What home battery storage costs in the UK

Storage quotes are unusually hard to compare, because the headline capacity is not the capacity you get to use, and different manufacturers are not consistent about which figure they advertise.

There is one number that fixes this. Work out the price per usable kilowatt hour for every quote, and the comparison becomes straightforward. Then work out what each cycled kilowatt hour actually earns on your tariff, which is arithmetic this page does with you, and you can judge any battery price without trusting a single brochure.

Nominal against usable capacity

Two figures describe every battery:

Nominal capacity is the total energy the cells hold.

Usable capacity is what the system will actually deliver, after the reserve it holds back to protect cell life. The share it will use is the depth of discharge, and it varies by product: some modern units advertise close to full depth of discharge, others hold back ten percent or more.

The gap between the two is not small, and it varies between products. A battery advertised on nominal capacity will look cheaper per kilowatt hour than one advertised honestly on usable capacity, while delivering less.

So: divide the installed price by the usable figure. A worked comparison of two illustrative quotes shows why this ordering matters:

• Quote A: 13.5 kWh nominal, 12.2 kWh usable, £6,500 installed. That is £533 per usable kilowatt hour.
• Quote B: 9.5 kWh nominal and usable, £5,000 installed. That is £526 per usable kilowatt hour.

On headline capacity and price, A looks like the better deal per kilowatt hour. On usable capacity they are the same price, and the decision moves to warranty terms and whether your household can actually cycle the extra capacity. If a quote does not state usable capacity, ask for it in writing before comparing anything.

What sits inside a storage price

• The battery unit itself, which is most of it.
• The inverter, or the battery side of a hybrid inverter. If you already have solar with a plain string inverter, adding storage may mean either replacing it with a hybrid unit or adding a separate battery inverter. This is a large part of why retrofitting costs more than fitting at the same time.
• Installation labour and the electrical work to connect it.
• Any consumer unit changes, and the mounting or plinth.
• Certification and notification, as with solar.

One line that should not be there: VAT. Battery storage installed in a home is zero rated until 31 March 2027, and since February 2024 that includes batteries retrofitted on their own, not just those fitted alongside new panels.

The important structural point: a substantial share of the price does not scale with capacity. As with panels, the marginal kilowatt hour is cheaper than the average, so larger units usually show a better price per usable kilowatt hour. That is an argument for not buying the smallest unit available, but only up to the size your household can actually cycle. Capacity you never use is capacity you paid for and did not benefit from.

What each cycled kilowatt hour earns

A battery earns money in two ways, and both are rate differences you can look up today.

Raising self consumption. Storing generation you would otherwise export, and using it after dark. Storing a kilowatt hour of solar means giving up its export value, and batteries lose some energy in the round trip, so the honest sum has three parts. At the current capped import rate of 26.11p, with Octopus Outgoing paying 12p for exports and a round trip efficiency near 87%, a stored kilowatt hour returns about 22.7p of avoided import, minus the 12p of export income you gave up: a net gain of roughly 10.7p per kilowatt hour cycled.

Tariff arbitrage. Charging from the grid when electricity is cheap and discharging at peak rates. On an EV tariff such as Intelligent Octopus Go, with off peak electricity at 8p, the same arithmetic gives 22.7p of avoided peak import for 8p of off peak charge: about 14.7p per kilowatt hour cycled, and available even with no panels at all.

Notice the uncomfortable result: on today’s rates, grid arbitrage on a cheap overnight tariff earns more per cycled unit than storing your own solar does. A battery that can charge from the grid, on the right tariff, is a different investment from one that cannot.

The ceiling on what any battery can earn

Multiply the per unit gain by the most the battery can cycle. A 10 kWh usable battery cycling fully once a day shifts 3,650 kilowatt hours a year: at 10.7p per unit that is about £390, and at the arbitrage rate about £540. Real households do less than a full cycle every day, especially in summer evenings and mild weather, so treat those as ceilings, not forecasts.

Set that against the price. If that 10 kWh battery costs £5,000 installed, an optimistic £450 a year of blended benefit still takes eleven years to repay, which is uncomfortably close to the length of a typical performance warranty, and capacity fades along the way, which our model degrades explicitly rather than ignoring. This is why batteries make sense for households with big evening consumption and wide rate gaps, and struggle to justify themselves for light users on flat tariffs.

Retrofit costs more than fitting at the same time

If you are installing solar now and think a battery is likely later, the choice of inverter today matters.

A hybrid inverter costs more up front but can run a battery when you add one. A plain string inverter is cheaper now and may need replacing or supplementing when storage arrives. The scaffolding and the site visit are also being paid for twice if the work is split, and the quote’s exclusions deserve the same scrutiny either way.

Whether the hybrid premium is worth it comes down to how likely the battery really is. It is a straightforward comparison to make, and worth making explicitly rather than by default.

Questions before you sign

• What is the usable capacity, and the price per usable kilowatt hour?
• What does the warranty guarantee, after how many years and how many cycles?
• Can it charge from the grid, or only from solar? This decides whether arbitrage, the stronger earner on current rates, is available to you.
• What happens in a power cut? Backup is usually not included unless specified.
• How much of my evening consumption would this actually cover, using my data?

Model it on your own pattern

The value of storage is almost entirely a function of your household’s shape: how much you use after dark, and how wide the gaps are between your rates. Generic payback figures for batteries are unreliable for exactly that reason.

Put your own consumption, your own rates and a realistic usable capacity into the ROI calculator, which applies degradation and replacement over the projection, and you will get an answer that belongs to your house rather than to the brochure.