Backup Power guide

Battery Backup Sump Pump Runtime Calculator: How Long Will It Run?

Updated Oct 18, 2026 · How we compare

Quick answer

It depends on how fast water comes in. Makers quote runtime at a 10% duty cycle, meaning the pump runs a tenth of the time. Basement Watchdog lists 80 hours for its Emergency (BWE) pump and 48 hours for its Special (BWSP) on one BW-27AGM battery.[2] Zoeller lists 48+ hours for the Aquanot 508.[1]
In a heavy storm it can be hours, not days. Zoeller lists up to 5.5 hours of continuous pumping on a full charge.[1] If the pump has to run nonstop, that's roughly the limit.
A bigger battery helps. Basement Watchdog lists 96 hours for the BWSP with two batteries.[2]

Estimate your inflow, pick a backup, and the calculator scales the maker's rating to your pit.

Battery backup sump pump runtime calculator

Backup pump Pump flow at 10 ft lift gal/min Continuous run per charge hours Water coming into the pit gal/min

Gallons pumped per charge (estimate)

Percent of time the pump runs

Hours of protection per charge

The calculator needs JavaScript. By hand: multiply the pump's gallons per minute by its continuous hours per charge and by 60 to get gallons per charge. Divide that by your inflow in gallons per minute, then by 60, for hours of protection.

Presets use each maker's flow at 10 ft of lift and its runtime rating.[1][2][3] The 3 gal/min inflow default is only an example. Measure your own pit (see below).

How the estimate works

A backup pump only runs when the float rises, so the battery lasts longer when less water comes in. The calculator does three steps:

Gallons per charge = flow at 10 ft × continuous hours × 60.
Share of time running = your inflow ÷ the pump's flow.
Hours of protection = gallons per charge ÷ your inflow ÷ 60.

This is our scaling of the makers' ratings, not a maker figure. It assumes a full, healthy battery and about 10 ft of lift. More lift means less flow per minute, and an older battery holds less charge. Basement Watchdog notes that runtimes vary with water flow.[2]

Measuring your inflow

During a wet spell, after the main pump shuts off, time how long the water takes to rise a few inches. Work out the gallons from your pit's size: a round pit 18 in. across holds about 1.1 gallons per inch of depth (π × 9² ÷ 231). Divide the gallons by the minutes. Use the heaviest rise you've seen, because outages often come with storms.

Maker ratings used

Backup	Flow at 10 ft	Maker runtime rating	Continuous hours used
Zoeller Aquanot 508[1]	About 34 gal/min	Up to 5.5 h continuous; 48+ h at 10% duty	5.5 (maker figure)
Basement Watchdog Emergency (BWE), one BW-27AGM[2]	16.7 gal/min	80 h at 10% duty	8.0 (10% of 80 h)
Basement Watchdog Special (BWSP), one BW-27AGM[2]	30.8 gal/min	48 h at 10% duty	4.8 (10% of 48 h)
Basement Watchdog Special (BWSP), two BW-27AGM[2]	30.8 gal/min	96 h at 10% duty	9.6 (10% of 96 h)
Wayne ESP25, Wayne 75 Ah battery[3]	1,680 GPH (28 gal/min)	Up to 26 h of protection at 5 gal/min incoming water	4.6 (26 h × 5 ÷ 28)

The last column is the only number we derive. The rest are quoted from each maker's page or sales sheet.

Making it last longer

Add battery capacity where the maker allows it. Basement Watchdog lists double the runtime for the BWSP with two batteries, but says a second battery isn't recommended for the BWE.[2]
Keep the battery healthy. Zoeller's runtime is for a fully charged battery.[1] An older or partly charged battery won't reach the rated hours.
Power the main pump during a long outage. A power station or generator can run the AC pump and save the backup battery. Our generator wattage calculator covers sizing.

Frequently asked questions

How long will a battery backup sump pump run?

It depends on how fast water comes in. Makers rate it at a 10% duty cycle: Basement Watchdog lists 80 hours for its Emergency pump and 48 hours for its Special pump on one BW-27AGM battery, and Zoeller lists 48+ hours for the Aquanot 508. At heavier inflow it can be only a few hours.

What does a 10% duty cycle mean?

The pump runs 10% of the time, for example 6 minutes in every hour. That happens when water flows in at one tenth of the pump's flow rate.

How long does a backup sump pump run continuously?

Zoeller lists up to 5.5 hours of continuous pumping for the Aquanot 508 on a full charge. Basement Watchdog's 10% duty-cycle figures work out to about 8 hours for the Emergency and 4.8 hours for the Special on one battery.

Does a second battery double the runtime?

For the Basement Watchdog Special, the maker lists 96 hours with two BW-27AGM batteries versus 48 hours with one, at a 10% duty cycle. It says two batteries aren't recommended for the Emergency model.

Sources

Zoeller Pump Company, Aquanot 508 Backup product page and sales sheet (FM3525). zoellerpumps.com/product/aquanot-508-backup/
The Basement Watchdog, Emergency (BWE) product page with BWE and Special (BWSP) specifications. basementwatchdog.com/product/emergency-battery-backup-pump/
WAYNE Pumps, ESP25 product page and specifications. waynepumps.com/product/esp25/