At the official median of £630 per kWh for batteries of 11 kWh or more (DESNZ, 2025/26), a 20 kWh battery costs about £12,600 installed. Most 20 kWh systems are stacked modules or two units, for example a Tesla Powerwall 3 plus an Expansion unit (27 kWh). In this site's model, going from 13.5 kWh to 20 kWh adds about £162 a year of saving for a London home with solar on Octopus Flux, and nothing at all for a battery-only home on Agile.
Who needs 20kWh
Twenty kilowatt-hours only earns its keep where a home moves that much energy most days. The households that can:
- Large homes using 6,000 kWh a year or more, with most use after dark.
- Electric car owners on an overnight EV tariff, where the battery carries the house through the day on cheap night-time energy.
- Heat pump homes, where winter heating adds several kWh a day of shiftable demand.
- Solar homes on Octopus Flux that export stored energy in the 4pm to 7pm peak. The export is limited by the connection: on a single-phase supply under G98, 3.68 kW for three hours is about 11 kWh.
- Three-phase properties, which can take more inverter power and more export.
How homes get to 20kWh
Most 20 kWh systems are built from several units or modules rather than one box:
- Tesla Powerwall 3 plus Expansion units. Each Expansion adds 13.5 kWh; up to four Powerwalls and three Expansions can be linked. Octopus prices an Expansion Pack at £3,773 when fitted with a new Powerwall 3, or £5,909 if added later.
- Modular LFP systems from Fox ESS, Sunsynk and others, where modules are added to one inverter up to its limit.
- Two separate batteries, each with its own inverter.
The rules that shape a large system
PAS 63100 caps a single battery enclosure at 20 kWh in a home, and the total at 40 kWh where batteries are inside, or 80 kWh outdoors or in a garage or outbuilding. The location rules (outdoors preferred; no bedrooms, escape routes, lofts or cellars without outside access) matter more with more units to place.
The network connection. The total inverter capacity that can export decides whether the installer notifies the network operator after installation under G98 (up to 16 A per phase, 3.68 kW single phase) or applies before installation under G99. Most large systems need G99, and the operator may approve them with an export limit. That limit caps how much stored energy you can sell back, which is part of why savings flatten at large sizes.
What it costs and what it saves
| Figure | |
|---|---|
| Official median, batteries of 11 kWh or more, 2025/26 | £630 per kWh, about £12,600 for 20 kWh |
| Octopus: Powerwall 3 plus one Expansion Pack fitted together (27 kWh) | From £7,999 plus £3,773 |
| Modelled saving, London, 4 kWp solar, Flux, 3,000 kWh a year: 13.5 kWh | £569 a year |
| The same home with 20 kWh | £731 a year |
| The same home with 25 kWh | £731 a year |
| Battery-only home on Agile, 13.5 kWh and 20 kWh | £342 a year either way |
The last rows show the diminishing return. In the model the saving stops growing past about 20 kWh for a solar home on Flux, because the export limit and the daily cycle are both used up, and past 10 to 13.5 kWh for a battery-only home, because there is nothing left in the house to shift. Put your own usage into the calculator: a household using 6,000 kWh or more, or charging a car nightly, will see a very different curve.
Before committing to 20kWh
Ask for quotes at two sizes, for example 13.5 kWh and 20 kWh, with the extra cost itemised. The size just below, 16 kWh, is mostly sold as hardware rather than a fitted system, and all the sizes in one table show where the saving stops rising. A middle option is 15 kWh, which a six-hour overnight window can fill even at single-phase power. If the extra capacity costs a few thousand pounds and adds £150 to £200 a year of modelled saving, it will not pay back inside a 10-year warranty unless your usage is well above average. Compare battery storage quotes at both sizes before you decide.
Questions people ask
How much does a 20kWh home battery cost?
At the official median for MCS-certified batteries of 11 kWh or more in 2025/26, £630 per kWh, a 20 kWh system is about £12,600 installed. Published examples include Octopus's Tesla Powerwall 3 from £7,999 plus a 13.5 kWh Expansion Pack for £3,773 fitted at the same time, about £11,800 for 27 kWh.
How long will a 20kWh battery last?
An average home using 3,000 kWh a year draws about 0.35 kW, so 20 kWh would cover more than two days of average use. In practice it is sized for homes using far more than that, where it covers a long evening, an EV top-up or a heat pump's peak-hour demand.
How many solar panels do I need for a 20kWh battery?
To fill 20 kWh from solar after the house has been supplied, you would need a large array, around 8 kWp or more, and even then only on good summer days. In winter a 20 kWh battery is filled mainly from cheap-rate grid electricity.
Can a 20kWh battery go in one unit?
Not under PAS 63100, which limits a single battery enclosure to 20 kWh of stored energy in a home. Larger systems are built from several enclosures, and the total is capped at 40 kWh if the batteries are indoors, or 80 kWh outdoors or in a garage or outbuilding.