At the official median for batteries of 11 kWh or more fitted in 2025/26, £630 per kWh, a 15kWh battery costs about £9,450 installed. In this site's model it saves about £626 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. Without solar, on Agile Octopus, the same home gains only an extra £7 a year going from 10 to 15 kWh: the extra capacity pays only where solar exports, an overnight tariff or high use can fill it.
Who a 15kWh battery suits
A battery earns only on the energy it moves each day. Fifteen kilowatt-hours is more than most homes use between sunset and morning, so this size makes sense only when something other than the evening load can fill it:
- Solar homes on an export-paying tariff. On Octopus Flux, stored energy exported between 4pm and 7pm is paid at the peak export rate, so capacity beyond the evening load still earns. In this site’s model that is where most of the gain from 10 to 15 kWh comes from.
- Heat pump or electric car homes. A home using 6,000 kWh a year needs about 9.9 kWh a day outside solar hours on average, which a 10 kWh battery cannot cover after losses.
- Homes on a long overnight window. A six-hour cheap rate fills 15 kWh at single-phase power; a three-hour one does not.
- Longer backup. If the installation includes backup hardware, 15 kWh runs essential circuits for longer in a power cut. Backup is a separate decision from savings, and it has to be quoted for.
If none of those applies, a battery of about 10 kWh usually captures nearly all of the saving for less.
A bigger battery also keeps the essentials running longer in a cut, but only if the installation includes backup hardware; see what each level of backup keeps on.
The products that reach 15kWh
There is no common 15 kWh single unit, so quotes at this size are usually built from modules. The usable figure is what matters:
| Configuration | Usable capacity | What to know |
|---|---|---|
| Three Enphase IQ Battery 5P | 15.0 kWh | AC coupled; each unit delivers 3.2 kVA, 9.6 kVA together; 15 years or 6,000 cycles at 60% |
| Sigenergy SigenStor BAT 6.0 + BAT 10.0 | 14.6 kWh | One to six modules per controller; 10 years at 60% (European terms, May 2025) |
| Three Sunsynk G5.3 | 14.4 kWh (16.0 kWh total) | IP20, so indoors only; 44 kg each |
| Three Sunsynk Elite | 13.8 kWh (15.4 kWh total) | One to six modules per stack; integrated heating |
| Three myenergi libbi 5 kWh modules | 13.8 kWh (15.4 kWh total) | With a 3.68 kW or 5 kW hybrid inverter; 54 kg per module, 162 kg for three |
| Tesla Powerwall 3 | 13.5 kWh | One unit with a built-in solar inverter; an Expansion Pack takes it to 27 kWh |
From the manufacturers’ datasheets and warranty documents. Between the top and bottom of the table there is a 1.5 kWh gap, about 10%, on systems that are all described as roughly 15 kWh. Ask every installer for the usable capacity and divide the price by it.
What a 15kWh battery costs
DESNZ publishes medians for 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, from 12,176 MCS-certified installations, up from £590 the year before. At 15 kWh that is about £9,450; at the mean of £670 per kWh, about £10,050.
The price per kWh falls as batteries get bigger, because the survey, labour, connection and paperwork cost much the same for any size: the same statistics put the median at £1,300 per kWh under 6 kWh and £890 per kWh for 6 to 11 kWh.
No large supplier publishes a 15 kWh price. The two published prices either side of it come from Tesla systems:
| Published price | Capacity | Per usable kWh |
|---|---|---|
| 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 |
| Powerwall 3 plus an Expansion Pack fitted at the same visit, Octopus | 27 kWh, £7,999 + £3,773 = £11,772 | about £436 |
A 15 kWh quote far above the median, or above the price of 13.5 kWh from a large supplier by more than the extra 1.5 kWh justifies, needs an explanation in writing.
What a 15kWh 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 | 13.5 kWh | 15 kWh | 20 kWh |
|---|---|---|---|---|
| London, 3,000 kWh, 4 kWp solar, Octopus Flux | £414 | £569 | £626 | £731 |
| London, 3,000 kWh, 6 kWp solar, Octopus Flux | £401 | £575 | £647 | £771 |
| London, 3,000 kWh, no solar, Agile Octopus | £335 | £342 | £342 | £342 |
| London, 3,000 kWh, no solar, Octopus Go | £489 | £503 | £503 | £503 |
| London, 6,000 kWh, no solar, Octopus Go | £425 | £671 | £777 | £977 |
| London, 6,000 kWh, 6 kWp solar, Octopus Flux | £369 | £524 | £594 | £804 |
| South Scotland, 3,000 kWh, 4 kWp solar, Octopus Flux | £379 | £511 | £563 | £664 |
Two patterns stand out. A 3,000 kWh home without solar stops gaining at about 10 to 13.5 kWh, because there is nothing left to shift. A high-use home on an overnight tariff, or a solar home on Flux, keeps gaining past 15 kWh. At the £9,450 median, 15 kWh pays back in about 15 years for the London Flux home with 4 kWp of solar and about 12 years for the 6,000 kWh home on Octopus Go, before allowing for the battery’s capacity fading.
These figures use September rates, which included 5% VAT; 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 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 battery allow, and on a single-phase home the inverter is often sized to the 16 A per phase limit, about 3.68 kW counting every inverter, that lets the installer notify the network operator after installation under G98 rather than apply first. At about 3.7 kW:
- Octopus Flux has three cheap hours, 2am to 5am: enough to put about 11 kWh into the battery.
- Intelligent Octopus Go, for homes with a compatible electric car or charger, has six, 11:30pm to 5:30am: enough to fill 15 kWh with time to spare.
- At 5 kW the battery fills in about three hours, but an inverter rated above 3.68 kW needs a G99 application before connection, which can bring an export limit.
The same limit caps what the battery can sell back at Flux’s evening peak: 3.68 kW of export for three hours is about 11 kWh a day, which is why the savings in the table flatten as batteries pass 20 kWh. Ask each installer which network route applies and what the inverter can charge and export at.
Where a 15kWh battery can go
PAS 63100, the fire-safety specification for batteries in homes, limits a single battery enclosure to 20 kWh, so a 15 kWh system can sit 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 total at this size: no bedrooms, escape routes, lofts or roof voids, and an interlinked alarm where the battery is somewhere rarely visited. Modular stacks are heavy as well as large, and three libbi modules weigh 162 kg before the inverter, so a garage floor or an outside wall is the usual home. PAS 63100 explained has the detail.
15kWh, 13.5kWh or 20kWh?
If your yearly use is under about 4,000 kWh and you have no solar, price the 13.5 kWh size or a 10 kWh battery first; on the figures above the last 5 kWh earns almost nothing without solar or a long overnight window. If you have a heat pump, an electric car or a large solar array on Flux, compare 15 kWh with a 20 kWh system, because the saving keeps rising and the price per kWh keeps falling. Just above it, a 16kWh battery mostly appears as retail packs sold without installation. Our analysis of whether a bigger battery saves more shows how each extra 5 kWh pays. Then ask installers for battery storage quotes at both sizes, with the usable capacity, inverter power and network route written on each.
Questions people ask
How much does a 15kWh 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 £9,450 for 15 kWh. Large suppliers do not publish a 15 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 15kWh?
Few single units do. Three Enphase IQ Battery 5P units give 15.0 kWh usable, a Sigenergy SigenStor BAT 6.0 plus a BAT 10.0 give 14.6 kWh, three Sunsynk G5.3 modules about 14.4 kWh, and three Sunsynk Elite or three myenergi libbi modules 13.8 kWh. Tesla's Powerwall 3 is 13.5 kWh on its own and 27 kWh with an Expansion Pack.
Can a 15kWh battery charge fully overnight?
Only with enough charging power or a long enough cheap-rate window. At 3.68 kW, the largest inverter a single-phase home can usually connect without applying to the network first, a battery takes in about 11 kWh during Octopus Flux's three cheap hours from 2am to 5am. Intelligent Octopus Go's six-hour window, 11:30pm to 5:30am, for homes with a compatible electric car or charger, is long enough to fill 15 kWh at that power.
Is 15kWh too big for my house?
For a home using around 3,000 kWh a year with no solar on a single overnight tariff, usually yes: in this site's model a London home on Agile Octopus saves £335 a year with 10 kWh and £342 with 15 kWh. It earns its size with solar on Octopus Flux, which pays a peak rate for stored energy exported between 4pm and 7pm, or with high use from a heat pump or electric car.
How long would a 15kWh battery run a house?
A home using 3,000 kWh a year averages about 8.2 kWh a day, so a full 15 kWh battery would run it for close to two days with nothing else. Day to day it is cycled instead: the same home uses about 4.9 kWh outside solar hours on an average day in this site's model and about 6 kWh in December, which is why the rest of the capacity needs a tariff or solar exports to earn anything.