Ready Home SystemsPractical home resilience guides for water, flood, air, and backup power decisions.
Planning tool

Sump Pump Battery Runtime Calculator

Estimate sump pump battery runtime from battery capacity, battery-side pump draw, standby watts, duty cycle, conversion efficiency, and reserve.

Estimated runtime Run the calculator

Planning estimate only: replace the example defaults with evidence for your exact battery, pump, controller, and inverter. Verify starting surge separately as a pass/fail requirement; this tool does not guarantee runtime.

Usable battery energy
—
Battery-side average power
—
Raw runtime before reserve
—
Planned runtime after reserve
—
Storm scenarios

Run more than one duty-cycle assumption

10% dutyRun the calculator
Your entered dutyRun the calculator
50% dutyRun the calculator

Example defaults only — replace them with model evidence for your exact equipment and observed storm conditions. Starting surge is a separate pass/fail check, not runtime energy.

Worked comparison

40 Ah vs 100 Ah: same pump, different stored energy

A 100 Ah battery has 2.5 times the rated amp-hour capacity of a 40 Ah battery at the same voltage. Runtime scales the same way only when every other input and the battery's delivered capacity remain equal.

Example assumptions

12 V, 80% usable capacity, 350 W pump load, 80% conversion efficiency, 10 W standby, 25% duty cycle, and 25% reserve.

40 Ah battery480 rated Wh, 384 usable Wh, and about 2.4 hours of planned runtime after reserve.
100 Ah battery1,200 rated Wh, 960 usable Wh, and about 6.1 hours of planned runtime after reserve.

This is a planning example, not a runtime promise. Battery chemistry, discharge rate, temperature, age, lift height, inflow, pump cycling, wiring, standby draw, and controller behavior can change the result. Enter exact-model evidence and test the installed system under load.

Planning workflow

How to estimate sump pump battery backup runtime

  1. Enter battery amp-hours, voltage, and a supported usable-capacity percentage.
  2. Enter running load watts, direct-model conversion efficiency, and battery-side standby watts.
  3. Choose an observed duty cycle and a reserve, then compare 10%, entered, and 50% duty scenarios.
  4. Read planned runtime as a range, and verify starting surge separately as a pass/fail requirement.
  5. Add alarms, testing, and a backup plan because real inflow and battery age can shorten runtime.
Evidence for the assumptions

Sources behind the runtime planner

Frequently asked questions

Is sump pump battery runtime guaranteed?

No. Runtime changes with storm inflow, pump cycling, lift height, battery condition, wiring losses, and charger health.

What duty cycle should I use for a sump pump?

Use several scenarios. A calm test may look like 10% duty cycle, while a severe storm may require 50% or more.

What else should I add besides a bigger battery?

Use a high-water alarm, regular testing, discharge checks, and a plan for long outages or fast inflow.

Before buying a bigger battery

Confirm pump wattage, float operation, discharge path, charger health, battery age, and whether the home needs a water alarm, generator plan, or secondary backup pump.

Compare backup systems

Related guides