Solar Battery Storage Size Calculator
Calculate the right solar battery size for your home.
Find out how many kWh of storage you need for energy independence.
A solar battery stores surplus energy generated during the day for use at night or during cloudy periods. Sizing it correctly avoids paying for unused capacity while ensuring you have enough storage to cover your evening demand.
Battery sizing formula: Usable battery size (kWh) = evening/night load (kWh) / depth of discharge (DoD)
Depth of discharge:
- Lithium iron phosphate (LFP): 90–95% DoD — highly recommended
- NMC lithium: 80–90% DoD
- Lead-acid: 50% DoD (avoid for solar use)
Worked example: Evening load (18:00–08:00): 8 kWh LFP battery at 90% DoD: Battery size needed = 8 / 0.90 = 8.9 kWh → buy a 10 kWh battery
Popular home battery sizes:
- 5 kWh: Small flat or supplemental storage
- 10 kWh: Average house, covers most evenings
- 15 kWh: Large house or EV top-up overnight
- 20+ kWh: Off-grid or EV-heavy households
Days of autonomy: If you want backup for cloudy days, multiply the daily load by desired days: 3 days × 8 kWh daily load = 24 kWh total storage needed
Cost guide (2025):
- LFP batteries: £500–£700 per kWh installed
- 10 kWh system: ~£5,000–£7,000 installed
- Most home batteries qualify for 0% VAT in the UK.