How to Size a Power Generator

List the power needs for a generator by the energy consumption of the appliances and other electrical fixtures that will be used. This information can usually be obtained off of the data plate on each appliance.

List each of the appliances and fixtures that will be used. Generators usually offer both 120 and 240 volts. The power capacity of the generator is measured in kilowatts. Light bulbs are already rated in watts, but other appliances and fixtures are usually rated with voltage and amperage. To convert to watts on a resistive circuit, multiply the voltage times the amperage, and this will give you the wattage.

Calculate the electrical load of the appliances that have an inductive circuit. This would be appliances that use coils, such as motors. The data plate will supply the amperage information, but when dealing with an inductive circuit, there are two factors to consider. One factor is the steady running amps, but when a motor starts, the amperage will momentarily jump three or more times the normal running amps of the motor. The generator must be of adequate capacity to handle this surge, but bear in mind that all appliances equipped with motors are not likely to come on at the same time. The steady running amps are listed as FLA. The starting amps are listed as LRA.

Figure out your total required wattage. After making your list of required wattage, add them up with a calculator. Don't forget to allow for surges from motors starting. You now have an idea of what size generator you need.