Select the Right Portable Generator after a Disaster

Publication Number 490-303, August 1996

General

Power outages can leave a home without power for lighting, cooking, refrigeration and pumping water. Portable generators can be bought to provide substitute power. However, the generator must be properly sized to start the appliances and equipment you want to run.

What To Do

(For 120-volt, plug-in appliances)

First, find the wattage of the appliance(s) you want to run by checking the nameplate. Motor-driven appliances may be listed in horsepower which must be converted to watts. Motors require four times as much power to start as they do to run. If the running wattage of a motor is 400, then the starting wattage will be 1,600. The following table gives some starting and running wattage for electrical motors:

Table 1

Watts Required

Motor, hp		To start		To run
1/6 1/4 1/3 1/2 1 5 7.5 10		1,000 1.500 2,000 2,300 4,ooo 18,000 28,000 36,000		215 300 400 575 1,000 4,500 7,000 9,000

Now determine how many appliances you want to run at the same time and add or total the wattage. The size of the generator you use must be such that it will start and run the necessary appliances. If you get a generator that is too small to run refrigerators and freezers, they will try to start, but the voltage will drop and their motors will overheat and burn out. If you cannot find the wattage, an estimate can be made from the following table:

Table 2

Typical Equipment Wattages

Essential home equipment		Typical wattage
	Refrigerator Freezer Furnace blower		400-800 600-1,000 400-600
Optional home equipment		Typical wattage
	Electric skillet Electric stove Washing machine Water pump Water heater Electric fan Central air conditioner		1,150-1,500 3,000-4,000 400 800-2,500 1,000-5,000 75-300 2,000-5,000
Farm equipment		Typical wattage
	Ventilator fans Silo unloader Feed mixing Feed conveyor Bulk milk cooler Electric fence		300-800 2,000-7,500 800-1,500 800-5,000 1,500-12,000 7-10

For example, if you want a generator to run a refrigerator and a freezer, the wattage (table 2) of the refrigerator would be 800 and the freezer would be 1,000. To select the correct size generator, you decide if both refrigerator and freezer are to start at the same time. If so, you would need (1,800 X 4) 7,200 watts. You would select the nearest larger wattage generator. If you can be certain both appliances will not start at the same time, you would only need 4,800 watts (to run the refrigerator while starting the freezer).

Installation

Install wiring and equipment to meet National Electrical Code requirements, local regulations and the requirements of the power supplier. Single phase standby generators are connected to the electrical line by a double-pole, double throw transfer switch. This prevents accidentally feeding power back into the utility lines where it can injure neighbors or utility workers servicing the lines. This type switch also protects the generator from damage when power is restored.

Put the switch in a water-tight box and properly ground it, the central meter pole is a common location. Install the switch between the watthour meter and the service disconnect (main fuse box). Note that the white (neutral) conductor is usually not switched, but some power suppliers require it be switched also. When the handle is up, the utility black and red conductors are connected to the load black and red conductors, respectively. In the down position, the load conductors are disconnected from the utility conductors and connected to the black and red conductors from the generators.

Based on information developed by Clemson Cooperative Extension following Hurricane Hugo. Revised for Virginia audiences by Virginia Cooperative Extension.

For more information, contact your local office of Virginia Cooperative Extension.

Visit Virginia Cooperative Extension.

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