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How can I store surplus electricity from my solar PV using batteries?

Posted by Helena Ripley on 29 October 2015 at 11:15 am

A domestic electricity store (ie. lithium-ion or lead-acid batteries) could be one way to make the most of the free clean electricity that you are generating from your solar PV panels. These batteries will be familiar to you – lead-acid is often used in car batteries and lithium-ion is used in mobile phones. Both of these technologies are suitable for storing electricity on a domestic scale.

Why store electricity locally?

Although exporting and importing electricity from the grid can be thought of as a type of electricity storage, it has some significant downsides: Firstly, you still need to import electricity from the grid (which might be the polluting kind - from coal). Secondly, if there is too much electricity going into the grid renewable sources are disconnected. Thirdly, the further electricity has to travel the more is lost during transmission.

How do I decide what type of storage system I need?

How do you make sure that you are getting a good deal in a relatively new market? How do you know you are being sold a battery of the right size at the right price?

You should take some time to do your calculations and think about how you use your electricity. Some of the questions you’ll need to ask yourself are: how much electricity do your solar panels produce? How much do you export to the grid? How much electricity do you use during the day? How much in the evening? If you produce a lot of electricity in the day but rarely use it, and then you consume a lot from the grid in the evening, you’ll want a larger storage capacity. Generally you’d expect to use 3 to 6kWh an evening and battery storage systems available in the UK range from 2kWh to 6kWh.    

What is the difference between lead-acid and lithium ion?

The lead-acid industry is mature and established whereas the lithium-ion one is relatively new, which is one of the reasons the prices vary. Also flooded lead-acid batteries release gases which need to be removed. (“Flooded” refers to the battery type.) Lithium-ion systems don’t have this issue and are able to cope with more discharge cycles, giving them a longer overall life. A disadvantage of lithium-ion is that they can have problems when over-charged or over-discharged.

What are the costs?

The cost of lead-acid batteries is about £395 to £730 per kWh. This means that for  3kWh in lead acid battery storage you would expect the price to be in the region of £1000 to £2000. A lithium-ion battery costs £570 to £1100 per kWh, which means that a 3kWh lithium-ion battery set-up ought to cost around £2000 to £3000.

However this isn’t the whole price: the storage system as a whole will cost about double that of the battery by itself.

What else do I need to know?

When choosing the right size battery, remember that you can’t get 100% of the electricity you put into a battery back out of it. This is due to heat, electrical losses and self-discharge. A lead-acid battery has an energy output of 85% to 90%; this means that 10% to 15% of the electricity put into the battery is lost. A lithium-ion battery has an output towards the lower end of that range- around 85%. Also batteries work best when they aren’t fully discharged: you’ll probably be looking at an optimum 80% depth of discharge.

The storage industry is expected to boom in the next few years, so we can expect prices to fall as the market expands and the technology improves. 


Photo: Ecohome Management

More information about Energy Storage on YouGen

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Related blogs

How do I make the most of my solar panels?

How can I store my solar generated electricity to use at night?

Need help with any Jargon?

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

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7 comments - read them below or add one


simonwellbyComment left on: 28 September 2016 at 2:22 pm

Advive please

would like to get battery storage. have 14 PV;s declared net capacity 2.73kw with single string to sunny boy 2500HF inverter. have been recommended the Grawett SP 2000 storage system, but told  this will not work unless a second string is added from the PV's,

does this make sense?

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Andrew Savage

Andrew SavageComment left on: 17 September 2016 at 8:12 am

Need advice. Battery Storage

i have a 5kW solar system. My Nissan Leaf has 28Kw of batteries.

These vehicles depreciate rapidly and are worth only £6K after 2 years - clearly this makes for very cheap battery storage. (a 6Kw Tesla Powerwall costs £6k)

What I propose is removing the batteries and scrapping the car body.

Can any one help with this?

Would also make an interesting article!




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JolneyComment left on: 29 January 2016 at 9:46 pm

In November i installed a Nedap Unifit Power Router with a 6.4 kw LG resu ex 48 volt l-ion battery  and the results so far are promising. On a sunny day, the battery takes 3 hours to be fully charged with my 3.9 KW solar system. The router, which monitors the grid, if exporting power it automatically takes that power and stores it to the battery. When importing power, it takes the power from the battery to feed the house.  All in, I utilise 90% of energy generated in winter.

Although there is not enough energy in winter to supply the battery every day, as the days get longer and spring starts to set in, I estimate even with a few hours of sun, the system will fully charge the battery to run over a 24 hour period, enabling us to be as close to self sufficiency as  possible.

At a cost of £5400, I estimate that system has a payback period of around ten years. When calculating a saving of around £600 a year in electricity costs.

Overall, the system will produce more energy than actually needed, estimating a self consumption of around 60% to 70% of the energy generated.

So I will also be looking put in an Immersun (or a cheaper equivalent), to store excess energy to heat water for the summer months.

This technology is new and relatively expensive at the moment. I have always been a pioneer and although it is expensive the payback period is still good.

When the system has been run for about a year I will post to you-gen website.

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JCapiroteComment left on: 18 November 2015 at 12:54 am


So, having read this article I have some questions.

If I am going to store energy provided by my 4KW solar PV instalation, will the power being provided by my panels be 12V?

If this is correct I assume I will need some batteries.

I have a nice pair of 170AH gel batteries, how much of the required system would these provide?

What other equipment would i need, bearing in mind I wish to claim the 50% feed in tariff for the energy produced, even though I fully intend to use as much as I possibly can myself.

I already use an Immersun unit diverting power to my immersion tank (which has been set to 70C to use as much as possible without the risk of overheating the system)

Is there a way to ensure the batteries are being charged after passing through my generation meter? I'm quite happy to use a battery charger plugged into the mains if this is the only viable method.

How much power could I conceivably expect from the batteries over an evening?

If i think of any more I will ask

Many thanks


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richmcComment left on: 30 October 2015 at 5:19 pm

I have a Wattstor system that can give me 6Kwh of stored power and an imertion diverter. To store all the excess power generated as hot watwer would mean a massive tank and you could never hope to use what you store, but combining the two is a great idea.

Usa as much as you need, export as little as you can.

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catchercradleComment left on: 29 October 2015 at 12:25 pm

Very tempted by the idea of having a stand alone system with 200W worth of pannels connected to batteries and an inverter for unning computer.s. (One Desktop and one laptop.)


Laptop would be on all the time and desktop on only when being used. The amount of usage should work quite well and I would expect to only very occasionally need to connect computers to the mains. This could also provide emergency backup for lighting which is now mostly LED.

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toneblueshawkComment left on: 29 October 2015 at 11:32 am

Storing "excess" electrical energy in a battery (as chemical energy) is not the only option. We store excess electricity generated by solar panels as heat. We have a device that uses any electricity that we cannot immediately use (that would otherwise be exported to the grid) to heat water via our existing immersion heater/hot water tank. The device - a box of electronics - detects when electricity would be exported to the grid and passes it to the immersion heater, which because it is a resistive load, can use whatever power is available up to the rating of the immersion heater (typically 3kW) which matches fairly well with our 3.5kW solar installation. This saves us gas (we would otherwise run a gas boiler to heat water), and helps us achieve a high rate of use for the electricity we generate.

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