Generator Size Calculator

Tap everything you'd run at the same time: the size you need adds up as you go.

Wattages from a utility's sizing guide (EWEB) and Generac. Free, and nothing you enter leaves this browser.

Generator safety (CPSC): run a portable generator outdoors only, at least 20 feet from homes and buildings, with the exhaust pointed away from windows, doors and vents. Never run one in a home, garage, basement, crawlspace or shed, even with the doors open: generator exhaust contains carbon monoxide, which you can't see or smell and which can kill within minutes. Put battery or battery-backup CO alarms on every level and outside sleeping areas, and test them monthly.

Never plug a generator into a wall outlet to power the house ("backfeeding"): CPSC says it can electrocute utility workers and neighbors and bypasses some of your home's built-in circuit protection. Look for generators with a CO shut-off; CPSC points to models certified to PGMA G300-2023 and UL 2201-2023.

How generator sizing works

  1. List everything you want running at the same time and add up the running watts. If a label only gives amps, multiply by 120 volts.
  2. Find the one item with the biggest extra surge when it starts. Motors (refrigerators, pumps, air conditioners, furnace fans) need a burst of power for a moment.
  3. Add that one surge to the running total. That's your starting (peak) watts. The method assumes only one motor starts at a time.

Generac says inverter generators are usually chosen for their compact size, quiet running and clean power for sensitive electronics, while conventional generators give more power for the money and come in a much wider range of sizes.

Generator wattage chart

Typical values. Your own appliances' labels are more accurate.

ApplianceRunning wattsExtra starting watts
Refrigerator/freezer7002,200
Microwave (625 W)625-
Microwave (1,000 W)1,000-
Coffee maker1,000-
Toaster850-
Toaster oven1,200-
Slow cooker250-
Blender1,000-
Sump pump (1/3 hp)8001,300
Sump pump (1/2 hp)1,0502,200
Well pump (1/3 hp)1,0002,000
Furnace fan blower (1/3 hp)7001,400
Space heater1,500-
Electric water heater4,500-
Window air conditioner (12,000 BTU)3,2503,950
LED light bulb (15 W)15-
Light bulb (60 W)60-
LED/LCD TV120-
AM/FM radio100-
Cell phone charger20-
Laptop60-
Computer with monitor800-
Hair dryer1,500-
RV air conditioner (15,000 BTU)1,800about 1,800 (estimate)

Sources: EWEB generator sizing guide (running and extra starting watts); slow cooker, blender, LED/LCD TV, hair dryer and RV air conditioner from Generac's wattage worksheet, which lists running watts only; the RV air conditioner's starting surge is estimated with Generac's rule that starting watts are typically twice the running watts.

Generator sizing FAQ

How do I figure out what size generator I need?
List what you want to run at the same time and add up their running watts. Then find the one item with the biggest extra surge when it starts (usually a motor such as a refrigerator, pump or air conditioner) and add that surge once. The total is the smallest generator that will do the job. If a label only lists amps, multiply by 120 volts to get watts.
Why do refrigerators and pumps need more watts to start?
Motors draw a short surge when they start. EWEB's chart lists a refrigerator at 700 running watts plus 2,200 extra to start, and Generac says starting watts are typically twice the running watts. The usual method assumes only one motor starts at a time.
Can I plug my generator into a wall outlet to power the house?
No. CPSC calls this backfeeding: it can electrocute utility workers and neighbors on the same transformer, and it bypasses some of your home's built-in circuit protection.
How far from the house should a generator be?
CPSC says to run it outdoors only, at least 20 feet from homes and buildings, with the exhaust pointed away from windows, doors and vents. Never run one in a garage, basement, crawlspace or shed, even with the doors open.

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