Sump Pump Battery Backup Runtime Calculator

A sump pump battery backup matters most during the storms that take out the power, which are exactly the storms that fill the sump pit fastest.
Sizing the battery for an outage requires knowing how much current the pump pulls, how often it runs, and how long the battery can deliver that current.

The math:

minutes_per_hour_pumping = cycles_per_hour × seconds_per_cycle / 60 amp_hours_per_hour = (pump_amps × minutes_per_hour) / 60 runtime_hours = (battery_Ah × usable_fraction) / amp_hours_per_hour

Battery capacity is quoted in amp-hours (Ah), which is simply how many amps the battery can supply for one hour. A 100 Ah battery delivering 5 amps lasts 20 hours on paper.

Typical current draw for a 12 volt direct current (DC) backup pump, rated in horsepower (HP) and gallons per hour (GPH):

• Small backup (1/4 HP equivalent, 1,500 GPH): 8-15 amps
• Medium backup (1/3 HP, 2,200 GPH): 15-25 amps
• High-output backup (1/2 HP, 3,300 GPH): 25-40 amps

Cycle frequency depends on the storm and the pit volume.
A normal rain event might fill a 15-gallon pit in 5-10 minutes, requiring a 30-second pump cycle to empty.
That is 6-12 cycles per hour with the pump running 3-6 minutes per hour total.
A severe rain event with high water table might cycle every 30-60 seconds, with the pump running half the hour or more.

A worked example.
Medium backup pump at 20 amps, cycling 8 times per hour, 30 seconds per cycle.
Pump-on minutes per hour: 8 × 0.5 = 4 minutes.
Average current draw: (20 × 4) / 60 = 1.33 Ah per hour.
Group 27 deep-cycle marine battery rated 100 Ah, usable 50 Ah (lead-acid limits to about 50% depth of discharge).
Runtime: 50 / 1.33 = 37.5 hours.

Same pump in a serious storm cycling 30 times per hour: 30 × 0.5 = 15 minutes/hour pumping.
Current draw: (20 × 15) / 60 = 5 Ah per hour.
Same battery: 50 / 5 = 10 hours.

The difference between “37 hours of backup” and “10 hours of backup” is just the storm intensity, not the battery.
That is the part most calculators do not model and most homeowners forget.

Lithium iron phosphate (LiFePO4) versus lead-acid.
A 100 Ah lead-acid deep-cycle battery is roughly 50 Ah usable; a 100 Ah LiFePO4 is 90+ Ah usable, so it nearly doubles the runtime for the same nameplate. Absorbent glass mat (AGM) batteries sit between the two at about 70%.
LiFePO4 also handles 3,000+ deep cycles versus 200-500 for lead-acid, so it lasts 5-10× longer in service.
The price gap has been closing: as of 2026 a 100 Ah LiFePO4 runs somewhere around $300-400 against roughly $200 for an equivalent lead-acid, though both move with the battery market and vary by region.
For sump pump backup that may sit unused for years between needing it, lead-acid often dies of self-discharge between events; LiFePO4 holds charge better and is the smarter long-term choice.

Three practical points.
Test the system every spring before storm season — battery, charger, float switch, and discharge hose all.
A backup pump that has not been tested in 18 months has a coin-flip chance of working when called.
Many backup systems have an alarm that sounds when the battery loses charge or the main alternating current (AC) pump fails; install one with a notification feature (smartphone app or local audible alarm), because you rarely know your basement is flooding until you go down to check.
And the backup pump’s discharge hose has to actually exit the house — many DIY installs route the backup discharge into the same line as the primary pump, which can siphon water back into the pit if the check valve fails.