What Size Solar Battery Do I Need?
The most common battery question — and the answer isn’t a round number. Here’s a simple way to size it to your home, plus a quick table matching household to capacity.
The short answer
Size your battery to cover your evening and overnight electricity use — the power you draw when the panels aren’t generating and would otherwise buy from the grid. For an average UK home using 8–10kWh a day, that usually points to a 5–10kWh battery.
Roughly: smaller or lower-usage homes suit around 5kWh; medium-to-large or higher-usage homes — especially with an EV or heat pump — suit 10kWh or more. The golden rule is don’t oversize: a battery bigger than your nightly use (or bigger than your solar can fill) just sits part-empty and never pays back. Match capacity to what you’ll actually use off-solar and what your solar system can realistically fill.
What this guide covers
“What size battery do I need?” is the question nearly everyone asks once they’ve decided they want storage — and it’s the one where buying wrong costs the most, in either direction. Too small and you export solar you could have banked; too big and you’ve paid thousands for capacity you never cycle. The good news: sizing it is straightforward once you know the one number that matters. Here’s the honest method.
The simple sizing method
Forget round numbers and sales patter — battery sizing comes down to one figure: how much electricity you use in the evening and overnight, when your panels aren’t generating. That’s the demand a battery exists to cover. Here’s how to find it:
- Step 1 — Daily use: take your annual electricity use in kWh (from a recent bill) and divide by 365. The UK average is around 8–10kWh/day.
- Step 2 — Evening/overnight share: estimate how much of that you use when solar isn’t generating — typically half to two-thirds for a normal home (mornings, evenings, night).
- Step 3 — That’s your target: that evening/overnight figure is roughly the useful battery capacity. There’s little point storing more than you’ll draw down each night.
A worked example
Say you use 3,650kWh a year → that’s 10kWh a day. If roughly 60% of your use is evening/overnight, that’s 6kWh you’d ideally cover from a battery each night. So a battery around 5–6.5kWh fits well — enough to run the evening and night on stored solar, without paying for capacity you’d never empty. A bigger home using 4,500kWh with an EV might land at 9–10kWh of nightly demand, pointing to a 10kWh battery. Same method, different answer — which is exactly why it’s about your usage, not a standard size.
Quick sizing table
If you’d rather start with a ballpark, here’s a rough guide by household — but treat it as a starting point, then refine with the method above:
| Your home | Annual use (rough) | Likely battery size |
|---|---|---|
| Small home / low usage / 1–2 people | Under ~3,000kWh | ~5kWh |
| Average home / 3 bed / family | ~3,000–4,000kWh | 5–10kWh |
| Larger home / higher usage | ~4,000–6,000kWh | ~10kWh |
| Large home + EV / heat pump | 6,000kWh+ | 10kWh+ (or modular) |
You’ll notice the average home spans 5–10kWh — that’s real, and it’s where the 5kWh versus 10kWh decision is closest. If you’re on that boundary, the tie-breakers are: your solar surplus (can you fill the bigger one?), whether you have or plan an EV or heat pump (push bigger), and your tariff (a smart tariff favours bigger — see below). Our dedicated 5kWh and 10kWh guides show exactly what each runs.
Why your solar system size matters too
Here’s the bit people miss: a battery can only be filled by solar energy your home doesn’t use directly. So your battery size and your solar array size are linked — a big battery needs a big enough solar surplus to fill it, or it’ll sit part-empty on an average day.
- A modest solar system (e.g. 4kW) generates a modest surplus — pairs naturally with a 5kWh battery.
- A larger system (5kW+) produces more spare energy — can justify a 10kWh battery.
- A big battery on a small array is a common mistake — you’ve capacity you can’t fill from solar (unless you’re topping up from a cheap off-peak tariff, below).
So size the two together. If you’re planning both at once, our battery guide and panel sizes guide help you balance them.
The smart-tariff twist
One thing changes the sizing maths: a smart tariff with cheap off-peak rates. Because you can fill the battery from cheap grid electricity overnight as well as from solar, it’s no longer limited by your solar surplus — so you can justify going a size up. Many people on these tariffs deliberately choose a battery big enough to store a full cheap-rate charge and run the whole home through expensive peak hours, earning from the price difference as well as from solar storage. If you’re on (or planning) a smart tariff, factor that in — it often nudges the answer from 5kWh toward 10kWh.
Can a solar battery be too big?
Yes — and it’s the more expensive mistake. If your battery is bigger than your nightly use, or bigger than your solar can fill, the extra capacity just sits there: part-empty, unused, and never earning back its cost. An oversized battery is the classic way to make storage not pay back. The honest guidance:
- Size to your genuine evening/overnight demand, not a round number or a salesperson’s upsell.
- Consider modular batteries (like Fox ESS or SolaX) — start at the right size and add capacity later if your usage grows (EV, heat pump), rather than overbuying now.
- When genuinely unsure, size slightly under rather than over — you’ll cycle it fully (better payback) and can expand later.
Sizing do’s & don’ts
✓ Do
- Size to your evening + overnight usage
- Use a real bill (annual kWh ÷ 365)
- Match battery to your solar surplus
- Go bigger if on a smart tariff or with an EV
- Consider modular to expand later
- Get an installer to confirm for your home
✗ Don’t
- Buy a round number without checking usage
- Oversize — it won’t pay back
- Put a big battery on a small solar array
- Ignore your tariff in the maths
- Let a salesperson upsize you needlessly
- Forget future EV/heat-pump plans
Frequently asked questions
What size solar battery do I need?
Size it to your evening and overnight usage — the power you draw when panels aren’t generating. For an average home (8–10kWh/day) that’s often 5–10kWh: ~5kWh for smaller/lower-usage homes, 10kWh+ for larger or EV/heat-pump homes. Match it to what you’ll actually use and your solar surplus.
How do I calculate it?
Annual kWh ÷ 365 = daily use. Estimate the evening/overnight share (often half to two-thirds) — that’s roughly your useful battery capacity. Then check your solar system can fill it. Little point storing more than you draw down each night.
5kWh or 10kWh?
Depends on usage: 5kWh for smaller homes/lower usage (under ~3,500kWh/yr), 10kWh for medium-large/higher usage or EV/heat-pump homes. Don’t buy more than you’ll cycle daily, or less than your surplus could fill.
Can a battery be too big?
Yes — an oversized battery sits part-empty and never pays back. Size to your nightly demand and solar surplus, and consider modular batteries to expand later rather than overbuying now.
Does my solar size affect it?
Yes — the battery only fills from solar you don’t use directly, so a bigger array feeds a bigger battery. 10kWh+ pairs best with a 4kW+ system; a small array suits a smaller battery unless you top up from a cheap off-peak tariff.
Should I size for a smart tariff?
If you have cheap off-peak rates, you can go a size up — you can fill the battery from cheap grid power overnight as well as solar, so it’s not limited by your surplus. Many size big enough to run through peak hours on a cheap-rate charge.
Size to your nightly usage — not a round number
The bottom line: The right battery size is the one that matches your evening-and-overnight usage and your solar surplus — usually 5–10kWh for an average home. Don’t oversize (it won’t pay back), factor in your tariff and any EV plans, and lean modular if you might expand. Get an installer to confirm the number for your actual home.
Get my free sizing check →The verdict
Sizing a solar battery isn’t complicated once you ignore the round numbers and focus on the one figure that matters: your evening and overnight electricity use. Work that out from a recent bill, match the battery to it, sanity-check it against your solar system’s surplus, and you’ll land on the right capacity — usually 5kWh for smaller homes and 10kWh for larger or EV/heat-pump ones.
The two mistakes to avoid: oversizing (paying for capacity you’ll never cycle) and ignoring your tariff and future plans (a smart tariff or a coming EV can justify going bigger). When in doubt, size slightly under and choose a modular battery you can expand. Best of all, get an installer to confirm the number against your real usage — a free sizing check does exactly that, with no pressure to buy more than you need.
Important
Going Solar UK is an introduction service, not an installer. We don’t design, supply, or fit battery systems ourselves. Sizing guidance, percentages and figures here are indicative 2026 generalisations and vary by home, usage pattern, solar system, tariff and battery — always get a site-specific assessment and quotes from MCS-certified installers before deciding on a battery size.
We may receive a commission when we introduce you to an installer, at no cost to you. Any contract for work is directly between you and the installer.