Most homes fit one of three setups: solar panels and a battery together, a battery added to existing solar, or a battery with no solar that charges on a cheap tariff. Backup in a power cut is an extra on any of them. For size, start from the electricity you use when your panels are not generating: in this site's model a medium home gets most of its saving from 5 kWh on a flat tariff and about 10 kWh on a time-of-use tariff.
The routes compared
The modelled figures are for a 10 kWh battery in a home using 3,000 kWh a year, with 4 kWp of solar where the setup has panels, using each region's rates for 26 September 2026.
| Setup | How it connects | Tariffs it suits | Modelled saving a year | Watch for |
|---|---|---|---|---|
| Solar panels and a battery | A hybrid inverter, or a battery with a solar inverter built in | Octopus Flux, or a flat tariff | £379 to £455 on Flux; £141 to £181 flat | A G99 network application if the inverter is above 3.68 kW |
| A battery for existing solar | Usually AC-coupled, with its own inverter beside the solar one | Octopus Flux (needs the solar MCS certificate) | As above, if the array is similar | Two inverters count together towards the G98 limit |
| A battery with no solar | AC-coupled, charging from the grid | Agile Octopus; an overnight EV tariff if you have an electric car | £222 to £335 on Agile; £452 to £527 on Octopus Go | Flux is not available without solar panels |
| Backup in power cuts | Any of the above plus a backup socket, backup circuits or a whole-home gateway | Any | No extra saving | A grid-connected battery switches off in a cut unless backup is installed |
The common sizes
Capacity is the biggest single driver of price, but a battery earns only on the energy it moves each day. The official median prices below are for MCS-certified installations in 2025/26; the savings are for London with 4 kWp of solar on Octopus Flux, where extra capacity keeps earning by exporting in the evening peak.
| Size | Usually suits | At the official median | Published price | Modelled saving (Flux, solar) |
|---|---|---|---|---|
| 5kWh | Low or medium use on a flat tariff with solar | £6,500 | from £3,947 (Octopus, battery only) | £174 a year |
| 10kWh | A medium home on a time-of-use tariff, with or without solar | £8,900 | from £5,336 (Octopus, battery only) | £414 a year |
| 12kWh | Solar on Octopus Flux, between the 10 and 13.5 kWh steps | £7,560 | no standard package price | £506 a year |
| 13.5kWh | High evening use, a heat pump or whole-home backup | £8,505 | from £7,999 (Octopus, Tesla Powerwall 3) | £569 a year |
| 16kWh | High use or a large array on Flux; mostly retail packs | £10,080 | no standard package price | £664 a year |
| 20kWh | Very high use, or trading stored power on Octopus Flux | £12,600 | no standard package price | £731 a year |
Without solar the curve flattens sooner. On Agile Octopus in London the model gives £202 a year for 5 kWh, £335 for 10 kWh and £342 for 13.5 kWh: once the battery covers a whole day's use, more capacity adds almost nothing. The sizing guide works through usage levels from 1,800 to 6,000 kWh a year.
Every system and size page
Solar panels + battery
A new solar system with a battery: shared hybrid inverter, combined prices and the tariff that makes it pay.
Read the guide →Battery for existing solar
AC-coupled retrofit, costs, modelled paybacks by array and tariff, the G98/G99 question and Feed-in Tariff panels.
Read the guide →Battery without solar
Charging a battery on cheap rates without panels: which tariffs allow it, a 12-month Agile back-test by region and honest payback.
Read the guide →Home battery backup
EPS sockets, backup circuits and whole-home gateways, what each keeps on, what backup adds to a quote and how long power cuts last.
Read the guide →5kWh batteries
The smallest common size: seven products compared on usable kWh, today's prices, why it costs most per kWh, and what it saves.
Read the guide →10kWh batteries
The common family size: five products compared on usable kWh and warranty, price per kWh, how long it runs a house and the payback.
Read the guide →12kWh batteries
Which products land on 12 kWh, what it costs at the national median, and why 13.5 kWh often costs about the same.
Read the guide →13.5kWh batteries
The Powerwall-sized battery: four installed prices, who gains over 10 kWh, run-times and the solar needed to fill it.
Read the guide →15kWh batteries
Three modules or one large unit: the products that reach 15 kWh, the median price, and the tariffs that can fill it.
Read the guide →16kWh batteries
A 16kWh pack is not a 16kWh system: what each costs, and why 13.5 kWh usually pays back sooner.
Read the guide →20kWh batteries
Large stacked systems: who can use 20 kWh, the PAS 63100 per-enclosure limit, network rules and diminishing returns.
Read the guide →Before you ask for prices
Decide the setup and a size range first, then ask each installer to quote the same thing so the prices are comparable. Our line-by-line checklist shows what each quote should include, and comparing two or three battery storage quotes for the same capacity shows what a fair price looks like. In London, the regional model puts a 10 kWh battery with solar on Flux at £414 a year; the regional pages give the figure for every part of Great Britain.
Questions about systems and sizes
Which kind of home battery system do I need?
If you are fitting solar panels too, a hybrid inverter or an all-in-one battery with a solar inverter built in is usually simplest. If you already have panels, an AC-coupled battery can sit alongside your existing inverter. With no panels, a battery on its own earns by charging at cheap rates. Backup is an add-on to any of these, not a system type of its own.
What is the most common home battery size?
In DESNZ's figures for MCS-certified installations in 2025/26, 38% of the batteries in the sample were under 6 kWh, 34% were 6 to 11 kWh and 28% were 11 kWh or more. The official price per kWh falls as size rises, from £1,300 to £630.
Can one battery store solar, charge cheaply overnight and provide backup?
Yes. A battery on a time-of-use tariff can store surplus solar by day and top up from the grid in the cheap hours, and with the right backup equipment it can also keep circuits running in a power cut. Each of those jobs takes a share of the same capacity, which is one reason homes that want all three choose larger batteries.
Which battery system saves the most?
In this site's model a 10 kWh battery in a home using 3,000 kWh a year saves most with an electric car on an overnight tariff (£452 to £527 a year across regions) or with solar on Octopus Flux (£379 to £455). The same battery with solar on a flat tariff saves £141 to £181. The tariff matters more than the system type.