Before you buy or hire
The Simple Runtime Formula
Battery watt-hours divided by pump watts gives a rough full-load runtime. Duty cycle matters because a sump pump usually runs intermittently rather than continuously.
Why Real Runtime Is Lower
Battery age, cold temperature, charger condition, wiring losses, float settings, lift height, and inflow rate can all reduce usable runtime.
Use Conservative Assumptions
If you do not know duty cycle, estimate several scenarios. A basement with water entering fast during storms should not use a sunny-day test as the only assumption.
Pair Runtime With Alerts
A large battery without an alarm can still fail quietly. Add a high-water alarm and a plan for someone to check the property.
When Bigger Is Not Enough
If outages are long and inflow is high, consider a generator-safe charging plan, dual batteries where supported, or a water-powered backup if the home has municipal water.
Try the Calculator
Use the calculator page linked from this article to estimate hours with your battery and pump numbers.
Sources Used
- Maintain Your Sump PumpFEMA
- Five Facts to Help New Homeowners Protect Against Water DamageNational Flood Insurance Program
FAQ
What is duty cycle?
Duty cycle is the percentage of time the pump is actually running. A 25% duty cycle means the pump runs about 15 minutes per hour.
Can I connect multiple batteries?
Only if the manufacturer supports it and the charger is designed for the configuration.
Should I size for continuous pumping?
For severe inflow, continuous or near-continuous pumping may be the safer planning assumption.
Related guides
Best Sump Pump Battery Backup: How to Choose Before the Storm
The best backup sump pump is the one that moves enough water for your pit, runs long enough for your outage risk, and alerts you before the basement becomes a claim.
Primary and Backup Sump Pump Maintenance Checklist
Maintain the sump system as one chain: pit, primary pump and float, backup pump and float, battery and controller, alarm, check valves, discharge, power, and the person who responds. Use installed manuals for intervals and procedures.
How Often Should a Sump Pump Backup Battery Be Replaced?
Replace a backup battery according to its system manual and condition, not a universal calendar. Age is one input; warnings, test performance, chemistry, charging, temperature, terminal condition, and the home's storm exposure determine how much margin remains.