Start with the lower of average daily solar exports and evening demand divided by efficiency. Compare usable capacity, allow for backup reserve and test both winter and summer conditions.
The simple sizing rule
Useful capacity is approximately the lower of daily export and evening demand divided by round-trip efficiency. If either side is small, a large battery may spend much of its life partly empty or partly full.
Adjust for the job the battery must do
Bill savings, outage backup and off-grid resilience are different objectives. Backup reserve reduces the capacity available for daily cycling, while a high-power load may require more inverter output without requiring more stored energy.
- Usable kWh
- Continuous and surge kW
- Backup reserve percentage
- Winter solar surplus
- Essential circuits
- Warranty throughput
Use ranges, not one perfect number
Run the calculator with a lower winter export case and a normal annual case. A sensible quote should explain how the proposed capacity behaves in both.