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Should I divert excess solar energy into my hot water cylinder?

Posted by Chris Jardine on 20 January 2015 at 4:48 pm

Most people don't use all the electricity they generate from their solar panels. Any excess is exported into the grid for someone else to use. But there are ways to keep and use more of your own electricity by using the surplus electricity to heat your hot water cylinder. 

Opinions differ on the benefits of doing this - even within companies. In this blog I discuss the pros and cons with our head of residential sales, Jon Cowdrill.

So what are these devices?

Jon: Well, basically, if you have a grid-connected PV system, then a lot of the time you’ll be exporting electricity back to the grid. However, you could maximise your benefit from installing PV by using all the output yourself, so a PV controller automatically diverts electricity that would otherwise leave your home, into your own hot water tank. When we talk to our customers, we’re certainly seeing a lot of demand for such products right now.

Chris: They’re certainly a ‘smart’ piece of kit, and technically very good. I think over the coming years we’re going to see the rise of smart grids and smart appliances, and this kind of flexibility and storage of energy is likely to become more important.

So why are people so keen at the moment?

Chris: I think it’s pretty clearly driven by the low price householders get from exporting electricity to the grid. If you have a solar PV system, you get paid 4.7p/kWh for any electricity you export to the grid. However, if you use the electricity yourself, you save about 15p/kWh off your bills. So there’s a clear financial imperative to consume the electricity you generate from solar if you can.

Jon: When I talk to customers, I’m seeing other motivations as well. The Big Six energy suppliers are deeply mistrusted right now, especially after massive price increases, and there’s often a motivation for people to be a little more autonomous. Others are just generally interested in new bits of technology and want to try out the latest thing.

There’s clearly an upfront cost. What are the economics of that like?

Jon: Well an average 3kW system might generate 2,500kWh a year, of which half might be exported – let’s say 1,250kWh. A 10p/kWh price differential between export and self-consumption, works out at £125 per year. With kit and installation at about £750, that’s a payback of 6 years, and after that you’re in profit. That’s not bad. In fact it’s comparable with the PV unit itself.

Chris: And if you’re out all day, with your appliances off, then your export would be even higher than Jon assumes. In this case the benefit of a PV controller could be even greater.

Jon: Indeed! So savings could be excellent depending on circumstance.

Chris: I have to add a very important caveat here: you’ll only be saving 10p from avoided imports if you currently heat your water with an electric immersion heater. If you currently use gas, you’ll be buying gas at about 5p/kWh giving you no financial savings at all! So I’d say it’s only worthwhile for places that already have electric immersion heaters, and maybe oil boilers. But if you’re on gas, you absolutely shouldn’t get an electric immersion heater put in – it won’t make sense in cost terms, even if you produce the electricity yourself.

Will a device like this reduce carbon emissions?

Jon: Yes, you are generating green electricity from your roof. If you use this to power lights and appliances you will reduce your carbon footprint a certain amount. But if you use otherwise surplus electricity to heat your hot water as well you will reduce your personal carbon footprint further. However, electricity is roughly three times more carbon intensive than burning gas, so if you currently heat water electrically, then you will reduce your personal carbon footprint more than if you currently heat with gas.

Chris: I think it also depends very much on where you draw the boundaries around this. For the householder, the situation is as Jon describes. But for the UK as a whole, the situation is different. Because electricity is three times more carbon intensive than gas, it would be better in carbon terms to let other people use it for lighting and appliances, rather than offset emissions from your comparatively clean gas heating. If you heat water with gas, and you divert with one of these devices, overall you’d save less carbon than if you let your neighbour use the surplus electricity!

So what does the future hold?

Chris: I think things will change when smart-metering of all households comes in, which should allow householders to receive a more realistic value for their exported electricity, with price maybe altering on a half hourly basis, in line with wider energy markets. So here electricity would be valued in line with how much other people require it, which is much fairer than the currently undervalued 4.7p/kWh for export

Jon: In the short-term, I still see demand for this kind of product, and so I suppose ultimately the market will decide. We’re certainly happy to install them if people decide that’s what they want to do. Hopefully this piece will help them form an informed and balanced opinion!

Joju Solar supply the ImmerSUN PV controller for this (see image above). Others on the market include iBoost, Solamiser and Optimmersion.

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About the author: Chris Jardine is technical director of Joju Solar and teaches on the MSc course in Environmental Change and Management at Oxford University.

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

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13 comments - read them below or add one


whistlewormComment left on: 30 May 2015 at 3:15 pm

How easy would it be to divert unused power to charge an electric vehicle in addition to or as an alternative to a hot water cylinder?

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SlotsComment left on: 30 April 2015 at 12:11 pm

I recently bought the Immersun MKII pictured above for £320 inc VAT from an electrical wholesaler and had a competent person fit it. The MK II is wireless CT enabled as standard but ships with a wired CT. It's more extensive wireless capability is not yet available.

Immersun are tied up with New Zealand Company Enasolar and on their version of the MKII the unit's wireless capability is fully realised and tied in with their products. Immersun are in the process of fully developing wireless connectivity to such things as smart phones and home routers. A MKIII is in development.

The manual shipped with the unit is for the MK I version. This can be confusing as there are two CT connectors available. An explanation of what the second CT does can be found on the Enasolar website in their MKII user manual.

The CT clamp comes with an unterminated 5 metres of unscreened two core cable. The lead was extended by a further 3 metres of twin cable without problem. The CT has a direction arrow but it can be mounted anyway round as the MKII sorts itself out during the install calibration process.

A menu setting allows you to set the amount of watts going back to the grid before it starts heating water. The default is 50 watts and I set this to zero. Exports measure as anywhere between 8W and 20W on a display sub menu when you do this.

I am really pleased with my purchase and the unit's versatility. The inability to wirelessly connect with a home router for viewing data is disappointing.


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Graham Marshall

Graham MarshallComment left on: 27 March 2015 at 2:28 pm

Completely agree on the principle of diverting unused PV power to heat hot water. Thereis, however, only so much hot water one can use. What do the experts think of dumping the heat from unused hot water back to the soil? If you have a GSHP then raising the temperature of the ground even by a few degrees must be beneficial.

I doubt much energy would be diverted back to the ground during the winter but since the ground is an excellent long term heat store and heat dumped back in the summer will surely leave a little in the autumn and help offset higher pump running costs in the winter.

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tony nicholson

tony nicholsonComment left on: 13 March 2015 at 7:38 pm

Well it cost me £500 for my immersun and I immediately put the number 2 relay output to good use, a 1500 watt oil filled radiator in the lounge and a 400 watt and 250 watt towel rail in the bathroom and toilet respective, this house has no central heating and two balanced flue heaters in the open plan downstairs living area.

I have the old FX night rate and that years ago was diverted from an 8.5 Kw central storage heater to a kitchen socket for the washing machine and tumble dryer but since going solar I have only used about 20 FX units on very rainy overcast days when we had to wash the clothes the immersion was switched from FX to standard as it needed a constant supply to the immersun unit.

The immersion was first switch to a twice a day boost then once a day now I leave it off as the immersun trickles anything from a 100 watts to 2.5Kw into the tank and I did not need special immersions as the unit does this by controlling the voltage to the heaters.

Next a switch from electric shower to power shower to use solar for my shower!

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joncowdrillComment left on: 2 February 2015 at 11:55 am

Hi there Folks,


ImmurSun is at the top end of the diverter market. If you need a transmitter as well the kit can cost in the region of £500. £750 would  easily cover the electrician's charges to install as well. If we installed one at the same time as the sollar array it would be a lot cheaper.

As mentioned, you can get cheaper versions of the ImmurSun however we find the ImmurSun is worth the extra investment.

All the best

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Solar Wind

Solar WindComment left on: 30 January 2015 at 9:22 am

Hi All,

I built one of these from scratch based on a article on the internet. My costs were very low but, anyone paying more than £300.00 would be way too much.

Mine has been working for over three years now and I installed it with a kWh meter, so I can confirm that I divert more than 1MWh (1,000kWh) surplus per year from my 3.67kWh system which generates 3.65MWh a year.

I do live in the south with a south facing roof.

During the winter diversion is low as you would expect but during the summer months all my hot water comes from diverted solar PV.

Last week ending January 25th I generated 41kWh of solar and 24kWh of that went to hot water.

This is a sound investment, but if like me you have Eco 7 where I only pay 6p per kWh then the savings are only £60.00 a year. So my pay back was only a year and I had the fun of building it.

Redards, Solar Wind.

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HarrytheWASPComment left on: 29 January 2015 at 7:54 pm

The diverters are being fitted for a lot less than this blog is stating. Anyone who has renewable energy and an immersion should get one, it tops up your water cylinder constantly and the bigger your water cylinder the better it is. Its the end of Jan and mine still diverted 3 kw from my solar today which means my airsource heat pump had a bit of time off. From March onwards my Immersun will do all the hotwater until about October .

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paul53Comment left on: 26 January 2015 at 8:30 pm

you  can  buy  a diverter  for  180 pounds i  fitted mine  myself  in  a  hour so  250  would  seem  reasonable  if  your  consumer unit is  up to date

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cannonballdazeComment left on: 26 January 2015 at 3:18 pm

Hi Cathy - installed September 2011 and get 3.39p - as do many people.

Any response to my query of Jon Cowdrill's inflated diverter costs ?

Cheers - Joe

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Cathy Debenham

Cathy DebenhamComment left on: 26 January 2015 at 9:03 am

@cannonballdaze When did you install? 4.7p is the current rate. However, people who installed in the early days of the feed-in tariff got 3p.

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paul53Comment left on: 24 January 2015 at 9:20 pm

a third of  electricity  is  lost in the  grid , a third  wasted by  poorly  insulated  houses  , poor lighting and  apliances left  on so  its  to  each  house holder  to max his  gain from investing  in solar

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nnw49Comment left on: 24 January 2015 at 4:15 pm

I've fitted one of these for someone and it seems to make a pretty good job of equalising the immersion use age to the surplus power available.

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cannonballdazeComment left on: 23 January 2015 at 10:52 pm

Hi, I don't get 4.7p export rate?

Plus you can get a diverter fitted for < £350 !

Please clarify.

Thx. Joe

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