How Long Will a Battery Run a Sump Pump? (Real Math)
Short answer: a sump pump only runs when the pit fills, so runtime depends entirely on how hard it's raining, not just battery size. A 1/2 HP pump draws ~800–1,050 W running and surges to ~1,500–2,400 W at startup, but in a typical storm it runs only a few minutes per cycle. Energy use ranges from near-zero (dry) to several kWh/day in a downpour.
| Battery (usable) | Light inflow (~10 cycles/hr) | Heavy storm (~30 cycles/hr) |
|---|---|---|
| 0.5 kWh | ~10–14 hours | ~3–5 hours |
| 1 kWh | ~24 hours | ~7–10 hours |
| 2 kWh | ~2 days | ~14–20 hours |
| 5 kWh | ~5 days | ~1.5–2 days |
Assumes a 1/2 HP pump running ~30 seconds per cycle at ~1,000 W, with ~85% inverter losses applied. Your pit's inflow rate is the real variable — measure cycles during a real rain.
What moves the number
- Inflow rate decides everything. Count how often your pump cycles during a heavy rain — that, not the nameplate, sets your runtime. A high water table or a failing check valve (water draining back) can double cycles.
- Surge needs ~2,000 W of inverter. A 1/2 HP pump's startup spike trips undersized stations; aim for a unit rated ~2,000 W continuous or more so it starts every cycle reliably.
- The cruel timing: the storms that flood your basement are the same storms that knock out power — and 2024 set a decade record for outage hours (backup power statistics). Backup here is insurance against a very expensive, very predictable failure.
- Dedicated battery backup pumps exist but use small lead-acid batteries good for only hours; a 1–2 kWh power station typically outlasts them and powers other loads too.
Sizing it right
Match the inverter's surge rating to your pump first, then size capacity to your worst realistic storm — a 2 kWh station covers most overnight events, 5 kWh handles multi-day systems. A little solar input keeps you ahead of a long wet spell. Compare your options in best portable power stations and total your loads in the battery size calculator. Weighing fuel for long outages? See battery vs generator.
More runtime guides: refrigerator, CPAP machine, and well pump.