Generator Size Calculator

How to use the generator size calculator

Enter the total running watts of everything you’ll power at once and the starting surge of the single largest motor-driven appliance, and you get the recommended generator size with 20% headroom built in. The surge field is the extra spike the biggest motor needs to start — an air conditioner, refrigerator, or well pump.

Running versus starting watts is the number-one generator-sizing mistake. Motor-driven appliances draw a brief surge — often two to three times their running watts — to get spinning. Size only to running watts and the generator stalls the instant the AC compressor or well pump kicks in. You add the surge of just the largest motor, because they rarely start at the same moment.

To total the running watts, add up everything you genuinely need on at once: lights, the fridge, a furnace fan, phone chargers, a few outlets. Be honest about “at once” — you seldom run the microwave, air conditioner, and well pump simultaneously. The appliance-energy calculator’s preset wattages make a good starting checklist for the essentials.

The 20% headroom matters because a generator shouldn’t run at 100% of its rating. Flat-out operation runs hot, stresses the engine, burns more fuel per watt, and leaves nothing for the next thing you plug in. Sizing to 120% of peak demand keeps it in its efficient, durable band and gives you margin. A generator run constantly at its limit wears out faster and runs less economically.

Treat the result as a target, then round up. Generators come in standard sizes — 3,500, 5,000, 7,500 watts and so on — so pick the next unit above the recommendation. For whole-home backup, a load calculation by an electrician and a transfer switch are the right route; this tool sizes a portable generator for the essentials you choose to keep running.

The formula

Peak demand is your running load plus the single largest starting surge; the recommended size adds 20% headroom on top:

peak demand = running watts + largest starting surge
recommended = peak demand × 1.20

Worked example with the defaults — 3,000 running watts plus a 2,200 W starting surge: peak = 5,200 W; recommended = 5,200 × 1.20 = 6,240 W. Round up to the next standard generator (likely 7,000–7,500 W).

Only the single largest motor’s surge is added, because motors rarely start at the same instant. Drop the surge to zero (no motor loads) and the recommendation is simply running × 1.2 = 3,600 W.

Frequently asked questions