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

Posted by Chris Rudge on 11 May 2011 at 9:28 am

Q: I have installed solar panels and am amazed at the power I am creating.  This is great in sunlight hours - but has anything been produced to store this power so I can use it in the evenings?  This is such a waste of free power, surely someone somewhere must have this on board.  It's not rocket science!!

A:  Storing all that 'free power' is something our customers often bring up. There are a couple of cost-effective of ways to do this:

1) You can consider using the excess generated power to run the immersion heater in your hot water cylinder. This device is usually installed for emergency use if your boiler broke down. However we have put in a couple of units to customers' solar PV inverters that know when the output power goes over 1kW and switches on a relay.

Read more about inverters for solar PV panels.

Wired to a standard 3kW immersion heater through one of our power reduction units, the immersion runs at 1kW and gently heats the water. Over a whole sunny day, you will get a full tank of hot water, at other times it will effectively pre-heat your hot water tank so the gas boiler does not have to run for long.

2) Battery backup. This option can be as simple or as complicated (expensive), as your budget & requirements stretch. A couple of our more experimental customers have installed their own twilight changeover systems that charge a large set of renewable energy lead acid batteries during the day, (on the free to use electricity), then at night the lighting circuits in the house switch over to using these batteries through a pure sine inverter. This can be extended to powering other devices in the house as well, and is dependent on your power requirements during the evening.

For a recent breakdown of the merits of solar batteries, and if they would be right for you, read our blog by our expert installer Ollie Gibbs.

However, decent batteries cost quite a lot of money and will need maintaining, plus replacing every 5 to 10 years, so is not an option to be taken lightly.

As of 2017, new solar batteries installed with PVs will be available at a lower tax rate! Read more here.

In rural areas, the battery backup option has real advantages that your house will have the option of backup power during power cuts. There are some quite sophisticated systems, such as Sunny Backup units, that do everything automatically, down to simple changeover switches.

There are a number of systems on the market that claim to maintain and manage your renewable power generation, but only batteries or the heating suggestion really do actually 'store' the power for you to use overnight.

Click here to read a follow up to this blog post.

Photo by Hepburn Wind


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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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29 comments - read them below or add one

Ian Nicholson

Ian NicholsonComment left on: 19 December 2014 at 6:47 pm

I have a first generation immersun, installed myself for around £250 (the summers FIT)  this is on to a standard 3Kw immersion occuping about 60L out of the 140L tank. On setting work I set up gas heating tank to 40C and then immersun to 65C. Never short of hot water. In a year it is diverting about 850Kw/h out 4100Kw/h production. This has reduced gas consumption by around 1000Kw/h circ £50 giving a return on investment at 20%. I am still exporting on sunny days and that will need much more power management, but return will be less. The key is use it when you are generating and invest the tax free FIT

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NacNacComment left on: 1 August 2014 at 7:33 pm

Hi all

This post is quite old, especially when considering that this type of technology is changing very fast.

The original topic was about storing the excess of energy produced in the day so it can be used at night.

Chris mentioned Battery backup. I was wondering if anyone has installed some ?

Are the cost still so high, do they need replacing every 5 or 10 years ?

Can they be installed when having the "free solar panels" scheme ? Technically the panels are own by a company who hire your roof and pay you by letting you use the electricity the panels produce.

It would be interesting to know what the latest update on those battery backup or solar storage and it is also called.



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michaelmaloneyComment left on: 10 March 2014 at 7:12 am

  This is one of the more informative articles I've seen about green energy storage. Thanks for the tips on this! I find that we really need to come up with a better method to really lock down all that energy - there's a lot of potential being waste (like the OP said) if we don't do something to proper harness all that's been generated. If not at the consumer level, then even more so for the levels at mass production.

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greenmanComment left on: 5 August 2012 at 5:06 pm

I have done something similar. I bought a 1kw full length immersion heater. Before that I found an affordable product that would intelligently optimise the use of my Immersion Heater.

The unit I bought is sold at and I bought their TP-S610A starter kit (they call it a Renewable Energy Monitoring Unit of their REMU I).

It monitors how much energy my house is consuming and how much power I am getting from my solar panels or any other renewable energy source. It automatically switches on your immersion heater (and up to 2 other appliances) when enough surplus energy is being generated.

It is £299. I think it will pay for itself as well, although it may take a year or two to get my money back. In the mean time it will make sure that my home is not consuming anymore power than I want to. The control unit will also record my homeís energy production and consumption and download it to a pc.

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thusharComment left on: 17 July 2012 at 7:39 pm

Investing less than 2 months worth of FIT payment for an automatic switch is worth it. We installed the solar pv immersion switch (Mark II) from that calculates the net difference between the production and house hold consumption to automatically turn on the immersion without importing any power from the grid. We also used the power reducer from to reduce our existing 3 kW immersion to a 1.5 kW - Help us to save on the plumbers cost and avoid the nasty job of changing the existing immersion. It works brilliantly. 

Had a chat with the customer support guy last day - told me they are working on the Mark III version with proportional controller and that too at a very affordable price tag. 

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LiammComment left on: 14 March 2012 at 8:22 am

All the above sound good, but why can we not for go the 3p export tariff and have a meter that goes backwards when we are exporting and we use this free pre generated energy when importing 

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Dave Thickett

Dave ThickettComment left on: 7 February 2012 at 2:16 pm

Hello Chris, I have a question about heating water by solar power to reduce my gas consumption. I want to connect the output of 2 x 250W solar panels connected in series to give 96v DC. Then connect 96v DC to my 3kW immersion heater which has 18.4 Ohms resistance. I know I need to bypass the thermostat to avoid contact arcing. Do you know if feeding DC to the heating element will have any adverse effects. Would this be cost efficient based on purchasing panels at ¬£1.10 per Watt? Thanks, Dave

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Bob Graham

Bob GrahamComment left on: 3 January 2012 at 3:37 pm

Am i right in thinking that to make a saving using an immersion heater you only need to be cheaper than the alternative eg. gas.

So if gas is half the cost of electricity then you only have to halve your net electric power to be comparable with gas. By net power I mean grid minus solar.

That is, for a 3kw immersion you need 1.5kw from the grid and 1.5kw from solar. So the break even point is 1.5kw, and I should switch my immersion on when the inverter is outputting 1.5kw.

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Rudge Energy

Rudge EnergyComment left on: 26 December 2011 at 7:49 pm

HI again JohnnyK

I've just checked my emails, and cannot find anything from you. So not sure what has happened with your previous email!

Mosauer.. Looking at your reference again , and looking into the background of Abeltronics power halver units. I see their unit is simply a half wave rectifier, that cuts off the negative going cycle of an AC sinewave, thus halving the power.

OK, I use the term 'simply' in the best possible way, as the idea is really good for mains based devices. However, if these would be used on renewable power systems, long term issues could arise.

On Wind Turbines: An imbalance of the bearings over a long period, which will cause premature wear. If you can imagine that only half the time of generation (50% on, 50% off) the turbine will be 'juddering' on and off of load.

On Solar PV systems: Similar to the above, but as no moving parts, will put the grid inverter through the same on-off routine 25 times second, which will definitely put the components through some stress.

As mentioned above, using these devices just for the grid, as was designed, they will work just fine as there will be many other power users absorbing power irregularities.

Chris Rudge 

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Rudge Energy

Rudge EnergyComment left on: 26 December 2011 at 7:10 pm

Hi JohnnyK

Apologies I missed the earlier post somehow, as I could have saved you all the hassle of coming to the conclusion that a standard 110V site transformer  for power reduction.

We have had a system using a site transformer on test all this year (2011) and have been pleased with the results. So much so, that if you have a 3.5 - 4kWp PV system and a hot water tank & immersion heater, you really do not need a solar thermal system.* Obviously there are some qualifications to this statement, but in brief, if you have a low daylight use of power (fridge, occasional washing machine, etc), then this method will work well. 

As you have also found, a bright day with broken clouds will cause some fluctuation, but this generally will not be a huge issue, and the benefits outweigh the downsides.

We have now started selling the auto relay unit that will be used in conjunction with the 110v transformer on a 3kW immersion heater, which if mains voltage is 230v will run the immersion at around 800 - 850W. From later Spring to early Autumn sunny days will provide a tank of hot water.

For more on this, see 

* This system WOULD NOT attract the upcoming RHI payments, and indeed is not really energy efficient, but does make effective use of your generated electricity rather than simply exporting at 3p a unit.

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JohnnyKComment left on: 26 December 2011 at 5:02 pm

Hi again guys,

Thought I'd better acknowledge the comment of John Twibell, about cutting up resistance wire & the effective changes in power ratings.  Thanks John.  I always check the good old Ohm's Law anyway to carefully calculate the resistance, voltage, current & power output of any potential redesign or test.

I've overcome the problem that I outlined a couple of months ago (of utilising surplus power, storing it, whilst not exceeding the PV power generated) & I thought I'd share this with you.  My solution is simple; if you lower the voltage, the power of an electrical heat load will drop.  So I purchased a 110v site transformer via Ebay (3.3kv size).  This transforms the 230v mains & PV output voltage down to 110v.  As the resistance of a heater element remains unchanged & constant, then when the heater is plugged into the 110v side of the transformer, it now draws only about 23% of its originally rated power (400w instead of 1750w).  Thus I can utilise the low amounts of power my PV system generates during the winter more effectively.

Unfortunately, during days of intermittent cloud (most days in the UK - lol), I still have to manually keep rather a close eye on power generated & power consumed.  At present I run around like a madman switching things on & off to keep within the low power generated during our winter months.  I would very much like to automate this, but without spending a couple of grand ¬£¬£s on an EMMA SP15 power management unit.  I had emailed Chris Rudge, but still haven't received any reply, about his more simple & much cheaper replay switching unit.  Chris, if you read this, would you contact me or email me please - thank you.

Regards to all & wishing you all a happy new year.


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mosauerComment left on: 16 November 2011 at 9:26 am

Further on the power halver unit. I have spoken with the guys at Ableltronics. It seems they are planning a development of this  enabling existing 3 kW immersion to be attenuated variably from 0-3 kw (early 2012 availability). This seems to me to be a preferred option if you can wait, especialy since there are remote controllability features to be offered as option.  Since it's winter there's no hurry I guess.

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john twibell

john twibellComment left on: 2 November 2011 at 6:11 pm

A quick warning to JohnnyK 

Be careful about what you do with the heating element. It sound like you are going to cut it into three. Cutting a 3kW element into 3 will not give you 3 x 1kW elements, you will get 3 x 9kW elements or a total of 27kW if you put mains across the cut up pieces. If you cut it into three each piece will have a third the resistance. If you put 240V across that it will pass three times the current of the original element. To reduce the heating power you need to add resistance by putting elements in series.  I use low power heating in my greenhouse by wiring heaters in series. 

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JohnnyKComment left on: 1 November 2011 at 10:51 pm

Hi Maurice (Mo),

The link (you requested) for the power halver from Abletronics is:-

I hope that it's alright to put it up on this blog thingy, (not sure what the rules are as yet).

I've just spent the afternoon wiring one of these power halvers to power two standard sockets & making them switchable, so that they will either allow full power OR half power.  Going to try making a portable multi-socket power halver thingy soon.  I think I'm a goin' nuts - lol.

Hope the link works for you Maurice.



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mosauerComment left on: 1 November 2011 at 8:29 pm

Hi all. Thanks for the stream of info. I guess how to requests  for leads on low cost means of soaking up excess energy by heating water will grow  and grow. Chris' system seems a nifty low cost little number, plus there are additional thoughts posted on his blog. I will give this system a try but will install low power immersion heater instead of transformer since the solution is tidier and the immersion is bottom fitted. I've been checking a number of british manufacturers to see who will supply, and found that the cost is, I think modest, for a non-mass produced 1kW incaloy unit (say ¬£35 +). One manufacturer does bespoke for not much more with the posibility of dual elements (say 1kW for winter and 2kw for summer) - prices little more than the cost of the transformer unit used by Chris. If details of manufacturers required I'm happy to supply.(



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Rudge Energy

Rudge EnergyComment left on: 30 October 2011 at 5:55 pm

Hi Malcolm

Sorry the email may not have got here. Maybe try for the full details.

We are currently awaiting a whole load of the current sense units in from the USA, and will have the units available soon after for £180.

Johnny K

It would be more elegant to use a 1kW immersion heater, and I would guess a non-mainstream part would cost far more than standard stock items.

It has been mentioned to me that by changing the immersion from 3kW to 1kW, would remove the possibility of heating water fast if need be via an override.

Chris Rudge 

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JohnnyKComment left on: 30 October 2011 at 8:30 am

Hi guys 'n' gals,

Just joined this forum as I've found your subjects of conversation very interesting.  I've just installed a 4kw solar PV system & am seeking methods of maximising its effectiveness, saving money & storing surplus energy generated during the day for later use during the evenings.

Could anyone tell me where I can purchase a 1kw 11" long. 2.25" BSP immersion heater for a "reasonable price" please.  The best I've found so far is on Ebay for around ¬£35; but that is still way more expensive than the local price of buying a std 3kw 27" immersion ¬£17.

Also I'm very interested in purchasing one of these devices mentioned above, that 'shunt' excess day power generated into a low powered immersion heater or storage radiator.

I can recommend the Abletronics Power Halver.  Wired one up yesterday & it works a treat.  Going to dismantle an old storage radiator soon & 'cut-up' the heating elements into three separate heating circuits, all with individual fused switches.  Also connect another power reducer to it.  Purpose to warm the house in winter without exceeding the level of self generated power from the PV system.

If anyone has any info upon my two requests above, would you be so kind as to email me please via . Many thanks.

Regards to all,


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Malcolm Comment left on: 22 July 2011 at 9:20 pm

Hi Chris,

Could you give me details of the unit please, I e-mailed you on the link provided 16.6.11 but it doesn't appear to be working.


Thanks Malcolm

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Marath0nmanComment left on: 27 May 2011 at 1:55 pm

Chris, We have a 3.96KWp PV system. In the last year it generated 4200 units of which 3200 were exported, much of which in summer months. How much of a contribution would 3200KWh make to hot water heating?  Is installing your device an alternative to having solar thermal panels fitted? As I understand it, typical domestic solar thermal panels say for a 2 adult household contribute a lot less than 3200KWh per annum, so it appears to me that we could do without the thermal panels altogether if we manage our PV output carefully.


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Chris Rudge

Chris Rudge from Rudge EnergyComment left on: 15 May 2011 at 12:41 pm

Hi Ian

The unit can be fairly easily installed, depending on the location of exisiting inverter and consumer unit. Probably best to email me  rather than getting into semi commercial discussion on the blog.

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ian day

ian dayComment left on: 15 May 2011 at 9:10 am

I have a 3.84kwp pv installation. I also have an oil fired heating instalation with a 3kw standby immersion heater. At the moment I am using the pv power to top up the hot water and switch the heater on and off manually; so its a bit hit and miss. Thus the auto system mentioned by Chris is of interest. What exactly does it comprise off ? What is the cost ? Is it easily retrofitted ?

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fox.martComment left on: 13 May 2011 at 3:29 pm

The cost is quite high, I suppose, but when installed when heating would be replacing oil or electric it's going to be quickest payback. There are loads of applications of the device too - I installed one today into a couple of panel heaters for some background heat in a community garden office, for example!

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Chris Rudge

Chris Rudge from Rudge EnergyComment left on: 12 May 2011 at 7:46 pm

Ah yes, they used to email us quite a bit with this product. I'll email you to discuss (rather than here), as they used to charge quite a lot for the unit, which somewhat outweighed the potential power savings.

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fox.martComment left on: 12 May 2011 at 6:44 pm

Thought it would be good to check. The product is called EMMA (Energy and Microgenerator MAnager) made by Irish company Coolpower Products ( We are Scotland distributor/installers for these when attached to PV systems.

As an engineer, they are a great piece of kit and work amazingly well. Have a look at their website, and I'm sure it will explain all and point you in the right direction to get hold of one! My email is if you want any further info/experience of the product from me - happy to help. 

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Chris Rudge

Chris Rudge from Rudge EnergyComment left on: 12 May 2011 at 9:27 am

Solar Clubs. I've not found these available at all in the last couple of years, though there is a plethora of companies selling fork lift batteries. These units will be far more managable, not requiring the use of lifting gear and can be picked up for almost scrap value. You will need a decent van to collect, as a standard 48v counterbalance forklift set weighs around a ton. Search online to see what is available in your area.

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Solar ClubsComment left on: 12 May 2011 at 6:17 am

I've heard that you can buy second-hand submarine batteries from the Royal Navy which are great for this purpose but haven't been able to find any for sale on the internet - is this the case, what do you think?

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Chris Rudge

Chris Rudge from Rudge EnergyComment left on: 11 May 2011 at 11:44 pm

Hi Fox Mart. I think it would be OK to mention suitable product sources here if your not blatantly plugging, so fire away!

The units we have put together use easily available CE marked parts, and is a step further from simply fitting timers to the immersion heater. Its really not rocket science. Under £100 in parts.

1kW immersion heater are available for old style 2 1/4 bsp thread, but new style tank threads in the UK seem to be limited to 3kW (unless someone knows otherwise).. 

We have been getting reports from one local customer who has not had his gas boiler running for about 2 months using this system, but of course depends on your lifestyle. Hot water useage varies wildly from house to house.  

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SolinvictusComment left on: 11 May 2011 at 10:52 pm

Hi Fox.mart.  We are an installation company based in Hertfordshire and have been seeking effective ways to help customers maximise their returns and I was very interested in your comment re the product you install which controls the immersion.  We have installed low power immersions for customers but they have to be manually controlled and if it is a top entry immersion only heats the top part of the tank.  A customer passed me some details about an extended 1 KW immersion designed for this purpose but we haven't bought one yet.  I tried to e.mail you for details as suggested but the email link didnt seem to be working.  Could you email me with details -  Thanks in anticipation  Rob

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fox.martComment left on: 11 May 2011 at 8:21 pm

Hi. Yes, a great way to do this is by storing the energy in hot water. Chris' method is great, however there is a product out there which we (solar and wind applications) have been installing on several sites which very accurately controls the power sent to an immersion heater to use all of the "excess" generation without importing power. Not sure if it's allowed to post product names on here, but feel free to message me for more information. The kit works really well - best used to displace oil or electricity which would otherwise be used to heat water. 

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