Standby Generator Engine

Just like an automobile, most standby generators have an internal combustion engine, complete with pistons, cooling fans, and spark plugs. But instead of propelling a pair of axles, the engine's sole purpose is to spin the alternator's rotor at 3,600 revolutions per minute, making a stream of of appliance- and light-friendly electricity in the process. Smaller units (those producing less than 16 kilowatts) generally have air-cooled engines; more expensive water-cooled engines will work harder and longer without complaint.

Just like a car, a generator has oil to change, filters to replace, and, if it's water-cooled, antifreeze to check. These tasks must be done every two years, even if you never use the unit in an emergency, or every 50 to 200 hours of running time if you do. If you'd rather not mess around under the hood, so to speak, sign a maintenance contract with a dealer who specializes in your brand of generator.

How Big Should It Be?

A generator's output, measured in kilowatts, needs to be enough to supply all the electrical devices you want to keep running in an emergency, plus a 20 percent cushion. That extra power means the engine doesn't have to operate at top speed to meet the expected demand. It also assures there's enough reserve power for the extra surge that electric motors, such as those in air conditioners and refrigerators, need to get started.

To figure out how big your generator should be, make a list of all the appliances you want to supply with emergency power, then use the online calculator at thisoldhouse.com/shortcuts. You can get a sense of your household's power needs from the list of common appliances below.

Running Watts
Start-up Watts
700
Refrigerator
2,800

750

Lights (10 at 75 Watts)
0
800
Television (large)
0
500
Gas furnace
(with 1U2-hp fan)
1,250

5,000

Central air conditioner
(3 ton)
12,500
2,000
Window air conditioner
6,000
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