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Can I use solar PV to heat my water?

Posted by Paul Hutchens on 5 March 2012 at 9:18 am

Q. Can you tell me what size PV panel would be required to run a standard domestic immersion heater?. I am not interested in returning power to the grid and want to dedicate the output from the panel to the immersion heater in my hot water thermal store.

A. A standard immersion heater is 3kW and so the obvious answer is you would require a 3kW solar panel array. However, as solar energy is variable, one cannot guarantee that there will always be enough energy to power the immersion at any time.

Also, 3kW of energy would heat the average domestic hot water cylinder in about 2 hours; so on a sunny day, depending on how much hot water you use, you may find that the solar PV heats the water with energy to spare. This spare energy may of course end up going back to the electricity grid.

Photo by Bradley Gordon

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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11 comments - read them below or add one


richmcComment left on: 31 October 2012 at 4:34 pm

Simple, I've not paid for water heating since May -

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charlestarkaComment left on: 9 September 2012 at 11:18 pm

I am still looking at using the DC output from the PV panels feed directly to an immersion heater.  The DC circuit will obviously be totally divorced from any other wiring and would be stand alone.  It is an unusual situation in that I have a very large demand for hot water and therefore do not need a thermostat to stop temperature excess.  In the unlikely situation water gets too hot I have other ways of loosing the heat.  Therefore in this situation it should surely work.

 Anyone any thoughts on this?


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D Leach

D LeachComment left on: 2 June 2012 at 12:06 pm

I try to divert as much generated electricity back into the house as possible as you get paid for it whether you do or you don`t. I believe simplicity is the key. The following is what we do in the main producing summer months when the heatings off. In the winter we go back to gas central heating which gives us hot water and try to mop up produced electricity via domestic use.

First fit 1kw  27" emmersion heater (just replace the exsisting) controlled by a timmer with battery back up. I believe that for at least 60-70% of the time I produce over 1 kw.

The remaining generated is maybe 400 watt at the very least going up to 999 watt. I guesstimate that this really equates to another 25-30%. So in reality the very worse case for me is that I buy in for 10-15% of the time. Cheaper than heating my water by gas.

I have the timer coming on at 11am and off at 3.30pm which really is the very best generating times for me. This provides me with a hot tank full of water every day which is what I require, no other back up.

Of course you can lengthen these times but you go further towards the times of under 1kw production

You can fit switches which will only come on when producing over 1kw but on a bad day I am left with no hot water. 110 transformers with 3kw immersion heater can be used but there is a loss and probably only produces say 700watt. Also cheaper 12" 1kw heaters can be used and put in the side of the tank but this is harder to fit and unless you can braze then the bolted gland is very expensive. Another method is a 12" 1 kw heater with an excelorater tube fitted and put in the top gland. This is difficult and if not done well could lead to rattles. Its cheaper and easier to fit 1kw 27" emmersion heater with no need for plugs, wiring drilling etc and more efficient.They cost about £50+ depending where you get them from. On Ebay I think they are £59.

There after we try to use washing machines and dishwashers etc between 9.30am and 11am and 3.30 onwards. If you work, then fit time swithes so that they come on when you want. These can be just cheap types.

Well this is what I do at the moment and it works very well. If you can manage with one tank of hot water a day. This is during the summer when I take more showers and less baths anyway. If you think I can readily improve this system for little extra money then please let me know.

Mr D Leach

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Keith_WalkerComment left on: 24 March 2012 at 6:04 pm


I have a 2.8kW rent-a-roof PV system.

Like everybody else, I also want to use all the energy that is available.

I'm in the process of developing a circuit that will automatically use the surplus power.

I'm also putting together a website to share and improve the circuit. The prototype looks like it will work well.

Watch for more detail as I develop the site.

Cheers, Keith

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Solar Wind

Solar WindComment left on: 21 March 2012 at 8:19 pm

There seems to be a lot of interest in diverting surplus solar power into your hot water tank rather than getting paid only 1.55p per kWh for exporting it back to your Regional Electricity Company (REC) via your local grid.

Many people quote 3.1p per kWh but this is wrong as you only get 3.1p for 50% of your total generation.

Diverting surplus solar is only worth £50 - £150 a year to the average home. £50 if you have economy 7 or similar and £150 if you don't. Of course future electricity prices are unknown.

There are three options, the first is to improve the efficiency of your home and reduce your energy needs. In this way you can probably save more money than you can divert into your hot water tank.

DC is not an option as it would burn out your thermostat which is not DC rated and there are other problems with rectification and control. Also you don't want to start rewiring your house for a £100 a year saving. What you want is a system that can use the existing house wiring, immersion and thermostat, which will keep the costs down.

So you need to lower the AC voltage to your 3kW immersion heater so you can supply it with less than 3kW which would probably never be available as surplus. There are I believe two ways to do this but whichever one you choose it needs to be automated as otherwise you will either miss the surplus or be importing not exporting.

So the second option then is to reduce the voltage using a transformer. If you use a suitably sized 110V Isolation transformer with the centre tap earth removed (perfectly allowable) you then have a 55V and 110V AC supply that you can toggle between giving 160W and 660W respectively. This is more than you need as you will be able to start heating water as soon as you have a spare 200W or more. Remember isolation transformers are not continuously rated so you will need one at least twice the size, so a 1.5kVA would be the smallest you could use for a 660W load, better 2kVA. Depending on transformer size you will probably want to change your circuit breaker from type B to type C to stop false tripping due to inrush current when switching on.

Third option is to use thyristor control to follow the surplus up to 3kW and back down again.

The problem is control of options two and three. There is information available on the internet using an Arduino microprocessor which is only £25 and this along with two efergy sensors, a hand full of components and a few solid state relays is capable of controlling the load. Do a search on OpenEnergyMonitor for more information.

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PeterBComment left on: 8 March 2012 at 3:43 pm

I simply added a timer to my immersion heater set for 3 hours in the afternoon (cost £4 on EBay) to top up the hot water. Simple and effective.

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Paul Hutchens

Paul Hutchens from Eco2SolarComment left on: 5 March 2012 at 7:32 pm

The problem with a DC system is that you will miss out on the valuable Feed in Tariffs - the main reason people are purchasing a solar PV system (or any renewable energy system at the moment).

EMMA is a good solution but it is expensive.

SMA are bringing out their Home Manager in Q2. It is configurable to set up channels (e.g. immersion, dishwasher) and direct PV energy to them when available. Bluetooth enabled “plug sockets” control the devices.

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Fred1Comment left on: 5 March 2012 at 6:14 pm

I generate about 2000 Kw per year. If I heat water with panel power using 75% ? of my generation

I  would save say 1500 Kw of gas at  ??? 4 p per Kw ie I would save say £60 per year . An EMMA though very expensive would technically capture/ direct the power, but at several thousand pounds looks very expensive. Anyone know of a cheaper system ???



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brummygitComment left on: 5 March 2012 at 4:40 pm

You can also install EMMA by Cool Power Products which will divert surplus generation to your existing immersion heater. This means you can time your water heating to only add heat that was not available during sunny hours if you wish.

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Rudge Energy

Rudge EnergyComment left on: 5 March 2012 at 10:54 am

As Rory says, the use of a DC immersion heater will be probably more advantageous. That way you would not need to pay out for a full sized PV system and associated grid inverter.

However it must be borne in mind that heating water like this is very inefficient as PV modules are only 14% efficient and you'd get a far better system with a simple bank of evacuated tubes and a pump station. Cheaper too!

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roryberginComment left on: 5 March 2012 at 9:31 am

What about using a 12V DC immersion, can these be used with a direct input from a smaller array than 3KWp? There are lots of these for sale on eBay for use with a wind turbine.

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