Sometimes much more, sometimes nothing. In this site's model a London home using 3,000 kWh a year with 4 kWp of solar saves £174 a year with 5 kWh and £414 with 10 kWh on Octopus Flux, so the second 5 kWh adds £240. On a flat tariff the same home gains £5. At Octopus's published prices the step from 5 to 10 kWh costs £1,389, which the extra saving repays in under six years on Flux and effectively never on a flat tariff.
Analysis · Published by SEO Dons Ltd · Updated 6 October 2026
The question, and how we tested it
A quote for 10 kWh is rarely double the price of a quote for 5 kWh. The survey, the labour, the electrical connection and the network paperwork cost much the same at either size, so the price per kWh falls as the battery grows. That makes the bigger battery look like better value. It is better value only if the extra capacity is filled and emptied often enough to earn its keep. If your question is whether 5 kWh is enough in the first place, is 5kWh enough for a house works through evening use by home size, and every size from 3 to 20 kWh is tabled side by side.
To find out when that happens, we ran this site’s savings model, the same one behind the calculator, for a London home at five levels of use: Ofgem’s new typical values from July 2026 (1,600, 2,500 and 3,800 kWh a year), the 3,000 kWh home the site uses as its standard example, and 6,000 kWh for a home with a heat pump or electric car. Each was run with a 5 kWh and a 10 kWh battery on four tariffs, using Octopus’s London rates for 26 September 2026 and PVGIS solar data for 4 kWp of panels. The saving is always measured against the same home with no battery on a flat tariff.
With solar panels: Flux pays for spare capacity, a flat tariff does not
| Annual use | Flux: 5 kWh | Flux: 10 kWh | What the second 5 kWh adds | Flat: 5 kWh | Flat: 10 kWh | What the second 5 kWh adds |
|---|---|---|---|---|---|---|
| 1,600 kWh | £186 | £438 | £252 | £120 | £120 | £0 |
| 2,500 kWh | £176 | £423 | £247 | £159 | £159 | £0 |
| 3,000 kWh | £174 | £414 | £240 | £169 | £174 | £5 |
| 3,800 kWh | £177 | £399 | £222 | £159 | £190 | £31 |
| 6,000 kWh | £185 | £374 | £189 | £132 | £212 | £80 |
London, 4 kWp of solar, modelled annual savings.
The two columns of extra savings point in opposite directions. On a flat tariff, the battery can only store surplus solar for the evening. A 4 kWp array’s surplus and a home’s evening use cap that at around 5 kWh for most households, so the second 5 kWh adds little or nothing until use reaches about 3,800 kWh a year.
On Octopus Flux, spare capacity still earns. Flux charges 15.29p per kWh from 2am to 5am and pays 28.6p per kWh for energy exported between 4pm and 7pm (London, 26 September 2026), so a battery larger than the evening load can buy cheap at night and sell at the peak. That is why the second 5 kWh adds most for the smallest users: they have the most room left over to sell.
Without solar: the tariff decides, and a small battery can lose
| Annual use | Agile: 5 kWh | Agile: 10 kWh | What the second 5 kWh adds | Octopus Go: 5 kWh | Octopus Go: 10 kWh | What the second 5 kWh adds |
|---|---|---|---|---|---|---|
| 1,600 kWh | £175 | £182 | £7 | £253 | £268 | £15 |
| 2,500 kWh | £196 | £285 | £89 | £236 | £419 | £183 |
| 3,000 kWh | £202 | £335 | £133 | £212 | £489 | £277 |
| 3,800 kWh | £210 | £379 | £169 | £175 | £524 | £349 |
| 6,000 kWh | £219 | £404 | £185 | £73 | £425 | £352 |
London, no solar, modelled annual savings.
Without panels, the battery earns by charging at a cheap overnight rate and covering the house at the dearer one, so the bigger the home’s use, the more the extra capacity has to do. The low-use home fills 5 kWh and has nothing left for the second 5 kWh.
Octopus Go shows the trap in buying too small. Its London day rate, 30.99p per kWh, is higher than the 26.35p flat rate, so the home pays more for everything the battery does not cover. With 10 kWh, a 6,000 kWh home on Go saves £425 a year; limited to 5 kWh a night, it saves just £73. A time-of-use tariff and an undersized battery is the worst of both.
The 12-month Agile back-test found the same pattern with real prices: sizes above 10 kWh added nothing for a 3,000 kWh home.
What the second 5 kWh costs, and how fast it pays back
Octopus publishes battery-only prices of £3,947 for 5 kWh and £5,336 for about 10 kWh (April 2026 prices), so the second 5 kWh costs £1,389. The official medians, £1,300 per kWh under 6 kWh and £890 per kWh for 6 to 11 kWh, put a typical 5 kWh installation at about £6,500 and a 10 kWh one at about £8,900: a step of about £2,400. For the 3,000 kWh London home:
| Set-up | Second 5 kWh adds a year | Payback at Octopus’s step (£1,389) | Payback at the median step (£2,400) | Payback of the first 5 kWh at Octopus’s price |
|---|---|---|---|---|
| Octopus Flux, 4 kWp solar | £240 | about 6 years | about 10 years | about 23 years |
| Octopus Go, no solar | £277 | about 5 years | about 9 years | about 19 years |
| Agile Octopus, no solar | £133 | about 10 years | about 18 years | about 20 years |
| Flat tariff, 4 kWp solar | £5 | never | never | about 23 years |
Simple paybacks, before capacity fade. The first 5 kWh carries the fixed costs of the installation, so on every time-of-use tariff here the second 5 kWh pays back two to four times faster than the first.
Where bigger stops paying
Run the model further and each set-up reaches a size beyond which extra capacity earns nothing:
- A flat tariff with 4 kWp of solar stops at about 5 kWh for a 2,500 kWh home, which saves £159 a year with anything from 4.6 to 15 kWh.
- Agile or Go without solar stops between 10 and 13.5 kWh for a 3,000 kWh home, which saves £342 on Agile and £503 on Go at both 13.5 and 15 kWh.
- Flux with solar keeps rising until about 20 kWh, where a single-phase connection’s limit of about 3.68 kW across the three-hour peak caps what the battery can export.
If you are pricing beyond 10 kWh, the 15kWh battery page runs the same model on larger homes and longer cheap-rate windows.
At the top of the common range, the 13kWh battery is the clearest example: in the same model its extra 3.5 kWh over 10 kWh earns about £155 a year with solar on Octopus Flux for a 3,000 kWh home, but an extra £14 a year at most without solar on Octopus Go.
Using this for your own home
Start with three facts: your yearly use from your bill, whether you have or will have solar, and which tariff you can join. If you are on a flat tariff with modest solar and no plans to change, the second 5 kWh is hard to justify. If you can join Flux, or you use more than about 2,500 kWh a year on an overnight tariff, the step from 5 to 10 kWh is usually the best-value money in the whole installation, which is why our page on the 10kWh battery treats it as the default size to price. Either way, get both sizes quoted and put each through the calculator before choosing.
Questions people ask
Does 10 kWh save twice as much as 5 kWh?
Not as a rule. In this site's model a 3,000 kWh home in London saves 2.4 times as much with 10 kWh as with 5 kWh on Octopus Flux with solar (£414 against £174), about 1.7 times as much on Agile Octopus without solar (£335 against £202), and almost the same on a flat tariff with solar (£174 against £169).
Why does a bigger battery save nothing on a flat tariff?
On a single-rate tariff a battery earns only by storing solar power you would otherwise export for less, and the home's evening use caps how much of that it can use. In this site's model a London home using 2,500 kWh a year with 4 kWp of solar saves £159 a year with anything from 4.6 kWh to 15 kWh of battery.
Does the extra capacity pay for itself?
Often faster than the first 5 kWh does. At Octopus's prices of £3,947 for 5 kWh and £5,336 for 10 kWh, the second 5 kWh costs £1,389. For a 3,000 kWh London home it adds £240 a year on Flux with solar, £277 on Octopus Go and £133 on Agile without solar in this site's model: paybacks of about 6, 5 and 10 years, against 19 to 23 years for the first 5 kWh on those tariffs.
Should I buy more capacity for a future electric car or heat pump?
If one is likely within a few years, price both sizes now. In this site's model a London home using 6,000 kWh a year on Octopus Go saves £425 a year with 10 kWh but only £73 with 5 kWh. Adding capacity later means another visit and can cost more: Octopus charges £3,773 for a Tesla Expansion Pack fitted with a new Powerwall 3 and £5,909 to add the same pack later.