Most UK homes with solar are best served by a battery somewhere between 5kWh and 10kWh, sized to capture a typical day’s surplus daytime generation for use in the evening – but the right number for your specific household depends on your solar system size, daily usage pattern, and whether you also want backup power during outages, not a single one-size-fits-all figure.
The Basic Approach: Match Battery to Daily Surplus, Not Total Usage
A common sizing mistake is trying to size a battery to cover your entire daily electricity usage – the more useful target is covering your typical daily surplus generation (what your panels produce beyond what you use directly during daylight), since that’s what would otherwise be exported at a lower rate rather than used at full value later. For a typical 4kWp system, daily surplus on a decent day often falls in the 4-8kWh range, which is why battery sizes in this bracket are so common.
| Solar system size | Typical daily surplus (summer) | Suggested battery size |
|---|---|---|
| 3-4kWp | 3-6kWh | 5kWh |
| 5kWp | 5-8kWh | 6.5-9kWh |
| 6kWp+ | 7-10kWh+ | 9.5-13kWh |
Factor In Your Actual Evening and Overnight Usage
If your household is genuinely home and active in the evening – cooking, lighting, entertainment, device charging – a larger battery lets you draw on stored solar rather than the grid for longer into the night. Households that go to bed early or are frequently out in the evening get less benefit from oversizing beyond their typical evening consumption window, since a battery sitting partially full with nowhere useful to discharge to isn’t adding value.
Sizing Differently If Backup Power During Outages Matters to You
If part of your motivation is having power during a grid outage (which requires the right inverter configuration, not just any battery – see our guide on whether solar panels work during a power outage), you’ll want to size around your essential loads specifically – fridge, some lighting, phone charging, perhaps a router – rather than trying to power your whole home. A smaller, carefully-targeted essential-loads battery setup is often more practical than oversizing a whole-home battery purely for occasional outage backup.
EV and Heat Pump Households Typically Need a Larger Battery
If you’re charging an EV mostly overnight on a cheap off-peak tariff rather than during solar generation hours, this specific load doesn’t actually benefit much from a bigger battery – smart EV charging schedules already often target off-peak grid rates rather than solar. A heat pump running through winter evenings, by contrast, can genuinely benefit from a larger battery, since heating demand often peaks precisely when solar generation is at its lowest.
Why Oversizing a Battery Isn\u2019t Automatically the Right Call
A battery larger than your typical daily surplus and evening usage combined simply won’t fill up regularly, meaning you’re paying for capacity you rarely use – battery cost (typically £3,000-£10,000 depending on size) doesn’t scale down proportionally at smaller sizes, so getting the sizing roughly right matters more for a battery than it often does for panels, where a slightly oversized array at least captures marginally more generation.
Modular Batteries Let You Start Smaller and Expand Later
Many modern battery systems are modular, letting you add capacity later if your usage pattern changes (an EV, a heat pump, or simply higher electricity prices making a bigger battery more worthwhile). Starting with a moderately sized battery and confirming your real-world surplus and usage pattern before committing to a larger system is a reasonable, low-risk approach if you’re uncertain.
Frequently Asked Questions About Solar Battery Sizing
Is a 5kWh battery enough for a typical UK home?
For many households with a 3-4kWp solar system and moderate evening usage, yes – a 5kWh battery captures a reasonable share of typical daily surplus. Larger solar systems, EVs charged during the day, or heat pumps often justify sizing up towards 8-10kWh instead.
Can a solar battery be too big for my system?
Practically, yes – if your daily surplus generation and evening usage never come close to filling a very large battery, you’ve paid for capacity that goes unused most days. It’s not harmful to the battery itself, just an inefficient use of your budget.
How do I find out what my actual daily solar surplus is before buying a battery?
If you already have solar without a battery, your inverter’s monitoring app typically shows generation vs consumption data that reveals your real surplus pattern. If you’re installing solar and a battery together, ask your installer for a modelled generation-vs-consumption profile as part of your system design.
