Sump Pump Size, Pit Volume, Cycles, Discharge Head, Freeze Protection, and Battery Backup
Pump horsepower for your inflow and your lift, the pit volume and how often it pumps, the discharge pipe size with friction loss and velocity, the freeze plan, and how many gallons a battery backup actually moves.
A sump pump is sized on two numbers: how fast the pit refills and how high and how far the water has to be pushed. Inflow is what you actually see: watch the pit on a wet spring day and time the level rise, then divide the volume it gains by the minutes. Vertical lift is measured from the water line at pump start to the highest point of the discharge line, and every elbow, check valve, and 40 ft of pipe adds head on top of it.
How the math works
- Duty flow: design the pump at about 2 times the measured inflow with a 10 gpm floor, then read the pump curve at your head. A pump sized at the inflow alone runs continuously and dies of duty, not of age.
- Total head: vertical lift from the water line to the top of the discharge line, plus pipe friction, plus about 1.5 ft for the check valve and elbows. Friction on PVC at 10 gpm is about 3.1 ft per 100 ft on 1-1/4 in, 1.2 ft on 1-1/2 in, and 0.6 ft on 2 in, scaling with flow to the 1.85 power.
- Horsepower: water horsepower = gpm times head divided by 3,960; brake horsepower = water horsepower divided by pump efficiency, about 0.34 on a small submersible. 1/6 HP is a light pit, 1/3 HP is the utility sump, 1/2 HP is a deep basement with a long run, 3/4 to 1 HP is a sump with a lift station.
- Cycle math: pit volume = 3.14 times (diameter divided by 24) squared times working depth in ft times 7.48. Drawdown on a 6 in level differential in a 24 in pit is about 15.7 gallons. Cycles per day = 1,440 divided by refill minutes.
- Backup: usable battery energy is about half of rated watt-hours, so a 12 V 100 Ah battery gives about 600 Wh, which is 600 divided by pump watts of run time at the duty flow.
What size sump pump do I need for a basement?
A 1/3 HP submersible that delivers 20 to 30 gallons per minute at 10 ft of head is the standard house answer, and it is the right pump when the pit refills slower than about one cycle every 20 minutes. Size on the measured inflow and your lift: 2 times the inflow at your head, read off the pump curve. A basement that takes in 1 gallon per minute on a spring thaw needs a bigger pump and a battery tower, not a bigger box store pump.
How many gallons per minute should a sump pump handle?
Between 3 and 30 gpm in a residential pit, and most basements sit between 0.5 and 3 gpm in wet season. Time the pit: measure the diameter and the level rise over ten minutes, convert to volume, and divide by the minutes. A pit that gains 4 inches in ten minutes in a 24 in pit is about 1.1 gpm.
Is a 1-1/2 in or 2 in discharge pipe better?
1-1/2 in on a short residential run, 2 in on a run over 40 ft or a lift over 12 ft. Friction on PVC at 20 gpm is about 12 ft of head per 100 ft on 1-1/4 in, 4.5 ft on 1-1/2 in, and 2 ft on 2 in, and head is what sizes the pump. Keep velocity between 3 and 10 ft per second so the line neither silts up nor scours its joints.
Why does my sump pump freeze in winter?
The discharge line freezes, not the pump. The line has to drain itself back into the pit when the pump stops: a loop-the-loop at the pump, a 1/8 in weep hole in the riser below the check valve, the horizontal run below the frost line, and a visible daylight end. A check valve with a full line above it and no weep hole is a frozen line in the making.
How long will a battery backup run a sump pump?
A 12 V 100 Ah deep-cycle battery gives about 600 usable watt-hours. A 1/3 HP sump drawing 250 watts runs about 2.4 hours on it, about 2,400 gallons, and about 15 pump cycles. A 1,000 gpm storm surge outlasts one battery, which is why the answer to a fast-filling pit is a second pump on a separate discharge with a float-activated tower.