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Battery Backup Runtime Calculator
Quick answer: Battery runtime is roughly amp-hours × volts × usable fraction × efficiency ÷ pump watts. Enter your battery and pump draw, then the share of time the pump runs in a storm. You get continuous pumping hours and clock-time coverage. Treat results as estimates; cold, ageing batteries and heavy inflow shorten them.
The math
A 100 Ah, 12 V battery holds about 1,200 Wh. Using 50% to protect lead-acid life and 90% efficiency leaves about 540 Wh. A 240 W pump uses that in about 2.25 hours of continuous running. If it runs 20% of the time, clock coverage is about 11 hours. LiFePO4 batteries often allow 80–90% usable capacity; check the maker's guidance.
Finding pump watts
Use the backup pump's rated running amps × battery voltage, or a manufacturer runtime table if one is published — trust the table over this estimate.
Worked example
Two 75 Ah batteries in parallel (150 Ah), 12 V, 50% usable, 90% efficient = 810 Wh. An 18 A pump (216 W) at 30% duty: 810 ÷ 216 ≈ 3.75 h pumping, ≈ 12.5 h clock time.
Illustrative note (2026-10-29): Figures in this example were prepared on 2026-10-29 for illustration and are not product data.
Limits
No backup guarantees a dry basement. Have a licensed electrician handle new outlets or circuits.