If you are looking to buy solar panels for your home, then you have probably come across solar batteries, which are basically large lithium battery packs.
Using the same technology as electric car batteries, they can store the electricity you generate during the day for use at night.
They can also soak up cheap overnight electricity to use during the day and act as a backup generator if your area suffers from power cuts.
They cost from £2,500 to £6000 depending on their size and brand, but installation can add about another £3,000, depending on whether you are having solar panels added at the same time.
Lloyd Greenfield, founder of Glow Green, a solar panel fitter, said: “Batteries are definitely suited to people who typically use their energy in the evening. So if you're a high daytime consumer, it might be a faster payback, actually, without the battery.”
Here, we make use of our connections in the solar power market to answer key questions and do the maths on whether batteries are a good choice for you.
How do they work?
Like a typical battery, solar batteries can be charged and discharged. In your home, they can be charged by solar energy, effectively for free, or from cheap overnight electricity if your tariff allows this. An inverter then converts the lower voltage of the battery to the 230v your home ring main uses.
You can get a huge range of capacities, but they typically range from about 5 kilowatt hour (kwh) to about 14kwh. You can also add capacity and use two or more at once.
How long does a solar battery last?
They are typically guaranteed for 10 years but can last for much longer.
How do I work out if they are for me?
Here is the tricky bit. It would be very useful to be able to work out how much it costs to store energy in the battery.
Just like any rechargeable battery, a solar battery has a finite life, largely down to how many times you can charge and discharge it.
So in theory, you can work out how many pence it costs to store and then use a kilowatt hour (kwh) of energy.
Once you have that, you’ll be able to see if it is worth it compared with power from your energy company, which from April will cost 27 pence under the price cap. For instance, if you could store a kwh of power for less than 20 pence, then storing sun power would make sense since power from the grid costs 7 pence more.
If you could store it for even less, then buying super-cheap overnight electricity on some tariffs for as little as 7.5 pence could make sense too, so long as the storage price plus 7.5 pence is less than the grid price.
But as we are about to see, this figure is not simple to arrive at, and even if it were, there are other things to think about.
To make things more difficult, batteries don’t just stop working after a certain number of cycles; instead they lose more and more of their ability to store energy.
Let’s do the maths anyway
With all of this in mind, let’s take a real-world example to get a figure, even if it isn’t set in stone, since it will give a rough indication of whether a battery is worth it.
Duracell’s Dura5 storage system is guaranteed for 10 years and 10,000 cycles. If we assume we can discharge the 5.12kwh battery by 90 per cent, that gives us 4.608kwh of useable charge per cycle. The unit can be bought for £2,280 including VAT but not including fitting.
For 10,000 cycles of 4.608kwh, a total of 46,080kwh of power charged and drained through its guaranteed life. If we take the price and divide it by this lifetime storage figure, we get a price of five pence per stored kilowatt hour.
This is a good price, because it means stored sunlight – which is free to generate – only costs 5 pence per kilowatt hour, less than a quarter of the current grid price. If you can buy power overnight cheaply – some tariffs offer prices as low as 7.5 pence – then buying and storing the power would cost just 12.5 pence per unit, way below daytime grid prices.
Looks good! I’ll get my bank card
Wait! There are quite a few caveats.
If the battery lasts longer than 10,000 cycles, the price per use is even lower. But towards the end of the life of the battery, it will keep less charge, meaning the price will creep up.
The price also doesn’t include fitting, which could double the cost, and bring about a final figure of about 10 pence.
Then, there is yet another thing to think about – with most batteries the guarantee is in years, not cycles. So, will you use it enough in 10 years to keep the price per kilowatt down?
Less usage should make the battery last longer, in theory, but with no guarantee after 10 years, that might prove cold comfort.
In real life, batteries may last longer than their warranties, just like any consumer product. It’s just that you aren’t financially protected if they don’t.
Mr Greenfield said: “We think batteries would last, typically, circa 15 years, maybe 20, depending on quality.”
The average home uses about 2,700kwh of energy per year, and so 27,000kwh in 10 years. That’s just under half the 46,080kwh in our example, pushing up the price per stored kwh if the battery dies on its 10th birthday.
If, however, you use a heat pump for central heating, your usage will be a lot higher, and it could make the battery viable again.
And then, of course, there’s yet another complication. How much can you get for selling the solar energy you generate back to the grid? You can get up to 15 pence from many companies, which could make storing it less attractive.
If you can sell the energy for 15 pence and buy it back when you need it for 27 pence – April’s energy cap charge – then you may not deem the difference as worth it.
If all this is making your head spin, and you’d be getting the device to try and save money, then a solar battery might not be for you.
But if you found all that maths a fun exercise and you’ve found a deal where you can store cheap sunlight and use it at night at a lower price than you buy from a utility company, then it could be a good option.
Are solar batteries worth it in the UK?
If you don’t mind taking a bit of a gamble that the battery will outlast its guarantee, and you use a lot of electricity, perhaps through a heat pump or electric car, or both, and you have a large solar array to feed it, then a battery could be for you.
This is because you will make the most of it in the guarantee period, charging and discharging it a lot, and getting a cheap price per stored unit of energy.
Mr Greenfield, of Glow Green, said: “If you're a working, busy professional, and you're out the house all day, and then you when you get in the evening, that's when you have a high consumption, whether it's plugging in an EV car or running lots of electrical appliances or lots of washing and tumble dryers, and high consumption appliances going in the evening, you'd definitely be suited for a battery!”
If you have lower usage or a smaller solar array, you may want to be more cautious.
Either way, you will need to ensure the battery is properly set up to soak up cheap sunshine during the day, and maybe cheap grid power at night, to avoid costlier energy.
How are they installed?
Solar batteries are typically bolted to a wall inside or outside your home. They need to be kept cool and accessible in an emergency.
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