Sump pump battery backup runtime calculator

Quick answer

Sump backup runtime depends heavily on how often the pump actually runs. A battery that looks enormous beside a quiet pit can disappear quickly when storm inflow forces long or continuous pump cycles. This calculator estimates the energy budget; it does not predict whether your pump can hydraulically keep up with incoming water.

How the estimate works

The calculator uses a transparent planning model:

• Modelled usable energy = entered battery Wh × entered efficiency factor × (1 − your reserve fraction)
• Average pump energy per hour = pump running watts × (run minutes per hour ÷ 60)
• Estimated runtime = modelled usable Wh ÷ average hourly Wh use

The math is intentionally simple enough to audit. The uncertainty belongs in the inputs and caveats rather than being hidden behind a fake precision slider.

Use the right pump number

For an AC sump pump, prefer exact manufacturer electrical data for your model. Horsepower alone is not a reliable running-watt or starting-watt conversion. For a dedicated DC backup pump, the controller or pump manual may publish current draw, battery requirements, pumping capacity, and runtime examples.

Starting current matters when deciding whether an inverter, portable power station, or generator can start an AC pump. It is not something to add as a blanket percentage to every runtime calculation.

Run minutes per hour matter more than a headline runtime

There is no defensible universal Canadian storm duty cycle. Watch your own system during wet conditions if it is safe to do so and record how long the pump runs in an hour. A backup system tested against 5 minutes of pumping per hour and the same system tested against 40 minutes per hour are solving very different problems.

Battery Ah and Wh

If your battery is labelled only in amp-hours, a starting energy estimate is Ah × nominal battery voltage = nominal Wh. That is not automatically the energy the system can deliver before its controller shuts down. Lead-acid discharge rate, battery condition, temperature, chemistry-specific limits, and controller settings can materially change usable energy.

Do not swap battery chemistry because the connector fits. Some backup controllers specify particular battery types, capacities, or charging profiles. Follow the backup-system and battery manufacturers' compatibility requirements.

What the calculator does not model

• pump flow at your actual head height
• friction losses and blocked or frozen discharge piping
• battery chemistry-specific discharge curves or Peukert behaviour
• battery age or temperature derating
• controller low-voltage cutoff
• AC motor startup compatibility with an inverter or generator
• changing inflow during the storm

What to do with the result

Treat the result as one input to a backup-system decision. If the energy budget barely reaches your planning window, that is not much margin. If your basement has high consequences or the pump runs aggressively during storms, consider additional independent layers rather than simply buying a bigger battery.

The full sump pump backup guide compares dedicated battery pumps, inverter systems, water-powered backups, generators, alarms, and shared failure modes.

When to call a pro

• Your primary pump struggles to keep up during normal wet weather.
• You cannot identify the pump, discharge path, or electrical arrangement.
• You are adding a second pump or modifying discharge plumbing.
• You are considering a water-powered backup connected to potable plumbing.
• You want an inverter, generator inlet, transfer switch, or other permanent electrical connection.