Generator Size Calculator

Size a portable or standby generator from the running and starting watts of what you'll power.

Appliances: name, running watts, starting watts	Fridge, 700, 2200 Sump pump, 800, 1300 Lights, 300, 0 Microwave, 1000, 0
Safety margin	20 %

Appliances: name, running watts, starting watts

Safety margin

Recommended generator

5.2 kW

Running load 2,800 W plus the largest extra starting surge 1,500 W = 4,300 W peak, plus a 20% margin. That's about 6.5 kVA at a 0.8 power factor.

Running watts

Fridge700 W (start 2,200 W)
Sump pump800 W (start 1,300 W)
Lights300 W
Microwave1,000 W

About the Generator Size Calculator

Appliances with motors (fridges, pumps, air conditioners, furnace fans) briefly draw 2–3 times their running power when starting. A generator must cover the total running load plus the largest single starting surge, assuming motors don't all start at once. Check each appliance's label for its exact wattage. Never run a generator indoors or in a garage: carbon monoxide kills.

The formula

Size = (Σ running W + largest (starting − running) W) × (1 + margin)

Typical appliance watts (check your labels)

Appliance	Running W	Starting W
Refrigerator	700	2,200
Sump pump (½ hp)	1,050	2,150
Window AC (10,000 BTU)	1,200	3,600
Furnace fan (½ hp)	875	2,350
Microwave	1,000	0
Space heater	1,500	0

Typical values from generator manufacturers' wattage guides; actual figures vary by model.

Worked example

Example: Recommended generator = 5.3 kW

Inputs: Appliances: name, running watts, starting watts: Fridge, 700, 2200; Sump pump, 800, 1300; Lights, 300, 0; Microwave, 1000, 0; Safety margin: 22%.
Formula: Size = (Σ running W + largest (starting − running) W) × (1 + margin)
Result: Recommended generator = 5.3 kW

Questions people ask

Running vs starting watts?

Starting watts are the brief surge as a motor spins up.

Do I need a transfer switch?

To connect to home wiring, yes, installed by an electrician; backfeeding can kill line workers.