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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