Battery Storage Sizing Guide
Choosing battery capacity should start with your actual electricity use - not a generic recommendation tied to a marketing-friendly kWh figure.
Start with your evening consumption
The most useful starting point is how much electricity you use between late afternoon and bedtime. This is the period when solar generation drops but household demand often peaks - cooking, lighting, heating controls and entertainment.
If you have a smart meter, half-hourly data makes this analysis straightforward. Without it, recent monthly bills combined with a rough breakdown of evening appliance use provides a reasonable starting point for discussion.
Three common sizing goals
Cover evening peak
Store enough solar surplus to reduce grid imports during your typical evening hours - often a modest capacity suits this goal.
Maximise self-consumption
Use more of your own solar generation year-round. Requires matching battery capacity to typical daily solar surplus, which varies by season.
Tariff arbitrage
Charge from the grid during cheap off-peak periods. Capacity should relate to what you can usefully discharge during expensive periods - depending on your tariff.
Why bigger is not always better
- A battery that rarely fills from solar surplus delivers limited benefit relative to cost
- Larger batteries occupy more space and may have higher standby consumption
- Over-sizing for backup power you may never use adds expense without proportional value
- Some systems allow modular expansion later - worth discussing if your needs may grow
Key principle: Battery sizing depends on actual consumption data. Marketing that recommends a fixed kWh figure for all homes should be treated with scepticism.
Solar surplus and seasonal variation
Summer months typically produce more solar surplus than winter. A battery sized for summer surplus may under-utilise in winter, while one sized for winter evening use alone may not capture all summer generation.
Your installer should explain the seasonal trade-offs rather than presenting a single annual figure without context.
Interaction with EV charging
If you charge an EV at home - particularly overnight - this affects battery strategy. You may prioritise solar charging the car during the day, storing surplus in the battery for evening use, or charging the car from off-peak grid power. These approaches require different capacity thinking.
See our solar with EV charging guide for integrated planning context.
Questions to ask during your survey
- What evening kWh figure did you use to recommend this capacity?
- How does the recommendation change between summer and winter?
- Can the system be expanded later if I add an EV?
- Does this include backup capability - and what circuits would be covered?
- What is the round-trip efficiency of the proposed battery?