What size generator do I need for a well pump?

The surge is the sizing number

A pump motor draws its heaviest current in the first moments of starting, before it is turning at speed. The calculator models that surge at 2.75× the running watts, and its generator recommendation is built from the surge: take the starting watts and round up to the next half-kilowatt step. A generator that can carry the pump's running load but not its surge will stall the motor or trip — the pump never starts, and the running-watts figure on the spec sheet never gets a chance to matter.

Running watts still earn their keep in the plan: they set the pump's share of the generator's continuous capacity while it runs, and they drive fuel use. But the purchase decision — which generator class to buy — is made by the starting number.

Sizes by horsepower, from the calculator

These are the calculator's outputs for its standard case — 100 ft of vertical lift, 50 PSI target pressure, 1.25 in pipe, 230 V supply:

• Notice the running column: even the 2 HP pump runs on less than a space heater. It is the surge column — and nothing else — that pushes these pumps into multi-kilowatt generator territory.
• Generators are sold on both a continuous and a surge rating. The calculator's recommendation keeps the surge inside the generator's headline size with headroom to spare, which is the conservative way to shop.

Depth and pressure are the hidden multipliers

The calculator does not size from horsepower alone. It builds the pump's total dynamic head — vertical lift, plus pressure head at 2.31 ft per PSI, plus pipe friction — and derives watts from the flow against that head. In the 1 HP example above, the head totals 218.6 ft: the 100 ft lift, 115.5 ft of it just from holding 50 PSI, and the rest friction. The pump is working against more than double the well's depth before friction is even counted.

Push the same 1 HP pump down a 250 ft well at 60 PSI and the head reaches 395.3 ft: running watts climb to 1,365 W, starting watts to 3,755 W, and the recommended generator to 4 kW — the size the table gives a 2 HP pump on a shallow well. Pipe size pulls the other way on friction: in the calculator's friction data, a 1 in pipe loses 6.3 ft of head per 100 ft at 10 GPM, against 1.2 ft for a 1.5 in pipe.

The pump shares the generator with everything else

The recommendation above covers the pump alone. On a real outage the generator also carries the refrigerator, lights, and whatever else made your essentials list — and two motors starting in the same moment stack their surges. The whole-home backup calculator totals a house's continuous and surge loads together; run the pump through it as part of the full list before treating the table's answer as your purchase. Then the generator fuel guide on this hub turns the chosen size and load into gallons and hours of runtime.