Quick Sizing Recommendations
| Solar System | Household | Recommended Battery | Approx. Cost |
|---|---|---|---|
| 3 kW | 1-2 people, low usage | 3-5 kWh | £2,500-£4,000 |
| 4 kW Most Common | 3-4 people, average usage | 5-10 kWh | £3,500-£6,000 |
| 5-6 kW | 4+ people, high usage / EV | 10-13.5 kWh | £5,000-£8,500 |
| 6+ kW | Large home, EV + heat pump | 13.5-20 kWh | £7,000-£10,000 |
How to Calculate Your Ideal Battery Size
Find your daily electricity usage
Check your smart meter or energy bill. The average UK home uses 8-10 kWh per day. Higher if you have an EV (add ~8 kWh/day) or electric heating.
Estimate your evening/night usage
Typically 50-70% of your total daily usage happens between 4pm and 7am (when solar isn't generating). For a home using 10 kWh/day, that's 5-7 kWh of evening demand - your battery target.
Match to your solar system's surplus
A 4kW solar system generates ~11 kWh on a good summer day, of which you might directly use 4-5 kWh. That leaves ~6 kWh surplus - a 5-7 kWh battery would capture most of it. In winter, surplus is lower, so don't oversize for summer alone.
Factor in future needs
Planning to get an EV? Add 5-8 kWh. Getting a heat pump? You may want a larger battery for tariff arbitrage (charge cheap at night, use during peak). Many battery systems are modular - you can add capacity later.
Don't oversize your battery
A common mistake is buying the biggest battery possible. If your solar system only produces 6kWh of surplus per day, a 13.5kWh battery will rarely be more than half full - wasting £2,000-£3,000 on capacity you don't use. Size your battery to match your actual surplus generation and evening consumption.
Do You Need Backup Power?
If you want your battery to power essential circuits during a grid outage, you'll need:
Essential backup (lights, fridge, internet)
Requires: 5+ kWh battery with EPS (Emergency Power Supply) function. Available on GivEnergy, Tesla Powerwall, and some Fox ESS models. Add £200-£500 for EPS wiring.
Whole-home backup
Requires: 10+ kWh battery, hybrid inverter, and a changeover switch. More expensive (£500-£1,000 extra) but powers your entire home during short outages.