be the cheap alternative that we have all got so used to.

From an environmental perspective our calculations and assessments assume that electricity comes from stinky, dirty coal and gas fired power stations. What if the electricity that we used was zero carbon?

What if that electricity came from our own, on site solar powered electricity or PV system?

We would then be provided with a free and zero emissions way of powering things – such as an immersion heater.

There is another benefit too. We all know - or you should by now if you read the YouGen blogs regularly which I am sure you do – that you get paid for all the electricity you use from the feed-in Tariffs. Excellent!

You produce free electricity too, for which – on average for a domestic consumer – you would have paid about 15p per unit from your energy supplier. If you cannot use it you export to the grid and get 3p per unit instead. Well I am no mathematical genius but I can work out without the aid of a calculator which I would rather do.

So what to do with the excess' electricity to maximise our investment? How about using to power your immersion heater and store the energy in the form of hot water than you can use later to bathe or shower with?

I am not sure that you would want to completely replace the connection to your boiler – in the winter or when it is dark you may be glad of it. But wouldn’t it make sense to top it up with free electricity when it is available?

You can also adjust your hot water heating controls to take best advantage of the free energy. Your installer can advise you about this to ensure that you get the best out of the solar power without ending up with a cold shower!

Photo by Ged Carroll

More information about using your solar PV electricity on YouGen 

Solar PV information page

How can we best use our solar generated electricity? 

Auto control enables the use of solar PV for immersion heater 

How to store your solar generated electricity to use in the evening

About the author: Paul Hutchens is founder and director of Eco2Solar, which installs solar systems around the UK.

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

Cathy Debenham

Cathy DebenhamComment left on: 26 April 2013 at 8:54 am

Hi Gary

If you click here and scroll down you will see Chris Rudge's reviews of three of the devices on the market including the immersun.

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GaryComment left on: 23 April 2013 at 10:30 pm

Hi all,

I am setting out on my Solar PV journey and thinking about some sort of proportionate (rather than manual or fixed) PV switch to divert excess power to the immersion heater.  One or two products are mentioned here but I have not yet found any comparison posting for the various products, either here or elsewhere on the net.

So far I have found the EMMA, which seems to have a reputation of being very expensive and looks quite large for domestic use.

Then there is the Immersun, which reads quite well; the Solic which reads similarly, S&G Wooldridge and the Solar Switch product.


I am sure there are more but has anyone tested/rated them?






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MarcusComment left on: 21 May 2012 at 11:39 am

The problem with connecting Solar Panels directly to a resistive load is there is no Max Power Point Tracking (MPPT).  Thus the resistive load is only a good match at a specific incident power level.  I looked at using a DC charge controller to do MPPT but they all require a battery to be connected and there are system losses involved in the voltage regulation.  Furthermore they would drop out if the input voltage drops below the set output voltage.  Finally their expense, although less than an inverter is still not cost effective.  However I did have an idea:

The power curve for solar panels, unsurprisingly is the inverse of that for a diode.  The panels in question will give a constant current output for a specific incident power up to a certain voltage whereupon the current falls away rapidly.  The maximum power point is the knee on this curve which for this panel is 30V for all incident solar power levels.  A Schottky diode when forward biased drops about 1V regardless of current.  Thus if I make a load of 30 Diodes it will match the Maximum Power Point of the panel.  The maximum current capability of the diodes is 10A and the panels under 1000W/sq m give about 8A at 30V. For the 4 panels below I would need 120 diodes in series but they are very cheap.  My idea now is to connect the diodes in series and sandwich them between 2 metal plates with some heat transfer paste. This could then be placed under the hot water tank acting as a preheater.  I might have to parallel 2 or 3 diodes in a string of 120 (slightly less to accommodate resistive loss of cable) so that the current is shared and a diode failure would not break the whole string. will it work and how much more efficient would it be than a plain resistive load?

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arwooldridgeComment left on: 13 May 2012 at 11:04 pm

I think the controller you need is here:-

Its unique in that it actually senses the net power available for export and diverts it, however small, to the heater, normally a hot water heater.

No need to fit a smaller immersion heater ,and never imports power for the heater. It just patienly waits for net surplus of pv generated over consumed power.

Will even power additional loads like towel rail heaters etc if purely resistive.

Operates even at 50 watts surplus power! These are great devices and should save its cost in less than a year.

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MarcusComment left on: 4 May 2012 at 10:46 am

Further to my last post, when using a Solar PV FIT system to power an immersion heater there is an alternative to using a transformer to downgrade a 3kW to 1.5kW, but only if you have a twin element immersion or 2 immersion heaters.  If you series connect the 2 elements the overall power becomes 1.5kW.  In that each element is only handling 1/4 of their rated power it would probably improve their longevity too.  I have done this on my twin element immersion and used the spare live feed to back connect the output of the thermostat to the neon in the switch.  This means I can actually see when the immersion is drawing power.  In that this involves rewiring the immersion heater and switch you should be competent with electrics before attempting it.   

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MarcusComment left on: 4 May 2012 at 2:58 am

I am looking to connect 4 x 225W Solar PV panels directly to an immersion heater and wondered if anybody has any experience of doing this? 

The panels at full power produce 4 x 30V (Pmax) which implies that a 16 Ohm element would be ideal, however for simplicity of supply it is intended to use a 230V 3kW element which has a resistance of 18 Ohms.  I am aware that ideally the load resistance should dynamically track the internal resistance of the panels but have no idea how much difference this will make.  Can anyone advise?  

I understand that there are concerns about contact arcing in the thermostat, although given the load is resistive and the maximum DC current 2/3 of the rated AC current believe this should not be a problem.

I should explain the panels are being offered to me really cheap(£100ea) and I live in a listed building so cannot easily add direct solar water heating to it.  The advantage of using solar PV is that I can site the panels some distance from the building behind some rocks. To be cost effective the system cannot include the cost of an inverter, hence the direct connection to an immersion heater.  I already have a 4kW Solar PV system on FIT connected to my supply but installed on the pitched roof of an outbuilding (where my meter is).

An incidental advantage of the above concept is I can use the Solar DC supply to the immersion to switch in by relay my main AC immersion heater to maximise my use of the remote 4kW Solar PV System.   

Grateful for any advice on this.  

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Cathy Debenham

Cathy DebenhamComment left on: 15 March 2012 at 11:18 am

Hi 50dwill. You may find this blog useful.

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50dwillComment left on: 14 March 2012 at 8:46 pm

Hi everybody i need help i have read with interest all the ways you try to control the spare energy from a pv aray, this is where i am up to. we have a 3.6 kw system and i want to use the spare energy to run a 3kw water heater via a 110v transformer, in the day only.

We are a low electric dependent house hold so has anybody got a up and running system they would like to share? have some electrical background so are willing to try any of your ideas


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The Suit

The SuitComment left on: 24 December 2011 at 5:33 pm

Been there done that have the T shirt and a three peace suit.


I have a 4kW PV System, gas central heating with a gas/electric hot water tank.


When I looked into using the excess power I was generating to heat my water I found that a device call an EMMA was on the market, but at £2000 the return on capital meant I might as well leave the money in the bank. This led me to design and build my own capable of up to 5kW output at a cost of under £100. This supplies us with all our hot water free from March to November, when I turned on the gas central heating. The gas heats the water to 60deg and the solar electric when available to 80deg giving us enough for two days. Warning for health and safety you must have a thermostatic mixing valve fitted.


I also fitted a 1.7kW electric storage heater (£20 from eBay) which is great for those cold evenings in late and early summer but when electric generation drops below 6kWh struggles to make a significant difference.






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Mach210Comment left on: 18 October 2011 at 10:57 am

As someone pointed out, a 3kW water heater uses more than most domestic pv, especially in winter with say 1kW pv max.  If the heater is still taking 2kW net, you are paying for this at 15p per kWH, not good economics.

The solution would be to step down the volts to the water heater, say 2:1 so it only draws half the current and 1/4 of the power, timed to run during the middle of the day.  And stat the temperature to a value just above the one controlling the gas heating, so pv provides the main input and gas comes on to top up.

I feel a search for a suitable (approved) transformer coming on . .

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webbyComment left on: 9 October 2011 at 11:31 am

I love the idea of the immersion heater on a timer for generating hot water for baths / showers - really simple

What I really want to be able to do is generate some background heat for my house during the day (other than installing electric storage heaters). We live in an old Edwardian house with no cavity walls. We have done all the insulation we can but it's always going to be a cold house in the winter as we are too mean to leave the heating on during the day.

I feel there is a real gap in the market having someway of converting my excess PV electricity generated during the day in to some latent background heat. It won't heat the house, but it will keep it from getting cold. I've discounted air source heat pumps as I just want a 'top-up' system

Any ideas?

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Cathy Debenham

Cathy DebenhamComment left on: 25 July 2011 at 8:03 am

Thanks for that Icarus. It's always great to get people's real life experience.

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IcarusComment left on: 23 July 2011 at 3:49 pm

Hi. I've detailed my own experiences in using PV panels to heat a 3KW electric immersion on my blog.

I used a timer to turn on the heater between 12:00 and 15:00. The heater has a thermostat and should automatically turn itself off once it’s reached the pre set temperature. Unfortunately, with a 2.66KWh system there is very little chance of me economically heating the water as on average in June/July 2007 I'm only generating 1.1KWs.

I'd love to tell you it's the way forward, but gas for the moment seems the cheaper option.

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Solent lecturer

Solent lecturerComment left on: 20 June 2011 at 4:35 pm make a combined Photovoltaic/thermal panel PV/T.  They have two types, one which optimises thermal performance and the other which focusses on electricity generation.  They were used in a Grand Designs project a few years ago and are among Kevin [McCloud]'s Green Heroes.

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AshtreexComment left on: 2 June 2011 at 11:10 am

We simply had a timer switch installed to activate our backup immersion heater (otherwise gas boiler hot water) for a couple of hours mid day. It's rated 2 kW. Our PV mostly generates about 2-3kW midday from May to Sept, less in the winter. Yes, the tank's well insulated and the water is still plenty hot in the morning. We'd never remember to switch on/off with the sun!

We use about 150 kWh electricity a month total in these months which seems to be a small fraction of what we used pre solar PV so we must be getting something right.

I looked at the EMMA brochure - I'm always up for a gizmo - it says the device costs only (!) about half of what a solar thermal installation would cost, so I'd try the cheap option first. Could add some storage heaters with a timer switch midday for winter maybe.

Yes we do lose a bit of FIT income by exporting to the grid, but part of our aim was to be good to the environment.

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Fred1Comment left on: 28 May 2011 at 7:34 am

All my roof electricity is used by myself or by people close to my grid connection. So what I generate reduces a power station somewhere.

I have a gas grid connection, the above thread seems to compare the additional option of either (a) Exporting and getting a small payment or (b) Storing the potential export amount as hot water, thus  reducing ELECTRICITY imports BUT having to pay for a very expensive electronic piece of kit.

Has anyone done the above comparison for the GAS alternate.????? My feeling is  that the cost of  extra electronic kit outweighs the ( additional Electric income- additional gas cost)

My feeling is that from an purely economics point of view it would be better to invest in more PV panels or consider the RHI, if it every is developed at a reasonable rate to be interesting...

From a carbon saving point of view every kW I produce off the roof helps , it does not really matter if I use it or someone else uses it...


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WestieComment left on: 27 May 2011 at 4:36 pm

Apologies if I have posted in the wrong place but I have just had installed a Solar PV 3.84kw system and have not yet registered with my supplier (Atlantic) . Very confused as whether to get an import/export meter fitted or not. Our total generation estimate nearly matches our yearly consumption and we are often away during the summer months.Would appreciate any help! Our heating is gas central heating coupled with wood burner in the winter and practically all the hot water we use is an electric shower.

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Cathy Debenham

Cathy DebenhamComment left on: 27 May 2011 at 4:12 pm

Hi Westie

Start by reading these blogs, if you haven't already:

Import and export meters and the feed-in tariff 

Energy companies & export meters

Then it's a matter of working out the cost/benefit. If you go for a deemed 50%, you'll get 3.1p x half your generated kWhs. If you go for an export meter you'll get 3.1p x exported kWh minus the cost of the export meter - see the second link above for costs. I hope that helps.

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LudgatemanComment left on: 27 May 2011 at 1:56 pm

Thanks Fox.Mart - EMMA isn't something I'd come across previously, but I'll certainly be looking into it further.

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fox.martComment left on: 26 May 2011 at 7:28 pm


This goes hand in had with the blog

As Paul Hutchens, Chris Rudge and Ludgateman above suggest, there are several methods of storing energy from your PV system whilst still getting full feed in tariff and saving on oil/gas etc. Makes a lot of sense to use all the power you can.

 Turning an immersion heater on when it's sunny is great, but does not guarantee that power will not be imported if the sun goes away.  Hi tech way is noted  in the link above - if you're a fan of gadgets (like me!) then worth a look.

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Norman Environmental Ltd

Norman Environmental LtdComment left on: 26 May 2011 at 3:28 pm

If you have an immersion then it is a simple case of setting the timer to the hours of best sunlight for your arrays, isn't it?

Could you even link the immersion switch to an irradiance sensor that is over-ruled by the stat?

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LudgatemanComment left on: 26 May 2011 at 1:01 pm

This is something I considered when first having Solar PV installed. Our bungalow is heated by a Parkray on which I burn my own felled and dried logs (nicely carbon neutral) or coal during the winter months. During the summer it’s rarely lit at all as the building is well insulated and stays comfortably warm for days. For hot water I have an electric immersion heater, and when our PV system was installed, I changed the manual on/off switch controlling it for an electronic “Electrisaver” switch from Horstmann Controls ( This simple electronic time switch operates for 30, 60 or 120 minutes at the push of a button. On suitably bright / sunny days, I simply press the button when leaving for work and am assured of plenty of free hot water when I get home, and all without the hassle of setting time switches etc. It also gives a quick boost at any time of day without having to worry about switching it off. It can equally easily be used to run electric or oil filled wall heaters, towel rails etc, and is a great way of helping you maximise your use of the free electricity coming off your roof. Highly recommended.

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