Storage
Battery Bank Sizing
Battery capacity is sold in nameplate kilowatt-hours, but you never get to use all of it. Depth of discharge, inverter efficiency, wiring losses, and cold weather each take a cut. This calculator works backwards from the energy you actually need to deliver and tells you what to buy.
What it asks for
- Daily load to back up (kWh/day) — Only the loads on the backed-up panel, not the whole house.
- Days of autonomy (days) — Grid-tied backup: 1. Off-grid: 2–3 depending on climate.
- Chemistry
- Usable depth of discharge (%) — LFP 90–100%. Lead-acid 50% max for cycle life.
- Inverter efficiency (%)
- Bank nominal voltage
- Cold-weather derate (%) — Unheated garage in the north: 10–20%. Conditioned space: 0.