Sump Pump Battery Backup Runtime Calculator

Storms that flood basements often knock out power at the same time. This calculator estimates how many hours a 12-volt battery backup can keep your sump pump working, based on battery size, how much current the pump draws and how often it has to run.

Your result appears here after you answer the questions and press the button.

This tool gives planning ranges only. It is an estimate, not a quote or inspection. A local provider who reviews the property sets the actual scope and price. Nothing you enter here is saved or sent; only the request form sends information.

Want a provider to look at it? Call or send a short request, and a local provider may contact you.

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The arithmetic

A battery's amp-hour rating says how much current it can supply over time. A 75 amp-hour battery could, on paper, deliver 7.5 amps for 10 hours. Two things shrink that number in real life. First, lead-acid and AGM batteries last much longer when they are not drained deeply; guidance from battery specialists is to avoid discharging them much past about 50 percent, so the calculator counts half the rated capacity as usable by default. Second, the pump does not run constantly. If it runs one minute out of every four, it uses a quarter of the current over time.

So the core formula is simple: usable amp-hours divided by pump current times the share of time the pump runs.

Finding your numbers

Battery. The amp-hour rating is on the label. If you have two identical batteries wired in parallel, enter two.

Pump current. Dedicated 12-volt backup pumps list running amps on the label or in the manual. If your backup is an inverter that powers your main pump, enter the pump's running watts. The calculator converts watts to battery current, allowing for inverter losses. The default efficiency is 85 percent; use your inverter's spec if you have it.

Run time. The best input is the share of time the pump runs during heavy rain. You can get it two ways: from your measured inflow and the pump's output at your head (the sump size calculator gives you both), or by timing how long the pump runs and how long it rests.

Worked example

A 75 amp-hour AGM battery, a 12-volt backup pump drawing 10 amps, and a pump that runs about 30 percent of the time. Usable capacity is about 37.5 amp-hours. Average draw is 3 amps. That works out to roughly 12 hours. Double the battery and you get about 24.

Why the real number may be lower

Batteries lose capacity with age and in cold basements, and a battery that was not fully charged starts behind. If the calculator shows your pump running nonstop, it means inflow is close to or above what the pump can move, and the backup may not keep up no matter how big the battery is. Test the backup at least once a year by unplugging the main pump and filling the pit with a bucket.

If the backup gave out and water is already on the floor, call (484) 275-9544 or send the request form. A qualified local provider may contact you.

FAQ

Is a water-powered backup an option?

Some homes on municipal water use water-powered backups that need no battery. They depend on water pressure and local code, so ask a plumber.

How long should a backup last?

Long enough to cover the outages you usually see during storms where you live. Many owners size for at least overnight.

Can I use a car battery?

Car batteries are not designed for deep cycling. Use a deep-cycle battery made for backup pumps.

Does the calculator account for charging?

No. It assumes the power is out and nothing is recharging the battery.

Sources

Battery University (Cadex), BU-804 "How to Prolong Lead-acid Batteries": limiting depth of discharge to extend lead-acid life. https://batteryuniversity.com/article/bu-804-how-to-prolong-lead-acid-batteries
Zoeller at Home, "5 Ways to Properly Size Your Sump Pump" (2023): pump flow at total dynamic head. https://zoellerathome.com/2023/09/15/properly-sizing-your-sump-pump/

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