What size solar battery do I need?
A bigger battery saves more, but each extra kWh saves less than the one before, and how much more depends mostly on how much your home uses. We gave the same 20 homes four sizes of battery, from 5 to 20 kWh, for a whole year, and measured what each saved.
Doubling the battery from 9.5 to 20 kWh added £62 a year.
The same homes with 4 kWp of panels, each battery charging and emptying at up to 5 kW. Saved is against the same home with no battery, on the same tariff.
| Battery | Agile with Outgoing Octopus, planned | Same tariff, left to itself | Go, no export payment, planned |
|---|---|---|---|
| 5 kWh | £364.71 | £145.12 | £444.18 |
| 9.5 kWh | £431.13 | £170.99 | £597.55 |
| 13.5 kWh | £460.48 | £176.89 | £662.44 |
| 20 kWh | £492.75 | £179.66 | £711.47 |
Saved a year by the average of 20 homes, 1 October 2025 to 30 September 2026, against the same home and tariff with no battery. Standing charges are left out, and no limit was put on export.
- Each extra kWh saved lessOn Agile with Outgoing Octopus, a year: £14.76 for each kWh from 5 to 9.5, £7.34 from 9.5 to 13.5, and £4.96 from 13.5 to 20. On Go: £34.08, £16.22 and £7.54.
- Left to itself, a bigger battery barely helpedFrom 9.5 to 20 kWh added £8.67 a year. Left to itself, a battery only stores spare sun and empties into the house before the next sun, so past about 10 kWh the extra mostly sits unused.
- Go gained most from sizeA cheap night is worth filling a big battery for, so on Go every size step paid more than on Agile with a fixed export rate.
A bigger battery paid for the homes that use the most.
How much a home used explained most of what a bigger battery added. Going from 9.5 to 20 kWh, planned, added a year:
9.5 to 20 kWh, by the home’s use
- Under 4,000 kWh a year: £43 on Agile, £30 on GoSeven homes, using about 10 kWh a day.
- Over 6,000 kWh a year: £97 on Agile, £260 on GoFive homes, using 17 to 36 kWh a day.
- The home using most, 13,014 kWh: £178 on Agile, £479 on GoAbout 36 kWh a day, enough to empty even a 20 kWh battery.
Ask what the extra kWh cost, and set it against what it saves.
A battery lasts about 10 to 12 years, by the Energy Saving Trust’s estimate. Over that, at this year’s prices, each extra kWh was worth:
One extra kWh over 10 to 12 years, average home
- From 5 to 9.5 kWh: £148 to £177 on Agile, £341 to £409 on Go
- From 9.5 to 13.5 kWh: £73 to £88 on Agile, £162 to £195 on Go
- From 13.5 to 20 kWh: £50 to £60 on Agile, £75 to £90 on GoCompare it with what your installer charges for each extra kWh between two sizes. For a home that uses a lot, the figures are higher; for one that uses little, lower.
Before you pick a size.
A year of 20 homes is strong evidence, not a quote. These are the things that change the answer.
All our guidesWhat size battery do most UK homes get?
The Energy Saving Trust puts a typical home system at about 10 kWh. In our year, each kWh from 5 to 9.5 kWh was worth twice as much as each kWh from 9.5 to 13.5 kWh, for the average home.
How do I work out my own use?
Your bill or your supplier’s app shows your kWh for the year. Divide by 365 for a day. The homes here used from 3,426 to 13,014 kWh a year, half of them over 4,713, more than Ofgem’s typical home at 2,500.
Does a bigger battery need a more powerful inverter?
Every battery here charged and emptied at up to 5 kW. A bigger battery often comes with more power, which lets it fill in fewer cheap hours and sell more in the dearest ones, so it may save more than these figures show.
Can I add more battery later?
Many home batteries are built from modules that can be added to later. Ask your installer whether yours can be, and what adding one would cost, before paying for size you may not need yet.
What would make my figures different?
Your use most of all, then your tariff, your panels and your export limit. No limit was put on export here; most homes may send 3.68 kW, which would cut what a large battery can sell. The panels faced south in a sunny year. Is a solar battery worth it at all?
What does Orza cost?
Nothing during the beta, and no card is needed to join. Whatever size your battery, Orza plans it every half hour for your own tariff and home.
Get the most from the battery you have.
Free during the beta. Connect in a few minutes, and start in Monitor only.