PumpendiumThe spec reference for home pumps

Calculator

Battery backup runtime calculator

Enter your battery's amp-hours, the backup pump's amp draw and how many minutes an hour it runs in a storm to estimate hours of protection.

Printed on the battery; group 27 deep-cycle batteries are often 75-100 Ah
From the backup pump's manual; it rises with lift height
Heavy storm: 10-20; constant inflow: up to 60

How the math works

Pumpendium mathPumping hours = battery Ah × usable share ÷ pump amps. Protection hours = pumping hours × 60 ÷ minutes pumping per hour. Lead-acid batteries are counted at 50% so the estimate does not rely on draining them flat; lithium iron phosphate is counted at 80%.
LimitsCold basements, older batteries and higher lift all shorten runtime. Makers state runtimes under their own test conditions, shown below as stated.

Questions

How long does a sump pump battery last in an outage?
It depends on the battery size, the pump's draw and how often water comes in. A 75 Ah battery feeding a 20 A pump that runs 10 minutes an hour lasts roughly 11 hours by this estimate.
Does a water-powered backup need a battery?
No. Water-powered backups use city water pressure instead of electricity, so they need adequate municipal water pressure and are not suitable for homes on wells.