At the official median for batteries of 11 kWh or more fitted in 2025/26, £630 per kWh, a 12kWh battery costs about £7,560 installed. A Tesla Powerwall 3 holds 13.5 kWh and is published from £7,795 at E.ON Next, so 12 kWh is not a cheaper way to buy a bigger battery. In this site's model it saves about £506 a year for a London home using 3,000 kWh with 4 kWp of solar on Octopus Flux, against £414 for 10 kWh and £569 for 13.5 kWh, a payback of about 15 years at the median price.
Where 12kWh sits in the size range
Most home batteries are built from 5 kWh blocks or a single large unit, so the catalogue has gaps. Between a 10 kWh pair and Tesla’s 13.5 kWh Powerwall 3 there is very little, and the 12 kWh label usually describes one of three things:
- Two larger modules. Two Sigenergy SigenStor BAT 6.0 modules give 11.7 kWh usable.
- A rounded nominal figure. A battery with a 12 kWh nominal rating can deliver less once the manufacturer’s reserve is taken out, so the figure to compare is usable capacity.
- A 13.5 kWh unit under another description. A Powerwall 3 is 13.5 kWh usable, and some quotes round it down.
If you are choosing between sizes rather than shopping a specific product, the whole range is laid out in solar battery sizes compared, from 3 kWh to 20 kWh and above.
| Configuration | Usable capacity | What to know |
|---|---|---|
| Two Sigenergy SigenStor BAT 6.0 | 11.7 kWh | Datasheet usable figure 5.84 kWh per module; one controller, modules added in steps |
| Fox ESS EVO 10, or two Enphase IQ Battery 5P | about 10 kWh | The common 10 kWh sizes; Octopus lists one from £5,336 |
| Tesla Powerwall 3 | 13.5 kWh | One unit with a built-in solar inverter; from £7,795 at E.ON Next |
From the manufacturers’ datasheets and the suppliers’ published prices. Capacity figures are usable, not nominal.
What a 12kWh battery costs
DESNZ publishes the median installed cost of every MCS-certified battery fitted in an existing home. For batteries of 11 kWh or more, the median between April 2025 and March 2026 was £630 per kWh, up from £590 the year before. At 12 kWh that is about £7,560; at the mean of £670 per kWh, about £8,040. Batteries of 6 to 11 kWh had a median of £890 per kWh, so the price per kWh drops sharply once a battery crosses 11 kWh, and a 12 kWh battery sits just over the line.
That makes the comparison with a Powerwall 3 awkward for the smaller battery:
| Published price | Capacity | Per usable kWh |
|---|---|---|
| Octopus, Fox ESS EVO 10 or two Enphase 5P | 10.24 kWh, from £5,336 | about £521 |
| Tesla Powerwall 3, E.ON Next | 13.5 kWh, from £7,795 | about £577 |
| Tesla Powerwall 3, EDF | 13.5 kWh, from £7,995 | about £592 |
| Tesla Powerwall 3, Octopus | 13.5 kWh, £7,999 | about £593 |
| DESNZ median, 11 kWh or more | 12 kWh | £630 |
A 12 kWh quote at the national median is within about £235 to £440 of a published 13.5 kWh Powerwall 3 price, for 1.5 kWh less battery. Where a Powerwall 3 is available at the large suppliers’ published prices, the 12 kWh quote has to be well below the median to be the better buy. The median is an all-installer figure that includes small jobs and complex ones, so it is a guide to what is typical, not a price any one installer should match.
What a 12kWh battery saves
This site’s model compares the home with no battery on a flat tariff against the same home with a battery on each tariff, at Octopus’s published rates for 26 September 2026. Annual savings by battery size:
| Home and tariff | 10 kWh | 12 kWh | 13.5 kWh |
|---|---|---|---|
| London, 3,000 kWh, 4 kWp solar, Octopus Flux | £414 | £506 | £569 |
| London, 3,000 kWh, 6 kWp solar, Octopus Flux | £401 | £501 | £575 |
| London, 3,000 kWh, 4 kWp solar, flat tariff | £174 | £174 | £174 |
| London, 3,000 kWh, no solar, Agile Octopus | £335 | £342 | £342 |
| London, 3,800 kWh, no solar, Agile Octopus | £379 | £417 | £431 |
| London, 6,000 kWh, no solar, Agile Octopus | £404 | £471 | £522 |
| London, 6,000 kWh, 6 kWp solar, Octopus Flux | £369 | £456 | £524 |
| South Scotland, 3,000 kWh, 4 kWp solar, Octopus Flux | £379 | £459 | £511 |
Three patterns matter. On a flat tariff nothing is gained past about 7.5 kWh, because the battery can only store surplus solar and the evening load is smaller than that. A 3,000 kWh home without solar on Agile stops gaining at 12 kWh, so the last 1.5 kWh of a Powerwall adds nothing there. And a solar home on Flux, or any high-use home, keeps gaining: the 6,000 kWh homes above save £456 to £524 at 12 to 13.5 kWh.
At the £7,560 median, 12 kWh pays back in about 15 years for the London Flux home with 4 kWp of solar, about 16 years for the 6,000 kWh home on Agile without solar, and about 22 years for the 3,000 kWh home on Agile without solar. A Powerwall 3 at E.ON Next’s £7,795 pays back in about 14 years on the same Flux home, because the extra £63 a year it saves over 12 kWh outweighs the extra price. These are simple payback figures before allowing for the battery’s capacity fading, and the model does not predict future tariffs.
These figures use September rates, which included 5% VAT on import; household electricity carries no VAT from 1 October 2026 to 31 March 2027, so import-side savings in that period are up to about 5% lower. The calculator runs the same model on your own usage, region and tariff.
Filling it: inverter power and the cheap-rate window
A battery charges no faster than its inverter and cells allow. On a single-phase home the total inverter power that can be connected under the G98 route is about 3.68 kW, so for a 12 kWh battery:
- Octopus Flux has three cheap hours, 2am to 5am, which put in about 11 kWh at 3.68 kW. A 12 kWh battery finishes the window about 1 kWh short of full unless solar tops it up the next day.
- Intelligent Octopus Go, for homes with a compatible electric car or charger, has six cheap hours from 11:30pm to 5:30am, plenty to fill 12 kWh.
- A 5 kW inverter would fill it in under three hours, but anything above 3.68 kW needs a G99 application before connection and can bring an export limit.
The same 3.68 kW also caps what the battery can sell back during Flux’s three-hour evening peak, about 11 kWh a day, which is why 12 kWh is close to the largest size that can empty completely in that window. Ask each installer which network route applies and what the inverter can charge and export at.
Where a 12kWh battery can go
PAS 63100, the fire-safety specification for batteries in homes, limits a single battery enclosure to 20 kWh, so 12 kWh fits in one enclosure or as a stack. Total storage is capped at 40 kWh inside the home and 80 kWh outdoors or in a garage or outbuilding. The location rules matter more than the capacity: no bedrooms, escape routes, lofts or roof voids, and an interlinked alarm where the battery is somewhere rarely visited. PAS 63100 explained has the detail.
12kWh, 10kWh or 13.5kWh?
Price all three. If you have no solar and use under about 4,000 kWh a year, a 10 kWh battery usually captures almost all of the saving, and the model shows 12 kWh adding £7 a year on Agile for a 3,000 kWh home. If you have solar on Flux, or use more than about 5,000 kWh, compare the 13.5 kWh Powerwall 3 size against 12 kWh on usable capacity and installed price, because the larger unit often costs a few hundred pounds more for 1.5 kWh more. Ask installers for written quotes at two or three sizes, with the usable capacity, inverter power and network route stated, and divide each price by the usable kWh before comparing.
Questions people ask
How much does a 12kWh battery cost in the UK?
The official median for MCS-certified batteries of 11 kWh or more fitted between April 2025 and March 2026 was £630 per kWh across 12,176 MCS-certified installations, about £7,560 for 12 kWh. The mean was £670 per kWh, about £8,040. No large supplier publishes a 12 kWh price. The nearest published single unit is Tesla's 13.5 kWh Powerwall 3, installed from £7,795 at E.ON Next, £7,995 at EDF and £7,999 at Octopus.
Which batteries come in 12kWh?
Very few are sold at exactly 12 kWh. Two Sigenergy SigenStor BAT 6.0 modules give 11.7 kWh usable, from a datasheet figure of 5.84 kWh each. The common single units either side are the Fox ESS EVO 10 and Enphase pairs at about 10 kWh, and Tesla's Powerwall 3 at 13.5 kWh. If a quote says 12 kWh, ask whether that is nominal or usable capacity and which module it is built from.
Is a 12kWh battery better than 10kWh?
It saves more only where there is energy to move. In this site's model a London home using 3,000 kWh a year with 4 kWp of solar saves £414 with 10 kWh and £506 with 12 kWh on Octopus Flux, an extra £92 a year. With no solar on Agile Octopus the same home saves £335 and £342, an extra £7 a year. The extra 2 kWh earns on Flux or in a high-use home, not on a flat tariff.
Can a 12kWh battery charge overnight?
Not in Octopus Flux's cheap window at the usual single-phase power. At 3.68 kW, the most that can usually be connected under the G98 route, the three cheap hours from 2am to 5am put in about 11 kWh, so a 12 kWh battery ends the window slightly short. Intelligent Octopus Go has a six-hour window, 11:30pm to 5:30am, but only for homes with a compatible electric car or charger.
How long would a 12kWh battery run a house?
A home using 3,000 kWh a year averages about 8.2 kWh a day, so a full 12 kWh battery would run it for about a day and a half with nothing else drawing on it. In a power cut the limit is usually the inverter's backup output, not the stored energy, and backup is a separate item on a quote; see what each level of backup keeps on.