How long will your battery backup sump pump actually run? This calculator estimates it from three things: your battery’s amp-hour (Ah) rating, how much current your backup pump draws, and how often the pump cycles during a storm. Enter your own numbers — the defaults are only an example.
Printed on the battery label. Common sump backup batteries are 40–100 Ah.
Lead-acid batteries shouldn’t be drained completely; older or cold batteries deliver less. Lower this for a battery more than 2–3 years old.
Check your backup pump’s manual or spec sheet for its DC amp draw. The default is only an example.
Time one pump-out of your primary pump during heavy rain.
Enter the same number as #4 in minutes (e.g. 0.33) if the pump never stops.
How the estimate works
The calculator uses a simple energy budget: runtime = (battery Ah × usable %) ÷ (pump amps × share of time running). If your backup pump draws 10 A and runs 20 seconds out of every 2 minutes, it is running one-sixth of the time, so a 75 Ah battery with 80% usable (60 Ah) lasts about 60 ÷ (10 × 1/6) = 36 hours. Run the same pump non-stop and the same battery lasts about 6 hours.
Two numbers matter most: how often your pit fills during a storm and how big your battery is. A pit that fills every 30 seconds will drain any battery quickly; a pit that fills a few times an hour barely touches it.
How to get your numbers
- Battery Ah: printed on the battery. Wayne and PumpSpy, for example, recommend a 75 Ah or larger deep-cycle battery for their backup pumps.
- Pump amps: listed in your backup pump’s manual or spec sheet as DC current draw.
- Cycle timing: during the next heavy rain, time how long your primary pump runs and how many minutes pass between starts. Your backup will cycle at roughly the same rate for the same inflow.
Ways to extend runtime
- Use a larger deep-cycle battery, or a second battery if your system supports one.
- Keep the battery on its charger and replace it on the schedule its manufacturer recommends.
- Fix the check valve if water drains back into the pit — backflow makes the pump cycle more often.
- For long outages on city water, consider a water-powered backup, which doesn’t depend on a battery. See our battery vs water-powered backup comparison.