What Size Solar Battery Do I Need?Sizing from your own use, not your roof
What size solar battery do I need? Match it to the energy you use when your panels are not working — for a medium-use home that is about 4.6 kWh, not the 10 kWh the Energy Saving Trust says a typical home system might be. Here is how to size it, and what each size adds, from a transparent model.
- A battery only saves money on energy you use: match it to your evening and night load, not to your roof
- Ofgem’s low, medium and high homes match batteries of about 3, 4.6 and 7 kWh (assuming 60% of use falls outside solar hours)
- In our model a 4 kW array and a medium home gain nothing from a battery above about 4.5 kWh; a 5 kWh battery adds £133 a year
- Past that size only a cheap-rate tariff can use more: on Octopus Go a 10 kWh battery beats a 5 kWh one only in a high-use home
- The Energy Saving Trust says a typical home system might be 10 kWh and that there is no exact formula
The rule: match the battery to what you use outside solar hours
A battery can only save money on energy you actually use. The useful question is not “how big can I go?” but “how much do I use when the panels are not working?” — the evening and night load the battery has to cover. The Energy Saving Trust says a typical home system might be 10 kWh and that there is no exact formula: your installer should advise from your needs and circumstances. Ofgem’s typical household figures give a starting point. Sources: Energy Saving Trust, "Battery storage" (page last updated 6 October 2026); Ofgem typical domestic consumption values, from 1 July 2026.
| Household (Ofgem typical use) | Daily use | Used outside solar hours (assumed 60%) | Usable battery that matches it |
|---|---|---|---|
| Low (1,600 kWh a year) | 4.4 kWh | 2.6 kWh | 2.9 kWh |
| Medium (2,500 kWh a year) | 6.8 kWh | 4.1 kWh | 4.6 kWh |
| High (3,800 kWh a year) | 10.4 kWh | 6.2 kWh | 6.9 kWh |
Daily use = annual kWh ÷ 365. The 60% outside solar hours is an assumption: if you work from home it is lower, and an electric car or heat pump raises it. Matching capacity = that energy ÷ 0.9 round-trip efficiency (also an assumption).
For a medium-use home that comes to about 4.6 kWh — a 5 kWh battery, not a 10 kWh one. The Energy Saving Trust’s 10 kWh is a typical home system, and plenty of homes use more, or have an electric car or a heat pump to feed.
To work out yours in three steps: take your annual electricity use from a bill and divide it by 365; multiply by the share you use outside solar hours (60% if you are out in the day, less if you work from home); then divide by 0.9 and round up to the nearest battery size. If your supplier’s app shows half-hourly use, your real evening and night load beats the 60% assumption.
What each size adds in a solar home
Take a 4 kW south-facing array on the Hampshire coast and a medium-use household that is out for part of the day (45% of its use falls in daylight). Our monthly model — PVGIS yields, bright and dull days, 90% battery efficiency, 26.32p import and 12p export — says each size adds:
| Battery | Adds, a year | Electricity covered | Full cycles a year |
|---|---|---|---|
| No battery | — | 43% | — |
| 2.5 kWh | £85 | 70% | 290 |
| 4 kWh | £129 | 83% | 280 |
| 5 kWh | £133 | 85% | 230 |
| 7.5 kWh | £133 | 85% | 150 |
| 10 kWh | £133 | 85% | 110 |
Solar alone earns £699 a year in this home. “Covered” = solar used directly plus battery output, as a share of annual use. Cycles = energy put into the battery ÷ its usable size. Sources: EU JRC PVGIS 5.3, PVGIS-SARAH3 2005-2023, 14% system loss; Ofgem energy price cap, 1 October to 31 December 2026; Octopus Outgoing, region H (OUTGOING-VAR-24-10-26, from 1 March 2026); Ofgem typical domestic consumption values, from 1 July 2026.
Above about 4.5 kWh the line goes flat: there is no more evening load left to cover and no more surplus left to store, so the extra capacity is bought, installed and idle. At Octopus’s published prices a 10 kWh battery costs £1,389 more than a 5 kWh one (£5,336 against £3,947) and adds nothing in this home. The fall in cycles is the other side of the same coin: a battery that is too big works fewer cycles a year than it could.
| Household (4 kW array) | 2.5 kWh | 5 kWh | 7.5 kWh | 10 kWh |
|---|---|---|---|---|
| Low | £91 | £97 | £97 | £97 |
| Medium | £85 | £133 | £133 | £133 |
| High | £76 | £124 | £148 | £148 |
What each battery size adds a year in a 4 kW south-facing solar home with 45% of use in daylight. A high-use home uses more of its own solar directly, so it has less surplus left to store at the smallest size but gains most at 7.5 kWh.
The shape is the same in every row: the benefit climbs, then stops. Where it stops depends on the evening load and on how much surplus the array leaves. With a 6 kW array and a high-use home the plateau is higher — £201 a year, reached at 7.5 kWh and unchanged at 10 kWh (£201). Beyond that, solar alone has nothing more to give a battery.
What the price says
Octopus’s published battery-only prices (April 2026) are £3,947 for 5 kWh and £5,336 for 10 kWh, and £7,999 for a 13.5 kWh Tesla Powerwall 3. On a flat 26.32p tariff the £133 a year a 5 kWh battery adds in the home above repays £3,947 in about 30 years, and a 10 kWh battery, which adds the same, takes about 40. The Energy Saving Trust puts it more gently: savings can be significant but will not always be enough on their own to justify the cost of the battery. Sources: Octopus Energy, battery-only installation (April 2026 prices, read 7 October 2026); Energy Saving Trust, "Battery storage" (page last updated 6 October 2026).
If you charge it at a cheap rate instead
A battery charged overnight at a low rate is limited by what you can use, not by the surplus. At Octopus Go’s night rate (8.21p, region H, read 7 October 2026) with no solar at all:
| Household | 5 kWh | 10 kWh | 13.5 kWh |
|---|---|---|---|
| Low | £165 | £165 | £165 |
| Medium | £258 | £258 | £258 |
| High | £282 | £392 | £392 |
Value a year = delivered kWh × (26.32p − 8.21p ÷ 0.9), capped by use outside solar hours (assumed 60%). Octopus describes Go as a smart EV tariff, so check you are eligible. Sources: Octopus Energy public product API, region H (read 7 October 2026); Ofgem energy price cap, 1 October to 31 December 2026.
Same lesson, at a cheaper rate: a 10 kWh battery only beats a 5 kWh one in a high-use home, and a medium-use home gains nothing from the extra capacity. A 13.5 kWh battery is for a household that would drain it: an electric car, a heat pump, or both.
What sizes are actually on the market?
Manufacturers make batteries in modules, so you buy what they make. Fox ESS EP units are 5.18 and 10.36 kWh nominal; SigenStor modules are 5.2 and 7.8 kWh usable; GoodWe Lynx Home F towers are 6.55, 9.83, 13.1 and 16.38 kWh usable. All three scale up by adding units — the EP11 to 41.6 kWh, GoodWe to eight towers — so starting right-sized and expanding later is a real option: add matching modules from the same generation. Sources: Fox ESS Battery Warranty Policy, UK & Ireland (V4.4); Fox ESS EP11 datasheet V1.4; Sigen Battery datasheet; GoodWe Lynx Home F PLUS+ product page.
Once you know the size, the next question is which one. Our best solar battery comparison reads each brand’s warranty from the manufacturer’s own document, works out how long the throughput floor really lasts under daily cycling, and says plainly when a battery will not pay. To run your own figures through the same model, use our solar panel calculator.
Frequently Asked Questions
Is 5 kWh enough for a solar battery?
For a medium-use home, usually yes. Ofgem’s medium household uses about 6.8 kWh a day, of which roughly 4 kWh falls outside solar hours, and a 5 kWh battery covers that.
Is a 10 kWh battery too big?
For most homes, yes. The Energy Saving Trust says a typical home system might be 10 kWh, but in our model a medium-use home gains nothing above about 4.5 kWh. A 10 kWh battery suits high-use homes, electric cars and heat pumps.
How many kWh does a house use per day?
Ofgem’s typical household values from 1 July 2026 are 1,600, 2,500 and 3,800 kWh a year for low, medium and high use: about 4.4, 6.8 and 10.4 kWh a day.
Can I add more battery capacity later?
Yes. Fox ESS EP11 scales to 41.6 kWh, SigenStor adds 5.2 or 7.8 kWh modules and GoodWe Lynx Home F goes to eight towers (131 kWh). Add matching modules from the same generation.
Does the inverter limit battery size?
The inverter and battery have to be compatible, and their power ratings matter as well as the capacity, so we size the two together at the survey.
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