Generator Size Calculator

Generator sizing ensures your generator can power all the devices you plan to run simultaneously without overloading. Undersizing causes shutdowns and can damage equipment; oversizing means unnecessary fuel cost.

The Formula:

Required wattage = Sum of running watts + Largest single starting surge

Running watts = steady-state power draw Starting (surge) watts = 2–3× running watts for motor-driven appliances

Common Appliance Wattage Reference:

Worked Example:

Emergency power needs:

• Refrigerator: 400 W running, 1,200 W start
• Window AC: 1,200 W running, 2,400 W start
• Lights: 150 W
• Phone chargers, TV: 200 W

Total running watts: 400 + 1,200 + 150 + 200 = 1,950 W

Now the surge, and this is the step almost everyone gets wrong. The window AC starts at 2,400 W, but 1,200 W of that is already counted in the running total, because the AC is one of the things running. What the generator has to find on top is the extra 1,200 W, not the whole 2,400.

Largest surge above running = 2,400 − 1,200 = 1,200 W

Peak demand: 1,950 + 1,200 = 3,150 W → a 3,500 W generator covers it

Add the derated-for-continuous-use margin and you are looking at 4,000 W, which is still a size below what the naive sum suggests. Over-buying a generator is not free: a bigger unit burns more fuel at the same load, costs more, and weighs more than one person can move.

Only one appliance is assumed to start at a time, which is why it is the LARGEST surge and not the sum of all of them. Two motors starting in the same second is possible but unlikely, and designing for it costs a whole size category.

Generator Size Categories:

Practical Tips:

• Never run a generator indoors or in a garage. Carbon monoxide kills silently, and a cracked door is not ventilation
• Run the generator under load. Light loads cause “wet stacking” in diesel units, where unburnt fuel glazes the cylinders
• Portable inverter generators are quieter and fuel-efficient for smaller loads