A generator must be properly sized to start and run the appliances and equipment you want to operate. When selecting a generator size, you must take into account the starting power required by motors and appliances that are driven by motors. Electric motors may be difficult for a generator because they often require two to six times their running power just to start them.

Follow these steps to estimate your generator size needed:

Identify all appliances to be operated by the generator
You will need to determine which appliances you want to have running by the generator during a power outage. Since the cost of a generator increases with size, you may want to think about only the necessary appliances you will need to use during an outage. Also, the greater the electrical load (and the larger the generator), the more generator fuel you will use.

List all nonmotor appliances wattages
Find the running wattage of the appliance(s) you want to operate by checking the nameplate. Most appliances have their wattages listed. Some appliances have only their amperages listed (sometimes designated with an "A" or "amps," such as 3.5 A or 3.5 amps).

To calculate watts, use the following formula: Volts x Amps = Watts
(single phase residential power)

Use 120 volts for small appliances such as radios and microwave ovens. Remember that larger appliances such as ranges and electric water heaters use 240 volts. Example: a television that draws 2.2 amps uses 2.2 A x 120 V = 264 watts.

List and calculate all motor and motor-driven appliance wattages
Determine the running wattage of electric motors and motor-driven appliances (such as furnace fans and well pumps) using the formula Volts x Amps = Watts.

Then calculate the maximum starting wattage, multiply the running wattage by three and use that value for sizing purposes. Some motors can require as much as five or six times the normal running wattage. Consult with the motor manufacturer to be sure.

List and total all wattages of appliances that will run simultaneously by the generator
Once all appliances have been identified and their wattages calculated, add these wattages to determine the size of generator needed.