Determining Your Stand-by Power Requirements

Make a list of the appliances and motors that must run during an electrical power failure. It is suggested for stand-by service that you include only essential equipment.

When you have completed the list, enter the number of watts required to operate each item. The watts required for operation are usually stamped on the nameplate of the product. (If only the ampere rating appears on the product, multiply the amps by the voltage to obtain the wattage.) Also, 1000 watts equals 1KW.

Remember that electric motors require more power when starting than when running. Starting one motor at a time, beginning with the largest down to the smallest, will reduce the size of the alternator you need.

Some motors require more starting current than others, and all require more starting current than running current. Split-phase motors are more difficult to start and require 5 to 10 times as much current compared to running current, dependent on the NEMA code letter rating. Motors with capacitor start require 2 to 4 times as much current to start compared to running current. Repulsion Induction Motors require 1 ½ to 2 ½ times as much current to start compared to running current. Determine your largest motor starting requirement by the HP rating and the NEMA code letter on the nameplate. Refer to the table below for the approximate wattage per HP by NEMA code letter. For the total starting wattage required, multiply this figure by the HP size of you motor.