Dim Your Bill

04Generators

Generator sizing calculator

Pick everything that must run at the same time. We add the biggest motor's starting surge and 20% headroom.

Checked

What must run at the same time
UseItemRunning / starting W
Refrigerator (modern, full size)200 / 1,200
Chest freezer100 / 600
Sump pump, 1/3 hp800 / 1,300
Well pump, 1/2 hp (240 V, needs a transfer switch)1,000 / 2,100
Gas furnace blower (needs a transfer switch)600 / 1,400
Window AC, 8,000 BTU700 / 2,000
Microwave (1,000 W cooking)1,500 / 1,500
Coffee maker900 / 900
LED lights (5 bulbs)45 / 45
Wi-Fi router and modem20 / 20
TV (55-inch LED)100 / 100
Washing machine500 / 1,200
Garage door opener550 / 1,100
Circular saw1,400 / 2,300

—running watts

Starting watts needed
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Size to shop for
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Adds every running watt plus the single largest motor’s extra starting surge, then 20% headroom. Anything hard-wired (well pump, furnace) needs a transfer switch or interlock installed by an electrician.

Why starting watts matter

Motors in fridges, pumps, air conditioners and garage door openers draw two to three times their running power for a second or two when they start. A generator has to cover that surge, but only for one motor at a time if you let things start one by one. That’s why the calculator adds only the single largest surge.

Watts are typical values from manufacturer charts. The nameplate on your appliance is better: look for watts, or multiply volts by amps.