Enter your pump's amperage, expected cycles per hour during heavy rain, and desired backup hours to size the right battery.
Sump pumps draw current only while actively pumping, not continuously — sizing around actual duty cycle (not full-time running) avoids drastically oversizing the battery.
A sump pump doesn't run continuously — it cycles on for a short burst, then rests, which means backup sizing should be based on total active pumping time during a storm, not the full outage duration. Power outages during heavy storms are exactly when sump pumps matter most, since flooding risk peaks at the same time grid power is most likely to fail — check the C-rate calculator to confirm your battery can deliver the pump's starting surge too.
Size the battery's Ah capacity around running current and duty cycle as shown here, but separately confirm the battery and inverter's max discharge current (C-rate) exceeds the pump's starting surge, which is typically 2-3× running current.
Most residential battery backup systems for sump pumps target 6-12 hours as a reasonable buffer for a typical severe storm outage, though multi-day outages would need proportionally more capacity.