Battery runtime depends on usable energy and the pump’s changing workload. A published “hours of backup” claim only applies to its stated battery, pump, head and operating pattern. Continuous pumping hours are different from elapsed outage hours.
Measure the duty cycle
Duty cycle is pump on-time divided by elapsed observation time. For example, 15 minutes of pumping in a 60-minute observation is 0.25, or 25%. That is a measurement example, not an assumed value for your basement. Heavy rainfall, a rising water table or discharge changes can substantially change the next hour.
Record a wet-period observation relevant to the outage you are planning for. If conditions become more demanding, recalculate. Do not extrapolate a quiet, dry-day observation into storm protection.
Calculate at one energy boundary
For an AC pump, a simplified average load is Pon × duty cycle + Poff × (1 − duty cycle). Estimated elapsed time is usable AC Wh divided by that average W. This assumes the usable-energy evidence covers the same operating conditions and system losses; do not subtract inverter loss a second time.
Hypothetical example: 900 Wh of verified usable AC energy, a 600 W pumping load, zero off-state AC load, and an observed 25% duty cycle give 150 W average and 6 hours. At continuous pumping, the same arithmetic gives 1.5 hours. Neither result is a prediction for a particular product. Idle draw, startup events and battery cutoff can invalidate a simplified estimate if the evidence does not include them.
EIA explains watts versus watthours. A nominal 1,000 Wh battery label is not evidence that 1,000 Wh reaches the pump.
Dedicated DC backup systems
For a DC backup pump, use the manufacturer’s battery and runtime guidance rather than adding an imaginary AC inverter. The Liberty 442 installation manual ties backup operation to battery capacity, charge and operating conditions. Battery amp-hours also depend on the specified discharge conditions; nominal volts × amp-hours is not a guaranteed runtime.
Make uncertainty visible
Keep separate records for ordinary and severe wet-period observations. State the assumed outage duration and any capacity allowance, with its reason. No percentage reserve guarantees that a flooded basement will be prevented. Ensure startup capability independently, even if the energy arithmetic looks favorable.
Enter actual on-time in the Backup Power Map. Review the load-measurement guide, power-station sizing and dedicated battery-system options.
By Tony Pham · Sources checked September 12, 2026. Educational planning guidance; no hands-on product testing claimed. Exact equipment instructions and local installation requirements govern.