Well Pump Backup Runtime & Recovery Calculator

Drawdown = P₁V/P₂ − P₁V/P₃, where P₁ is pre-charge pressure, P₂ is cut-in pressure, P₃ is cut-out pressure (all in absolute pressure, i.e. gauge pressure plus atmospheric), and V is the tank's total volume. As the tank fills with water, the air cushion above it compresses and its pressure rises; drawdown is the difference in that air volume between the cut-in and cut-out pressure points — not the tank's total physical size.

Most of a bladder or diaphragm pressure tank's volume is taken up by the compressed air cushion, not water — the acceptance factor (drawdown ÷ total tank volume) is typically only around 25–33% for common residential pressure settings. A '44-gallon' tank might deliver only 12–14 gallons of actual usable water before the pump needs to cycle back on, which is exactly why tanks should be sized by required drawdown, not by the number printed on the shell.

The pre-charge (the air pressure in an empty tank before any water enters) should be set about 2 psi below the pump's cut-in pressure. If pre-charge is too high, the tank accepts very little water before reaching cut-in, drastically reducing usable drawdown; if set correctly, the full pressure range between cut-in and cut-out is available for water storage.

Most residential well pumps should run for at least 1 minute per cycle (2 minutes or more for 2 HP and larger motors) to avoid short-cycling, which stresses the motor windings and starting components with repeated high-current startup surges. Drawdown volume divided by pump flow rate gives the minimum run time, corresponding to essentially no water being used while the pump runs. When household demand continues during a cycle, the tank refills at the net rate (pump flow − usage rate), so the actual pump-on time is longer. The short-cycling warning therefore uses the conservative no-demand minimum.

Unlike a sump pump (driven by external inflow), a well pump's cycling is driven by household water demand — higher usage means more frequent cycling and a higher duty cycle, while lower usage stretches out the time between cycles. During pump-off time, the tank empties at the household usage rate; during pump-on time, it refills at pump flow minus that continuing usage. For a steady continuous-equivalent demand, this gives a duty cycle equal to average usage rate divided by pump flow rate. The calculator then applies that duty cycle to the usable battery energy to estimate outage runtime.

Mathematically, drawdown does increase as pre-charge approaches cut-in — but that number becomes misleading in practice. Standard guidance is to keep pre-charge at least about 2 psi below cut-in; set any closer and tank pressure can crash abruptly to zero right as the pump is about to turn on, rather than declining smoothly through the cycle, even though the calculated drawdown figure still looks favorable. This calculator flags it if your entered pre-charge gap is too small for exactly this reason.

Because most of a pressure tank's volume is the compressed air cushion that pushes water out, not water storage itself — the acceptance factor (drawdown as a fraction of total tank volume) is normally only around a quarter to a third of the nameplate size for typical residential pressure settings, which is a well-known point of confusion when people assume a tank's printed size directly represents its water capacity.

30/50 psi is the more common residential default and gives slightly more usable drawdown per gallon of tank size at the same tank; 40/60 psi delivers somewhat higher water pressure at fixtures (useful in multi-story homes or where pressure loss to distant fixtures matters) but yields a lower acceptance factor, meaning a larger tank is needed for the same usable drawdown. Check what your existing pressure switch is set to, or what your installer specifies, before changing it.

A simple starting point is your average daily water use (from a utility bill in gallons, or an estimate of roughly 80–100 gallons per person per day for typical US households) divided by 1,440 minutes in a day — this gives a continuous-equivalent average rate, not the much higher instantaneous flow rate while a specific fixture is running, which is the right basis for a duty-cycle backup runtime estimate.

Well pump duty cycle is driven by household water demand, which for most homes averages out to a fairly low continuous-equivalent rate even though individual fixtures draw much more water while actually running — a sump pump's duty cycle, by contrast, is driven by continuous groundwater inflow that doesn't pause the way household water use does, which is why sump pumps often show a higher duty cycle for a comparably sized pump and storage volume.