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.