For a London home using 3,000 kWh a year with 4 kWp of solar on Octopus Flux, this site's model gives annual savings of £174 for 5 kWh, £414 for 10 kWh, £569 for 13.5 kWh and £731 for 20 kWh. Without solar on Agile Octopus the saving stops rising at about 10 to 12 kWh, at £335 to £342. The best-value published prices are at 10 to 13.5 kWh: Octopus lists 10.24 kWh from £5,336 and E.ON Next lists a 13.5 kWh Powerwall 3 from £7,795, both about 13 to 14 years' simple payback on the Flux home.
Every size in one table
Capacity below is usable energy, the figure that matters, built from the manufacturers’ datasheets. Savings are this site’s model for a London home using 3,000 kWh a year, at Octopus’s published rates for 26 September 2026, against the same home with no battery on a flat tariff. “Flux” is Octopus Flux with 4 kWp of solar; “Agile” is Agile Octopus with no solar.
| Size | How it is usually built | Saving a year, Flux | Saving a year, Agile | Read next |
|---|---|---|---|---|
| 3 kWh | No common single product at this size; modelled | £89 | £134 | Cost by size |
| 5 kWh | One Fox ESS EVO 5 or one Enphase IQ Battery 5P | £174 | £202 | 5kWh batteries |
| 6 kWh | Sigenergy SigenStor BAT 6.0 (5.84 kWh usable) | £220 | £236 | Cost by size |
| 7.5 kWh | No common single product at this size; modelled | £294 | £287 | Cost by size |
| 10 kWh | One Fox ESS EVO 10, or two Enphase IQ Battery 5P | £414 | £335 | 10kWh batteries |
| 12 kWh | Two Sigenergy BAT 6.0 (11.7 kWh usable) | £506 | £342 | 12kWh batteries |
| 13.5 kWh | Tesla Powerwall 3, one unit | £569 | £342 | 13.5kWh batteries |
| 15 kWh | Three Enphase IQ Battery 5P | £626 | £342 | 15kWh batteries |
| 16 kWh | Few installed products; retail packs sold as hardware | £664 | £342 | 16kWh batteries |
| 20 kWh | Four Enphase IQ Battery 5P, or stacked modules | £731 | £342 | 20kWh batteries |
| 27 kWh | Powerwall 3 plus an Expansion Pack | see the 13.5kWh page | see the 13.5kWh page | Powerwall 3 |
The shape is the point. On Flux the saving keeps rising with size because stored energy is exported at the peak rate. Without solar on Agile it stops at about 10 to 12 kWh because a 3,000 kWh home has nothing more to shift. A battery bigger than the home can use is paying for capacity that earns nothing.
What each size costs
Published prices exist only where a large supplier sells the product (a brand such as Duracell publishes hardware prices through retailers, not installed prices):
| Published price | Capacity | Per usable kWh | Simple payback on the Flux home |
|---|---|---|---|
| Octopus, Fox ESS EVO 5 or Enphase 5P, from £3,947 | 5 kWh | about £790 | about 23 years |
| Octopus, Fox ESS EVO 10 or two Enphase 5P, from £5,336 | 10.24 kWh | about £521 | about 13 years |
| Tesla Powerwall 3, E.ON Next, from £7,795 | 13.5 kWh | about £577 | about 14 years |
| Tesla Powerwall 3, EDF, from £7,995 | 13.5 kWh | about £592 | about 14 years |
| Tesla Powerwall 3, Octopus, £7,999 | 13.5 kWh | about £593 | about 14 years |
Payback here is the price divided by the model’s annual saving, 5 kWh against £174, 10 kWh against £414 and 13.5 kWh against £569, before allowing for the battery’s capacity fading. It shows the pattern, not a promise: it assumes the home and tariff of the Flux row, and a home without solar or without a time-of-use tariff pays back much more slowly.
For sizes nobody publishes a price for, DESNZ’s median installed cost for MCS-certified batteries in existing homes fitted in 2025/26 is the guide: £1,300 per kWh under 6 kWh, £890 per kWh for 6 to 11 kWh and £630 per kWh for 11 kWh or more. At those medians 12 kWh is about £7,560, 16 kWh about £10,080 and 20 kWh about £12,600. These are medians for each band, not a price list, so a median for 10 kWh is above one for 12 kWh even though real quotes rise smoothly with size, and the median covers every kind of installation, not only the cheapest. The cost guide goes through the bands in detail.
Which size fits which home
The what-size guide sizes by household; the short version, from the model:
- Flat tariff, with solar. A battery can only store surplus solar for the evening, and in this site’s model a 3,000 kWh home gains nothing beyond 7.5 kWh. Price 5 to 7.5 kWh and treat anything larger as unproven.
- Time-of-use tariff, with solar. Octopus Flux pays a peak export rate for stored energy between 4pm and 7pm, so the saving keeps rising to 13.5 kWh and past it. This is where 10 to 16 kWh earns.
- No solar. The saving is the gap between cheap and expensive electricity, and a 3,000 kWh home stops gaining at about 10 to 12 kWh on Agile. A heat pump or electric car home, using 6,000 kWh or more, keeps gaining to 20 kWh.
- Backup matters most. Capacity is the smaller question; the backup output and the hardware are the bigger ones. What each level of backup keeps on sets them out.
The what-size guide turns this into a table by household use and tariff, and the calculator runs the model on your own figures.
Limits that apply at every size
A battery charges no faster than its inverter allows. On a single-phase home the total inverter power connectable under the G98 route is about 3.68 kW, so a battery fills at roughly that rate: 11 kWh in Octopus Flux’s three cheap hours from 2am to 5am. A larger inverter needs a G99 application before connection. The same 3.68 kW caps how much the battery can sell back in the Flux evening peak, about 11 kWh a day, which is why the savings table flattens beyond 20 kWh.
PAS 63100, the fire-safety specification for batteries in homes, limits a single battery enclosure to 20 kWh and total storage to 40 kWh inside the home and 80 kWh outdoors or in a garage or outbuilding, with rules that keep batteries out of bedrooms, escape routes and lofts. PAS 63100 explained covers the detail.
Whatever size you choose, ask each installer for the usable capacity, the inverter’s charge and export power, the network route, and the price per usable kWh, in writing, and compare at two sizes. The quote form is the place to start a request.
Questions people ask
What are the standard sizes of solar batteries?
There is no single standard. Most home batteries are built from modules of about 5 kWh, so installed sizes cluster at about 5, 10 and 15 kWh, with Tesla's Powerwall 3 as a single 13.5 kWh unit that can be doubled to 27 kWh with an Expansion Pack. Modular ranges such as Sigenergy's 5.84 kWh and 8.76 kWh modules add intermediate steps, and the figure to compare is always usable capacity.
What size solar battery do I need for a 4kWp system?
In this site's model a 4 kWp array in London makes about 16 kWh a day in June and under 5 kWh in December, and a home using 3,000 kWh a year needs about 4.9 kWh a day outside solar hours on average and about 6 kWh in December. That puts a flat-tariff home at about 5 to 7.5 kWh, and a home on Octopus Flux, which pays a peak export rate, at 10 to 13.5 kWh. The what-size guide works it through by household.
Does a bigger solar battery always cost more?
In total, yes, but not per kWh. The DESNZ median for MCS-certified batteries fitted in 2025/26 was £1,300 per kWh under 6 kWh, £890 for 6 to 11 kWh and £630 for 11 kWh or more, because the survey, labour, connection and paperwork cost much the same at any size. These are band medians, so a median for 10 kWh is above a median for 12 kWh even though real quotes rise smoothly with size.
Can I add more battery later?
Often, but check before you buy. Octopus lists a Tesla Expansion Pack at £3,773 when fitted with the Powerwall 3 and £5,909 when added to an existing one later. Modular batteries such as Enphase's 5P and Sigenergy's modules can usually be extended, up to the inverter's limit and the PAS 63100 limit of 20 kWh per enclosure and 40 kWh inside the home.
Is a bigger battery better for a power cut?
Only if the installation includes backup hardware, and then the limit is usually the inverter's backup output, not the stored energy. A home using 3,000 kWh a year averages about 8.2 kWh a day, so a full 13.5 kWh battery holds about a day and a half of average use, but what can run at once is set by the backup output in kW, which is a separate line on a quote.