To find the ideal battery storage capacity, consider using this formula: Total Daily Usage (kWh): Multiply your average daily usage by the number of days of backup you want. Add Losses: Multiply the total by 1. Battery sizing is goal-driven: Emergency backup requires 10-20 kWh, bill optimization needs 20-40 kWh, while energy independence demands 50+ kWh. Usable capacity differs from total capacity: Lithium batteries. Home batteries store electricity from your solar system or the grid for use during outages, when the grid is most expensive, or at night when it is dark. Follow it, and you turn daily kWh into a bank that carries evening peaks, cold snaps, and busy shifts.