Sump pump battery backup calculator

Quick answer

The important number is not "battery size" by itself. It is how often the pump runs, how many watts it draws, and whether the system still works when the power is out. This calculator gives you a planning estimate — not a flood protection guarantee.

How the estimate works

The calculator uses three simple steps:

• Usable battery energy = battery Wh × efficiency × (1 − reserve fraction)
• Hourly energy use = pump watts × (run minutes per hour ÷ 60)
• Estimated runtime = usable Wh ÷ hourly Wh use

The result is compared against your storm target duration to flag whether the setup is likely enough, marginal, or insufficient.

Why pump cycling matters

A sump pump does not run continuously — it cycles on and off as water enters the basin. The run minutes per hour input captures this behaviour. In light rain, a pump might run 5 minutes per hour. During heavy storms, 15–20 minutes per hour is common.

If the pump runs 30+ minutes per hour, you may have a high-risk water problem that needs professional diagnosis before a bigger storm hits.

What battery backup can and cannot do

• Can: Keep your sump pump running during a short power outage if sized appropriately. Buy time until power returns or you can deploy a generator.
• Cannot: Replace a working primary sump pump. Handle foundation drainage issues. Protect against sewer backup. Guarantee your basement stays dry.

Battery backup is one layer in a multi-layer approach that includes grading, gutters, downspout extensions, a working primary pump, an alarm, and a backup plan (generator, water-powered backup, or second battery).

When to call a pro

• If you are not sure how to wire a battery backup to your pump system.
• If the pump runs more than 30 minutes per hour during storms.
• If you need a backup pump installed in the basin alongside the primary.
• If you are considering a permanent generator or transfer switch for pump power.