Sump Pump and Backup Runtime Calculator

Usable basin volume = (π/4)·D²·h, where D is basin diameter and h is the vertical distance between the float switch's on and off levels. Fill time = volume ÷ inflow rate (how long it takes water to rise from the off level back up to the on level); pump-down time = volume ÷ (pump flow − inflow), using the net removal rate since inflow continues while the pump runs. Cycle time is simply fill time plus pump-down time.

Duty cycle = pump-down time ÷ cycle time — the fraction of each cycle the pump actually spends running. This is the key number that makes battery backup runtime very different from a naive (battery capacity) ÷ (pump power) estimate: a pump with a 20% duty cycle draws its full running power only one-fifth of the time, so a battery lasts roughly five times longer than a continuous-run calculation would suggest.

Frequent starts (commonly cited guidance suggests keeping starts under about 15–20 per hour, with at least a 1-minute run time and ideally several minutes of rest between starts) stress a pump motor's windings and starting components, since each startup draws a current surge well above normal running current. Short cycling is usually a sign the pump is oversized for the basin, or the basin/float range is too small for the actual inflow — widening the float switch range or using a larger basin both increase cycle time and reduce short-cycling risk.

Usable energy (Wh) = (Ah per battery × bank voltage × number of batteries) × (usable depth of discharge ÷ 100) × (inverter/system efficiency ÷ 100). Average power demand = pump running power × duty cycle (the pump's effective continuous draw, accounting for the fact it's not running all the time). Runtime = usable energy ÷ average power demand. Depth of discharge matters because most battery chemistries shouldn't be fully discharged — deep discharge shortens battery life and, for lead-acid batteries particularly, a full-capacity assumption significantly overstates real usable runtime.

Since average power demand scales with duty cycle, and duty cycle depends on the ratio of inflow to net pump-down rate, reducing inflow (better exterior grading, redirected downspouts, French drains) reduces the pump's average power draw directly — often more cost-effectively than simply adding more battery capacity to compensate for heavy, uncontrolled inflow.

A bigger basin (or a wider float switch range) increases the usable volume between on and off levels, which increases both fill time and pump-down time proportionally — the duty cycle percentage itself doesn't change, but the cycle time gets longer and the number of starts per hour drops, directly reducing short-cycling stress on the motor without needing a different pump.

Because inflow rate directly drives duty cycle — heavy rain or snowmelt can increase groundwater inflow into the basin dramatically compared to dry-weather baseline seepage, which increases the pump's duty cycle (and therefore average power draw) correspondingly. A battery that comfortably lasts a day or more during light, intermittent inflow might last only a few hours during a genuine heavy-storm event, which is exactly why backup sizing should be checked against a realistic worst-case inflow rate, not just typical conditions.

Use the actual flow at your system's real total dynamic head (vertical lift plus pipe friction losses plus any check valve loss), not the pump's maximum rated flow at zero head — pump curves show flow dropping substantially as head increases, and using the nameplate maximum will significantly overestimate real pump-down rate and underestimate cycle time and duty cycle.

It depends heavily on battery chemistry — lead-acid (AGM/flooded) batteries are typically limited to 50% depth of discharge for reasonable cycle life, while lithium (LiFePO4) batteries can often tolerate 80–100% depth of discharge with minimal life impact. Always use the specific manufacturer's recommended usable depth of discharge for your actual battery type rather than a generic assumption.