How this is calculated
A sump pump is a burst load: runtime over the outage is cycles per hour × seconds per cycle × hours. At the defaults (6 cycles of 25 seconds, 24 hours) the pump actually runs just one hour — about 800 Wh for a ⅓ HP pump. The battery figure adds a 25% margin, because the storm that takes the grid down is rarely the storm you measured on.
The number that disqualifies cheap hardware is the surge: an across-the-line pump motor kicks at roughly 2.5× running watts — 2,000W for a ⅓ HP pump — and it does so dozens of times a day, so the inverter needs that as repeatable headroom, not a one-off peak rating.
Solar deserves honesty here: the pump works hardest exactly when the sky produces least. Panels refill the battery between weather systems; the battery alone carries the outage. Size the battery for the storm and treat the panel as the recovery plan.
- Count your real cycles: tape a night’s storm on your phone, or count float trips for ten minutes of heavy rain and multiply.
- Check the pit and check valve before upsizing the battery — a leaking check valve doubles cycle counts for free.
- On municipal water, a water-powered backup ejector is the zero-electricity belt to the battery’s braces; on a well, the battery is the plan.
Sump, fridge, furnace fan and comms together are what an outage really asks — size the whole backup with the home backup power calculator, or read standby generator vs home battery.
Quick answers
How many watts does a sump pump use?
~800W at ⅓ HP, ~1,050W at ½ HP, ~1,500W at ¾ HP while running — with a 2–3× start-up surge each cycle.
How long will a 2 kWh battery run one?
About two heavy-storm days at typical cycling — the constraint is the inverter clearing the 2,000W+ surge, not the energy.
Why not just solar panels?
Storms and solar don’t overlap. The battery carries the outage; panels refill it when the sky clears.