Backup Sump Pump Runtime Calculator

Size your emergency sump pump protection with a quick runtime estimate. Provide your backup battery capacity and the pump's watt draw, then optionally adjust voltage, inverter efficiency, and duty cycle to see how long the system can keep water moving during an outage.

Amp-hour rating of the backup battery bank.
Average wattage while the sump pump is running.
Defaults to 12 V for typical backup systems.
Defaults to 85% if blank.
Defaults to 100% (continuous run) when blank.

Runtime estimates assume healthy batteries and consistent pump performance. Confirm sizing with a licensed electrician before relying on backup power for flood protection.

Examples

  • 120 Ah battery, 600 W pump, 12 V, 85% efficiency, 40% duty cycle ⇒ Available energy after losses: 1,224.00 Wh • Estimated runtime before recharge: 5.10 hours (306.12 minutes) • Duty cycle assumed: 40.00% • Runtime expressed as 0.09 days of intermittent pumping at the entered duty cycle.
  • 200 Ah battery, 450 W pump, optional fields blank ⇒ Available energy after losses: 2,400.00 Wh • Estimated runtime before recharge: 5.33 hours (319.80 minutes) • Duty cycle assumed: 100.00% • Runtime expressed as 0.22 days of intermittent pumping at the entered duty cycle.

FAQ

Can I add multiple batteries?

Yes. Multiply the amp-hour rating by the number of batteries wired in parallel and enter the combined capacity.

How do I adjust for partial discharge limits?

Reduce the amp-hour input to the usable capacity. For example, a 120 Ah battery at 80% usable capacity should be entered as 96 Ah.

What if my pump surge wattage is higher?

Use the average running watt draw, but ensure your inverter is sized for the startup surge so the runtime estimate remains achievable.

Does this cover AC and DC pumps?

Yes. Enter the appropriate voltage and efficiency—DC pumps can leave efficiency at 100%, while AC pumps typically use an inverter efficiency between 80% and 90%.

Additional Information

  • Available watt-hours multiply amp-hours by voltage and inverter efficiency so losses are accounted for upfront.
  • Duty cycle represents the fraction of each hour the pump actually runs; lower duty cycles extend total hours of coverage.
  • Lead-acid batteries should not be fully discharged—rerun the model with a reduced capacity to reflect an 80% depth of discharge.

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