Generator Wattage Calculator

Electrical appliances require two types of power measurements: running watts and starting (or surge) watts. Running watts represent the continuous power an appliance needs to keep operating, while starting watts refer to the temporary surge of electricity required to fire up motors inside devices like refrigerators, air conditioners, and sump pumps. The core formula multiplies the operating current (amperes) by voltage to find baseline wattage, then applies a power factor (PF) adjustment for inductive loads. To find your total requirement, the calculator sums up all simultaneous running watts and adds the single largest starting wattage surge you expect to experience at any given moment.

Imagine you are preparing a backup power plan for a sudden grid failure. You need to run a refrigerator (800 running watts, 1,200 starting watts), a sump pump (1,000 running watts, 2,000 starting watts), a window AC unit (1,200 running watts, 2,200 starting watts), and 5 LED light bulbs (60 running watts total). To find the total running wattage, sum them all up: 800 + 1,000 + 1,200 + 60 = 3,060 running watts. Next, identify the single largest starting surge, which belongs to the AC unit at 2,200 watts. Add this surge to your total running watts minus the AC running wattage: 3,060 minus 1,200 equals 1,860, plus the 2,200 starting surge gives you a total required generator capacity of 4,060 watts.

Always account for inductive motor loads, as compressors and pumps require significantly more power to start than they do to run. Avoid running a portable generator near its absolute maximum threshold for extended periods, as this accelerates engine wear and reduces fuel efficiency. Finally, remember to factor in altitude and temperature extremes, which can reduce a combustion engine's net power output by up to 3.5 percent for every 1,000 feet above sea level.