What size power station do you need to run a sump pump for 24 hours?
Short answer: 2,259 Wh of rated capacity — that's the ~3–4 kWh — extra-large class. Sump pumps fail exactly when storms knock the power out. The surge draw is the sizing constraint.
The math, appliance by appliance
Appliance
Avg draw
Hours used
Energy
Sump pump (1/3 HP)
800 W
2 h
1,600 Wh
Energy needed per day
1,600 Wh
÷ 0.85 inverter efficiency
1,882 Wh
× 1.2 headroom (ageing, cold, overrun)
2,259 Wh
Recommended: ~3–4 kWh — extra-large class. Output check: 800 W continuous, 1,800 W surge.
Fridges and freezers use their average cycling draw for energy and full compressor draw for the output check — that's why the numbers differ from the label.
The bare energy need is 1,882 Wh after inverter losses; the extra 20% covers battery ageing, cold weather, and outages running long. A smaller unit works on a good day — this size works on a bad one.
What about output watts, not just capacity?
The station must sustain 800 W continuous and survive a 1,800 W start-up surge. Nearly every unit in the ~3–4 kWh clears both comfortably.
Could solar keep this running indefinitely?
Roughly 533 W of panels replaces a day's usage with ~4 hours of decent sun at real-world (75%) efficiency. Half that solar doubles your runtime instead of sustaining it.