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Light Duty
A 5-kilowatt portable generator is adequate for operating essential equipment in most homes, including an electric furnace or free-standing heater. A generator of this size will also run some lights and a refrigerator. When it is in regular operation, it should be operating at about 50 percent of its load capability and will use about 1/2 gallon of gas per hour at that rate. Five-kilowatt generators are available that run on other fuel sources in addition to gasoline, most notably diesel. If more appliances are required, such as a separate freezer, either a larger generator should be used or the appliances can be alternated by plugging some in and unplugging others temporarily. This has the effect of having the appliances take turns using power, limiting the load at any one time.

Medium Duty
Generators that can produce 10 to 12 kilowatts of power per hour are commonly chosen for their ability to operate a heat source, such as a furnace, a refrigerator, freezer, TV and a computer simultaneously. Note that a surge protector is recommended for any sensitive electronic equipment such as computers and TVs and is also a good idea for any appliances to be run with the generator. Generators in this size range offer such fuel choices as natural gas and propane as well as diesel and gasoline.

Heavy Duty
When a generator will be expected to run an entire household during a power outage, it will need to be larger than 12 kilowatts, sometimes much larger. A large, heavy-duty generator capable of producing 25 kilowatts or more is required for situations where it must run normal household appliances as well as a well pump, a water heater, microwave oven, stove and other common items. These generators are available in models that can operate on any of the same fuels as the smaller models. They also feature such options as automatic start and are commonly connected directly to household wiring via a transfer switch that sends the power from the generator into the home without the need for extension cords. 

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