Choose what you need to keep running during an outage. The calculator adds up running watts and the largest starting surge — the two numbers to compare against any generator's spec sheet.
Add the running watts of everything you'll run at the same time, then add the single largest additional starting wattage among them. Your generator's running (rated) watts must cover the first number, and its starting (peak) watts must cover the total.[1]
Only one motor usually starts at a time, which is why only the largest starting surge is added.[2]
Quick start:
Add the appliances you need to run during an outage.
JavaScript is required for the interactive calculator. Appliance wattage reference:
Essentials
Appliance
Running W
+ Starting W
Light bulb, 60 W
60
—
LED light bulb (15 W)
15
—
Refrigerator / freezer
700
2,200
Water & pumps
Appliance
Running W
+ Starting W
Sump pump, 1/2 HP
1,050
2,200
Sump pump, 1/3 HP
800
1,300
Electric water heater (240 V)
4,500
—
Well pump, 1/3 HP
1,000
2,000
Heating & cooling
Appliance
Running W
+ Starting W
Furnace fan blower, 1/2 HP
800
2,350
Furnace fan blower, 1/3 HP
700
1,400
Space heater
1,500
—
Window AC, 12,000 BTU
3,250
3,950
Kitchen
Appliance
Running W
+ Starting W
Coffee maker
1,000
—
Microwave, 1,000 W
1,000
—
Microwave, 625 W
625
—
Toaster
850
—
Toaster oven
1,200
—
Electronics
Appliance
Running W
+ Starting W
Computer with monitor
800
—
Laptop
60
—
Phone charger
20
—
AM/FM radio
100
—
TV
400
—
Worked example
A refrigerator, a 1/2 HP sump pump and a 1/3 HP furnace fan, using the utility's typical values:[1]
Appliance
Running W
+ Starting W
Refrigerator / freezer
700
2,200
Sump pump, 1/2 HP
1,050
2,200
Furnace fan blower, 1/3 HP
700
1,400
Total
2,450
largest: 2,200
You'd need a generator rated for at least 2,450 running watts and 4,650 starting watts (2,450 + 2,200).
Running watts vs. starting watts
Running (rated) watts are what an appliance draws continuously. Starting watts are the extra burst a motor needs for the first two to three seconds — refrigerators, pumps and furnace blowers all need it; lights, heaters and electronics don't.[2] A generator lists both numbers: rated (continuous) output and maximum (starting) output.
How many watts to run a house?
One generator maker's guide puts essential items in a typical home at an average of 5,000–7,500 watts.[2] That range is a rough starting point — central air, electric water heaters and well pumps can push you well past it, and many of those run on 240 V. Use the calculator with your real appliances instead.
Before you plug anything in: the values above are typical figures, not your appliances. Check the nameplate or manual for exact watts. Never connect a generator to your home's wiring through an outlet — use a transfer switch installed by a licensed electrician. Why this matters.