Generator Sizing Calculator

Size a backup generator for your home or jobsite by totaling running and starting wattages of all your appliances.

Results

Visualization

How It Works

Generator sizing requires knowing both running watts (continuous load) and starting watts (surge when motors kick on). The largest motor in your load list will create a temporary surge of 2-3x its running wattage. A 25% headroom above peak starting watts protects the generator from overload.

The Formula

Starting W = Running W + Largest Motor x (Surge - 1). Recommended = Starting W x 1.25.

Variables

  • Running W — Sum of all appliance running wattages
  • Largest Motor — The highest-wattage motor load (fridge, pump, furnace, etc.)
  • Surge — Motor starting multiplier (typically 3x for most motors)
  • 1.25 — 25% safety headroom above peak starting load

Example

Fridge (700W) + sump pump (1,500W) + furnace (700W) + lights (500W) + microwave (1,200W) = 4,600W running. Largest motor is the sump pump at 1,500W x 3 = 4,500W surge. Starting watts = 4,600 + 1,500 x 2 = 7,600W. With 25% margin: 9,500W. Get a 10,000W generator.

Tips

  • Always size by starting watts, not running watts -- a generator that cannot handle motor surge will trip its breaker.
  • Consider which appliances truly need to run simultaneously during an outage.
  • Inverter generators produce cleaner power for sensitive electronics but cost more.
  • For whole-house generators, a licensed electrician must install a transfer switch.
  • Run your generator at 50-75% load for best fuel efficiency and engine longevity.