Generator Wattage Calculator

Size a generator so it can both run your loads and handle motor start-up surges.

How the Math Works

The Generator Wattage Calculator uses a straightforward formula to determine the minimum generator capacity you need: size = running watts + largest starting surge. Running watts represent the continuous power consumption of your appliances when they're operating normally, while starting surge (or starting watts) is the brief but higher power spike that occurs when motor-driven devices like compressors or air conditioners kick on. By adding these two values together, the calculator ensures your generator can handle both the steady load and the temporary peak demand simultaneously.

Practical Applications

To use this calculator practically, first make a list of all devices you want to power and locate their running watt and starting watt ratings, typically found on the nameplate or in the manual. Sum the running watts of everything you might run at once, then identify the single largest starting surge among those items. Add that starting surge to your running total to get your minimum generator size. For safety, consider rounding up to the next common generator size and accounting for additional safety margins or future expansion.

Day-to-Day Use

This calculation prevents the frustrating scenario of buying a generator that's too small for your needs, which would result in tripped breakers or damaged appliances during startup. Whether you're preparing for hurricanes, camping, or running a workshop, knowing your true power requirements ensures reliable backup power when you need it most. It also helps you avoid costly mistakes of oversizing, which wastes money on unnecessary capacity while Undersized generators can fail when their starting surge capability is exceeded, creating dangerous power outages precisely when reliability matters most.

Worked example

3,000 running + 2,200 surge → 5,200 W.

FAQ