Size a battery to the electricity you use when your panels are not generating, plus a margin for winter. In this site's model a home using 2,700 kWh a year (Ofgem's medium figure until June 2026) gets its whole saving from 5 kWh on a flat tariff with solar, and from about 10 kWh on a time-of-use tariff without solar. Beyond that, extra capacity only earns by exporting stored energy at the peak on a tariff such as Octopus Flux, which on a standard single-phase connection tops out at around 20 kWh.
Guide · Published by SEO Dons Ltd · Updated 6 October 2026
Start from your usage, not your roof
A battery saves money in two ways: by storing solar power you would otherwise export cheaply, and by storing cheap-rate grid power to use instead of dearer power later. Either way, it only earns on energy that goes in and comes back out each day. Capacity that never empties earns nothing.
So the useful size is roughly the electricity your home uses when your panels are not generating, which in this site’s model is about 60% of a home’s use, and more in winter. Ofgem’s 2023 typical domestic consumption values give three reference homes. Ofgem lowered them on 1 July 2026 to 1,600, 2,500 and 3,800 kWh, and the daily figures fall in proportion:
| Annual use | Average per day | Used outside solar hours (average day) | Used outside solar hours (December day) |
|---|---|---|---|
| 1,800 kWh (Ofgem low, 2023) | 4.9 kWh | 3.0 kWh | 3.6 kWh |
| 2,700 kWh (Ofgem medium, 2023) | 7.4 kWh | 4.4 kWh | 5.3 kWh |
| 4,100 kWh (Ofgem high, 2023) | 11.2 kWh | 6.7 kWh | 8.1 kWh |
| 6,000 kWh (electric heating or heat pump) | 16.4 kWh | 9.9 kWh | 11.8 kWh |
Your own figure is better than any average. A smart meter records use every half hour, and most suppliers show it in their app or online account, so you can see exactly how much you use after dark. If you charge an electric car directly in an overnight cheap window, leave that out: the car does not need battery capacity.
What the model says, size by size
This site’s savings model compares a home with no battery on a flat tariff against the same home with a battery on each tariff, using London rates for 26 September 2026 and PVGIS solar data. For a home using 2,700 kWh a year, with 4 kWp of solar where stated:
| Battery | Flat tariff, with solar | Octopus Flux, with solar | Flux, no export trading | Agile, no solar | Octopus Go, no solar |
|---|---|---|---|---|---|
| 5 kWh | £165 | £175 | £156 | £199 | £226 |
| 7.5 kWh | £165 | £298 | £163 | £278 | £392 |
| 10 kWh | £165 | £420 | £163 | £308 | £453 |
| 13.5 kWh | £165 | £576 | £163 | £308 | £453 |
| 20 kWh | £165 | £718 | £163 | £308 | £453 |
Annual saving against the same home with no battery on a flat tariff. Go is Octopus’s overnight tariff for electric car owners. Other regions differ by roughly a tenth either way.
Three things stand out:
- On a flat tariff, 5 kWh is enough. The battery can only store surplus solar for the evening, and a medium home’s evening use fills about 5 kWh. Anything larger sits partly empty. Is 5kWh enough for a house? sets out evening use by home size and the winter case.
- Without solar, the useful size is about a day’s use. On Agile or an overnight tariff the battery charges in the cheap hours and runs the home through the dear ones, so the saving climbs until the battery covers a whole day, around 10 kWh for a medium home, then stops.
- On Flux, bigger keeps paying only through trading. Flux pays a high export rate from 4pm to 7pm, so a battery can charge cheaply overnight and sell in the peak. Without that trading the Flux saving stops at 7.5 kWh (£163). With it, capacity keeps earning until the export limit bites: a standard single-phase connection under G98 exports at up to 3.68 kW, about 11 kWh across the three-hour peak, which is why the modelled saving stops rising at around 20 kWh.
Where extra capacity stops paying
Taking the point where the next size up adds less than 5% to the annual saving:
| Annual use | Flat tariff, with solar | Flux with solar, own use only | Agile, no solar | Octopus Go, no solar |
|---|---|---|---|---|
| 1,800 kWh | 5 kWh | 5 kWh | 7.5 kWh | 7.5 kWh |
| 2,700 kWh | 5 kWh | 5 kWh | 10 kWh | 10 kWh |
| 4,100 kWh | 7.5 kWh | 10 kWh | 13.5 kWh | 13.5 kWh |
| 6,000 kWh | 10 kWh | 13.5 kWh | 20 kWh | 20 kWh |
These are modelled sizes for an average pattern of use, not a prescription. A household that is out all day and busy all evening needs more; one that is home all day and uses its solar directly needs less. The calculator runs your own usage, region, solar and tariff.
With no solar the ceiling is lower again. A 12-month back-test on real Agile prices found that a 5 kWh battery earned 94% of what a 10 kWh one did for a home using 3,000 kWh a year, so for home battery storage without solar the smaller size is usually the better buy.
When a bigger battery makes sense
- You plan to trade on Octopus Flux, charging cheaply and exporting at the peak. It works, but the saving depends on Flux’s peak export rate staying where it is, and tariffs change: the Tesla Energy Plan has been retired, and Intelligent Octopus Flux is currently limited to Octopus’s own solar and battery customers. See the best tariff for a home battery.
- You have a heat pump or electric heating, which adds several kWh a day of winter demand a battery can move to cheap hours.
- You want whole-home backup, where a reserve kept for power cuts takes a share of the capacity. See home battery backup. Ofgem’s outage figures, worked through in how long UK power cuts last, show how much of a battery the essentials take in an average cut.
- You expect your use to grow. Adding capacity later is possible with modular systems but can cost more: Octopus charges £3,773 for a Powerwall 3 Expansion Pack fitted at installation and £5,909 to add one later.
Power matters as well as capacity
Capacity (kWh) is how much a battery stores; power (kW) is how fast it can deliver it. A battery that cannot supply your evening peak will leave part of it on the grid however large it is. An Enphase IQ Battery 5P delivers 3.2 kVA, a Fox ESS EVO 10 comes in versions from 5 kW to 10 kW, and a Tesla Powerwall 3 can be set from 3.68 kW to 11.04 kW. An electric shower drawing 7.5 to 10.5 kW is beyond most home batteries. Match the power to the appliances you run at the same time in the evening, and check the backup output separately if you want power in a cut.
Price per kWh falls as batteries get bigger
Installation costs much the same whatever the size, so larger batteries cost less per kWh. The official medians for MCS-certified batteries fitted in 2025/26 were £1,300 per kWh under 6 kWh, £890 for 6 to 11 kWh and £630 for 11 kWh or more. That makes a small battery the hardest to pay back: at Octopus’s published £3,947 for about 5 kWh, the model’s £165 to £199 a year means a payback of about 20 to 24 years. It is also why a 13.5 kWh battery at the median price (about £8,500) can pay back faster than a 10 kWh one (about £8,900), provided the home can use or trade the extra capacity.
Limits on how big you can go
PAS 63100, the fire-safety specification for batteries in homes, allows at most 20 kWh in any single battery enclosure, and 80 kWh in total outdoors or in a garage or outbuilding (40 kWh elsewhere). Few homes come close. The more common limit is the network: above 3.68 kW of combined inverter output on a single-phase home, the installer must apply to the network under G99 before connecting.
Turning this into a quote
Work out your use outside solar hours, decide which tariff you can join, and read off the size where the saving levels out. Then ask installers to quote that size and one step either side, so you can see the price of each extra kWh. Comparing two or three battery storage quotes on the same usable capacity is the quickest way to see what a fair price looks like; our 5kWh, 10kWh, 13.5kWh and 20kWh pages cover each size in detail.
At the large end, the 13kWh battery page shows which homes gain from 13.5 kWh over 10 kWh, and the solar battery cost guide shows what each size cost on average in 2025/26. The solar battery sizes table lists every size from 3 to 20 kWh with its price and modelled saving, including the 12kWh and 16kWh steps. Once you know the size, the best solar battery guide picks a battery for each situation.
Questions people ask
What size battery do I need for a 4kW solar system?
It depends more on your evening use than on the panels. In London a 4 kWp array makes about 16 kWh a day in June but under 5 kWh a day in December, according to PVGIS, so a battery charged only from solar is full in summer and nearly empty in winter. For a medium home on a flat tariff, 5 kWh captures the whole modelled saving; on Octopus Flux, extra capacity keeps paying up to about 20 kWh, mostly through exporting stored energy at the peak.
Is a 10kWh battery big enough for a house?
For most homes, yes. A home using 2,700 kWh a year uses about 7.4 kWh a day, of which about 4.4 kWh falls outside solar hours in this site's model, rising to about 5.3 kWh in December. 10 kWh covers that with room for cheap-rate charging on a time-of-use tariff.
How much electricity does a UK home use per day?
Ofgem's typical domestic consumption values, revised from 1 July 2026, put a medium home with a single-rate meter at 2,500 kWh a year, about 6.8 kWh a day, with low use at 1,600 kWh and high use at 3,800 kWh. The 2023 values used in this guide's tables were 1,800, 2,700 and 4,100 kWh. Your own bill or smart meter data is a better guide than any average.
Can I add more battery capacity later?
Often, if the system is modular, but it can cost more. Octopus charges £3,773 for a Tesla Powerwall 3 Expansion Pack fitted with a new Powerwall and £5,909 to add the same pack later. Check the most the battery and inverter can support before you buy.
What is the difference between kWh and kW for a battery?
kWh is how much energy the battery stores; kW is how fast it can deliver it. A 10 kWh battery with a 3 kW output can run 3 kW of appliances for about three hours, but cannot run a 9 kW electric shower at all.