Generator Sizing Calculator — Standby kW

It depends entirely on which loads you intend to run, not on the square footage. A typical essential-circuits set — furnace or air handler, well pump, refrigerator, lighting and outlets, sump pump — runs around 9,000 to 10,000 W, which usually lands on a 20 kW generator once the motor-starting surge is accounted for. Whole-house coverage with central air and electric cooking often needs 24 kW or more.

Because starting a motor draws several times its running current for a second or two. If your loads total 9,200 W but the air conditioner alone needs 15,000 W to start, the generator has to supply the other 4,200 W and that 15,000 W inrush at the same moment — 19,200 W. On any house with an air conditioner, heat pump, or well pump, the starting requirement usually governs rather than the running total.

Running watts is the steady draw once a load is up to speed. Starting watts (or surge) is the brief inrush a motor demands to overcome inertia — commonly 2 to 3 times running watts for a typical induction motor, and higher for a hard-starting compressor. Resistive loads like heaters, lights, and ovens have no surge at all: running and starting watts are the same.

Often, yes — and it is frequently cheaper than the next generator size up. A soft starter or an inverter-driven compressor reduces inrush dramatically, which can move the governing requirement from the starting path back to the running path. If this calculator tells you motor starting is what set your size, a soft starter on the largest motor is the first thing to price.

Yes. A permanently installed standby generator requires a transfer switch so utility and generator power can never be connected simultaneously — that is a life-safety requirement for line workers as much as for the equipment. An automatic transfer switch starts the set and switches over on its own; a manual switch is cheaper but needs someone present. Backfeeding a panel through a receptacle is extremely dangerous and never permitted.

Essential circuits is usually the better value — a smaller set, a smaller gas supply, and a lower install cost. Whole-house makes sense where central air conditioning matters in summer, where the house is all-electric, or where an occupant depends on medical equipment. A middle route is whole-house coverage with a load-shedding module that drops the air conditioner when the range is in use, letting a smaller generator serve everything.

For sizing purposes, treat 1 kW as 1,000 W of load. Two caveats: generator ratings are usually given for standby duty rather than continuous prime duty, so a set rated 20 kW standby should not be run flat out indefinitely — which is exactly what the margin in this calculator protects. And a set's rating derates with altitude and high ambient temperature, typically a few percent per 1,000 feet.