Sump Pump Backup Runtime Calculator: Hours of Protection

A battery backup is only useful if it lasts longer than the power cut. This calculator converts your battery capacity, the backup pump draw and the share of time it will be running into an honest runtime in hours, and also shows the worst case where the pump never gets to rest.

When this tool helps most

Use it when choosing a backup system, and again when the battery is a few years old and you are deciding whether to replace it.

How to use this tool

1. Find the battery capacity in amp-hours and the system voltage.
2. Set a usable depth of discharge: 50 percent is normal for flooded lead-acid.
3. Enter the backup pump power draw in watts while it is running.
4. Estimate the duty cycle by timing how often your primary pump runs during heavy rain.
5. Leave the efficiency figure unless your installer gives you a better one.

Worked example

A 100 amp-hour 12 volt battery taken to 50 percent, driving a 350 watt pump at a 30 percent duty cycle with 85 percent efficiency, gives about 4.9 hours of protection. If the storm is bad enough that the pump never stops, the same battery gives about 1.5 hours. Both numbers are worth knowing before you rely on it.

Understanding your results

The runtime figure is the planning number. The continuous figure is the one to check against a serious storm. If the gap between them worries you, the answer is usually a second battery rather than a bigger pump.

Common mistakes to avoid

• Assuming a battery delivers its rated amp-hours after several years of service.
• Guessing the duty cycle instead of timing the primary pump.
• Forgetting that cold reduces battery capacity, and outages often happen in cold weather.
• Never testing the backup, so a dead battery is discovered during the outage.

How to get the most accurate result

• Measure your own basement or crawl space with a tape measure instead of using typical sizes.
• Use the numbers printed on your product labels, manuals and data sheets.
• Change one input at a time to see which assumption matters most.
• Print or save the result and bring it to your installer or contractor.

Next steps

Confirm the primary pump is right too, with the Sump Pump Sizing Calculator.

Frequently asked questions

Why only 50 percent of the battery?

Repeatedly discharging a flooded lead-acid battery below about half its capacity shortens its life sharply. Lithium chemistries tolerate deeper discharge; use the figure the manufacturer states.

What duty cycle should I use?

Time your primary pump during real heavy rain. If it runs 20 seconds in every minute, that is about a 33 percent duty cycle.

Is a water-powered backup better?

It needs no electricity at all, which is a real advantage. It does use municipal water while running and is not an option on a private well.

This tool is for planning and education only. It does not store your entries. Structural judgement belongs to a licensed structural engineer, radon mitigation design to a certified mitigator, and mold over roughly 10 square feet to a remediation professional. New circuits are a job for a licensed electrician. Read our disclaimer.