The common way to size a home battery is to divide your yearly use by 365 and buy something close to a day’s worth. That over-sizes almost every home. A battery earns money only on the kWh it moves out of the expensive hours, so the right size is set by how much you use in those hours and how cheaply you can refill.
The figures below come from Fortyeight’s model, which replays a year of half-hourly Octopus prices against a household’s use. These are modelled homes. To see your own figures, run it on your own meter data. Not financial advice — a model, so check real quotes.
Size to the expensive hours, not the year
A home using 2,500 kWh a year (Ofgem’s medium typical value since 1 July 2026) averages about 6.9 kWh a day. But not all of that is worth shifting. In the model’s load shape:
| Modelled household | Daily average | Used 16:00–19:00 | Used 16:00–23:00 |
|---|---|---|---|
| Low user, 1,600 kWh | 4.4 kWh | 0.8 kWh | 1.8 kWh |
| Typical, 2,500 kWh | 6.9 kWh | 1.3 kWh | 2.9 kWh |
| High user, 3,800 kWh | 10.4 kWh | 1.9 kWh | 4.3 kWh |
| Heat pump, 2,700 + 3,500 kWh | 17.0 kWh | 2.8 kWh | 6.1 kWh |
These are yearly averages. Winter evenings are heavier, and a heat pump home’s winter day is far heavier than its summer one. Even so, the typical home’s whole evening fits in a 5 kWh battery with room to spare on most days. Every extra kWh of capacity beyond that only earns money on the few days it is needed, or from the smaller gap between night and daytime prices.
What the model says: 5 vs 10 vs 13.5 kWh
The table shows what each size adds per year over the best tariff the same home could join without a battery, on the best tariff the household is eligible for. All homes are in London. Inverters are 3 kW for 5 kWh and 5 kW for the larger sizes.
| Modelled household | Tariff | 5 kWh | 10 kWh | 13.5 kWh |
|---|---|---|---|---|
| Low user, 1,600 kWh | Agile | £131 | £137 | £137 |
| Typical, 2,500 kWh | Agile | £187 | £210 | £211 |
| Home worker, 3,500 kWh | Agile | £228 | £275 | £283 |
| High evening user, 3,800 kWh | Agile | £246 | £303 | £313 |
| EV, 2,700 + 2,000 kWh charging | Intelligent Octopus Go | £334 | £541 | £541 |
| Heat pump, 2,700 + 3,500 kWh | Agile | £298 | £384 | £414 |
| Heat pump, same home | Cosy | £35 | £209 | £215 |
| 4 kWp MCS solar, 3,500 kWh | Flux | £220 | £448 | £594 |
For the typical home, doubling from 5 to 10 kWh adds £23 a year, and going on to 13.5 kWh adds £1. At the prices we use for illustration, those steps cost about £3,000 and £2,100. Only the Energy Saving Trust’s £4,600 for a 5 kWh system is a published figure. Our £7,600 for 10 kWh and £9,700 for 13.5 kWh are assumptions, because no verified figures exist for those sizes.
A larger battery clearly pays its way in three cases:
- EV on Intelligent Octopus Go. Six cheap hours at 6.93p against 37.93p for the rest of the day in London. A 10 kWh battery can carry the whole home through the day, but 13.5 kWh added nothing more.
- Heat pump. Heavier evening use, especially in winter, keeps a 10 kWh battery working. On Agile, even 13.5 kWh added £30 a year over 10 kWh.
- MCS solar on Flux. Flux pays 30.31p for exports between 16:00 and 19:00 in London, including exports from the battery, so spare capacity can be sold rather than sitting idle. This is the only case in the model where 13.5 kWh added a lot (£146 a year over 10 kWh).
Usable vs nominal capacity
Battery specifications often give two figures: nominal (or total) capacity, and usable capacity. The battery management system holds some charge back to protect the cells, so you never get the nominal figure. Fortyeight assumes 95% of nominal is usable, so a “10 kWh” battery gives 9.5 kWh. A charge-and-discharge round trip then loses about 10% (we assume 90% round-trip efficiency). For comparison, Tesla’s UK Powerwall 3 datasheet quotes 13.5 kWh and 89% efficiency from solar to battery to home.
When comparing quotes, put the usable kWh side by side, and check whether the efficiency figure is round-trip or one-way.
Inverter power: kW is not kWh
Capacity (kWh) is how much the battery stores. Power (kW) is how fast it can charge or discharge. A 10 kWh battery with a 3 kW inverter takes over three hours to fill and can supply at most 3 kW at once. If the kettle, oven and heat pump run together at 6pm, the rest comes from the grid.
In the model, power mattered less than you might expect:
| Household and battery | 3 kW inverter | 5 kW inverter |
|---|---|---|
| Typical, 10 kWh, Agile | £207 | £210 |
| High user, 10 kWh, Agile | £298 | £303 |
| Heat pump, 13.5 kWh, Agile | £405 | £414 |
The power rating matters more for paperwork than for savings. A battery inverter counts as generation. Up to 16 A per phase (about 3.68 kW single-phase) it is connected under G98, and the installer notifies the network operator within 28 days. Above that, G99 applies, and the network operator must accept the application before installation. If you have or plan solar, the inverter ratings are added together, which often pushes a system into G99 or needs an export limiter.
Cycles and warranty
A bigger battery cycles less. In the model, a typical London home on Agile ran its battery for about 259 full cycles a year at 5 kWh, 153 at 10 kWh and 115 at 13.5 kWh. Fewer cycles mean less wear per kWh of capacity, but you are paying for capacity that mostly sits unused.
Some tariffs push the other way. On Cosy, with three cheap windows a day, the heat-pump home’s 5 kWh battery did about 527 full cycles a year. So check the warranty’s terms on throughput as well as years. Tesla’s UK Powerwall warranty is 10 years with 80% capacity retained. It allows unlimited cycles for solar self-consumption, time-of-use shifting and backup, caps other uses at 37.8 MWh, and requires an internet connection throughout. The Energy Saving Trust gives a typical battery life of 10 to 12 years.
Check the price for the size you are quoted
Whatever size you are quoted, the test is the same: does the extra saving justify the extra price? Enter a quote and the saving for that size:
The default is the typical London home with 10 kWh: £210 a year against an illustrative £7,600 never pays back within 25 years. To pay back in 10 years it would need to cost no more than about £1,900.
A sizing checklist
- Download a year of half-hourly data. The smart meter data guide shows how.
- Add up what you use in your tariff’s expensive hours on an ordinary winter weekday. That is your upper size.
- Choose the tariff first. The tariff comparison shows which ones you can join.
- Ask for quotes on two sizes, and compare the extra saving with the extra price.
- Check usable capacity, inverter kW, the G98 or G99 route and the warranty’s throughput terms. The installer question sheet lists them.
If your home has a heat pump, the heat pump guide covers winter sizing. If you are still deciding whether to buy at all, start with are home batteries worth it.
Model assumptions: Octopus prices fetched 25 October 2026, with Agile import prices from 25 October 2025 to 24 October 2026, on a 5% VAT basis (bills from 1 October 2026 to 31 March 2027 carry no VAT, so are about 4.8% lower); the battery plans 36 hours ahead knowing exactly how much the home will use, so results are an upper-middle estimate; a cycle only runs when it saves at least 2p/kWh. Fortyeight is not affiliated with any manufacturer or supplier.