Home Generator Sizing Calculator

Add up the running watts of everything you want to power at once, then add the extra starting surge of the single largest motor load (such as a refrigerator or well pump), since only one big motor typically starts at a time. Add a headroom margin so the generator is not run flat out. The result is the minimum starting watts the generator must supply.

Running watts is the steady power a device draws once operating. Starting watts is the brief surge a motor needs to spin up, often two to three times its running watts. Appliance labels and nameplates list both. A generator must supply the total running watts plus the biggest single starting surge at the same time.

Motor-driven appliances rarely all start at the exact same instant, so the worst case is usually your running load plus the surge of the one largest motor kicking in. This calculator adds the single biggest starting surge to the total running load, which is the standard sizing assumption for backup generators.

Running a generator continuously at its maximum rating shortens its life and leaves no room for an extra load. A margin of around 20 to 25 percent above the calculated peak is common, so the generator runs at a comfortable load. Enter the margin you prefer; the calculator applies it to the peak demand.

Watts equal volts times amps for a resistive load (and times the power factor for motors). If an appliance lists amps and volts instead of watts, multiply them to estimate running watts. For backup sizing, work in watts and confirm the generator and any transfer switch are rated for your voltage and total load.