Battery Backup Runtime: The Math That Matters

Realistic backup runtime comes from three numbers. Battery capacity, rated in amp-hours (Ah), tells you how much charge you're storing. Pump draw, in amps, tells you how fast the pump spends it. Duty cycle — the fraction of time the pump is actually running versus waiting — depends entirely on how much water is entering your pit during that particular storm. A pump drawing 10 amps from a 75 Ah battery has a theoretical continuous runtime around seven and a half hours, but pumps rarely run continuously; if your pit only demands pumping a third of the time, effective runtime stretches toward a full day.

Published runtime figures usually assume gentle conditions: a fresh, fully charged battery, modest inflow, and an ideal cycle pattern. Real batteries lose capacity as they age, cold basements reduce effective capacity further, and a severe storm can push the duty cycle far higher than the brochure assumes. Treat any runtime claim as a best case, and size your system so the best case is comfortably longer than the longest outage you consider plausible in your area. Some systems accept two batteries in parallel to double capacity — worth knowing before the forecast, not during it.

Most backup systems run on deep-cycle lead-acid batteries in two flavors. Maintenance-free (AGM or sealed) batteries cost more upfront but need no topping off, tolerate basement installation well, and are the lower-hassle choice for most homeowners. Traditional wet cell (flooded) batteries cost less and can deliver strong capacity, but they need periodic distilled-water top-offs and proper ventilation. Either way, plan on replacing the battery roughly every three to five years and testing the system quarterly — a backup pump with a dead battery is no backup at all.

The two battery backup pumps in our catalog take different approaches to the same job — compare their published pumping rates at head height, controller features, and battery requirements side by side before choosing. Whichever you pick, pair it with the battery capacity the manufacturer recommends and put the quarterly test on your calendar.