Choose the right sump pump capacity for your basement.
Sump pumps are sized from measured inflow, not guesswork. Work out how many gallons an inch of pit depth holds — pit area in square inches divided by 231 — then time how fast the water actually rises during heavy rain, and multiply. An 18 inch pit holds about 1.1 gallons per inch, so a 2 inch per minute rise is 2.2 gpm, or 132 gallons an hour. Add a safety factor of 50% because the storm you must survive is worse than the one you measured. The catch is that the flow figure on the box is at zero lift; a pump rated 3,000 gph delivers perhaps 2,000 at 10 ft of head, so always read the pump curve at your total dynamic head, which is the vertical lift plus about 1 ft of friction per 25 ft of pipe.
Sump Pump Size
Gallons per inch = pit area sq in / 231; inflow gpm = gallons per inch x rise rate; required gph = inflow x 60 x (1 + safety%); total head = lift + pipe run x 0.04
Whichever meets your measured inflow at your actual head. A 1/3 hp pump handles most basements with under 10 ft of lift; 1/2 hp is for higher lift or heavier inflow, and it is not automatically better — an oversized pump short-cycles.
If a flooded basement would be costly, yes — storms and power cuts arrive together, which is precisely when the pump is needed. A water-powered or battery backup is cheap relative to one flood.