The short answer
A backup pump runs only when water comes in, so runtime depends on the battery's amp-hours, the pump's current draw and its duty cycle (the share of time it runs). Makers quote runtime at a stated duty cycle: Basement Watchdog says its BW-27AGM gives up to 80 hours on the BWE system at a 10% duty cycle.
Makers' figures
- Basement Watchdog BW-27AGM: up to 40 to 80 hours depending on system, at a 10% duty cycle.
- Wayne ESP25n: up to 10,000 gallons per charge on Wayne's WSB1275 75 Ah battery.
Estimating your own
Usable amp-hours divided by the pump's amps gives hours of continuous running; divide by your duty cycle for hours of protection. The runtime calculator does this and lets you change the assumptions.
SafetyBatteries lose capacity with age. Check the charger's status regularly and plan to replace a lead-acid battery every few years.
Products mentioned
Questions readers ask
How long will a sump pump battery last in an outage?
From several hours to a few days, depending on battery size and how often the pump runs.
What is a duty cycle?
The share of time the pump runs; 10% means 6 minutes in every hour.