Generator Wattage Calculator
Size a backup generator for a power outage by adding up the running and starting watts of the appliances you must keep on.
How it works
Peak = sum of running watts + largest single starting surge · Recommended = peak × (1 + reserve%)
Motor-driven appliances like fridges, pumps and air conditioners briefly draw two to three times their running wattage when they start, so a generator has to cover the running load of everything plus the surge of the biggest motor kicking in. Because motors rarely all start at the same instant, adding just the single largest surge — not every surge at once — sizes the generator realistically. A reserve on top gives headroom so the generator is not pinned at 100% for hours, which shortens its life and risks overload.
Worked example
One fridge (700 running / 2200 starting), one sump pump (800 / 1600) and six 100 W lights run at 700 + 800 + 600 = 2,100 W. The largest surge is the fridge at 2200 − 700 = 1,500 extra watts, so peak is 3,600 W. With a 20% reserve, aim for about 4,320 W — a 4.3 kW or larger generator.
Frequently asked questions
What size generator do I need for a house?
It depends on what you must keep running. A few essentials — fridge, some lights, phone charging, a furnace fan — often fit within a 3,000-5,000 W portable generator. Powering central air conditioning or an electric range pushes you toward 7,500 W or a whole-house standby unit. Add up your specific loads rather than guessing.
Why do starting watts matter?
Appliances with electric motors draw a large surge for a fraction of a second when they start — often two to three times their running draw. If the generator cannot supply that surge, the appliance will not start and the generator may trip. Sizing to peak (running load plus the biggest surge) prevents that.
Should I get a bigger generator than I need?
A modest reserve is wise so the generator is not maxed out constantly, which runs hot and wastes fuel. But a hugely oversized generator costs more, burns more fuel at light load, and can suffer from "wet stacking." A 10-25% reserve above your peak is a reasonable balance.
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This calculator is for educational purposes. Double-check important results before acting on them.