How This Calculator Sizes Your Runtime
Designing a reliable solar energy storage system requires matching the capacity of the battery bank with the electrical load requirements of your home. Unlike grid-tied solar systems that feed excess power back to the utility grid, off-grid or hybrid solar installations rely on batteries to supply power during nighttime hours or grid outages.
This calculator works from a battery you already own or are evaluating, rather than solving for a target capacity. Enter the battery's nominal capacity and Depth of Discharge (DoD) to find usable capacity: Usable Capacity = Nominal Capacity × DoD Enter your critical load in watts to find daily consumption: Daily Usage (kWh) = (Critical Load (W) × 24)/1000 The calculator then compares your daily solar recharge against that usage to find your net daily energy balance, and — if recharge falls short — how many days the usable capacity lasts before running out.
To model the charging source for this storage setup, you can size your solar panels using the solar inverter sizing calculator or estimate overall system returns with the solar panel payback calculator. Achieving balance between solar generation and battery capacity ensures maximum energy resilience.
For a 13.5 kWh battery (Tesla Powerwall-class) at 95% DoD, powering an 800W critical load with 10 kWh of daily solar recharge: usable capacity is 13.5 × 0.95 = 12.825 kWh, and the 800W load draws 800 × 24 / 1000 = 19.2 kWh/day — more than the 10 kWh/day the panels can replace. That leaves a net daily deficit of 9.2 kWh, so the battery alone (ignoring solar) would run the load for about 16 hours, and with solar still contributing, the system stretches to roughly 1.4 days before depleting.