Generator Sizing

Sizing a Generator

To determine the size of the generator needed, total the rated watts of the appliances and fixtures you’ll want to operate during an outage. Some loads are easy to determine -- a 100-watt light bulb, for example, uses 100 watts. Ten 100 bulbs would require 1,000 watts, or 1 kilowatt (KW). The power requirements for appliances are often provided in the operating manual. These specifications are also stamped on the “face plate” along with serial and model numbers.

While the power needs of individual appliances vary, those that produce heat or use large motors tend to require higher wattage input.

An average 14 cubic-foot refrigerator with automatic defrosting, for example, requires about 615 watts while an electric range at 12,220 watts needs nearly 20 times as much power. Electric space heaters and water pumps are usually rated at 1,000 to 1,800 watts (1-1.8 KW) and electric water heaters require 2,500 to 4,500 watts (2.5-4.5 KW).

As you can see, a household’s power requirements can quickly add up.

Motors Require Extra Calculations

Power needs for motors, such as those powering well pumps or furnace fans, are more difficult to determine. Electric motors require more current when starting than they do in continuous operation-as much as three to six times more.

Motors are rated by horsepower (hp). A one-horsepower motor requires about one kilowatt to run and a surge of five kilowatts to get started. An electric motor rated for two-horsepower needs twice as much power. So while a motor-powered appliance may run with only one kilowatt, the generator must be able to provide five kilowatts to get it started. Again, those ratings appear on the motor’s face plates. Without sufficient starting power, motors may overheat, burn out or trip the generator’s circuit breaker.