Electrical generators use watts to rate the generator's output. Watts is a combination of voltage times the electrical devices load capacity in amperage. Sizing a generator correctly is a crucial process for efficient operation. A generator that is sized too small will not provide enough power for the equipment you want to run on the portable electrical supply device. You must have the electrical equipment specifications in order to calculate the correct generator for the output usage.

Calculator and pen
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Step 1

Understand the generator can only output a finite amount of power. That amount of power will be listed as watts. A generator may be listed as 3,000 watts delivering 120 volts.

Step 2

Find the amperage that the generator can output at 120 volts. Wattage is equal to volts time amperage (w = v X a). Divide wattage by volts to find the amperage (a = w/v). In this example 3,000 watts divided by 120 volts is equal to 25 amperes. Some generators may be a dual voltage type and also output 240 volts.

Step 3

Find the amperage available from the same generator but at the higher voltage. Divide 3,000 watts by 240 volts. Only 12.5 amperes are available by the same generator at the higher voltage. Take into consideration what you are going to operate with the generator. In other words, the total load from the devices cannot exceed the generators overall output.

Step 4

Look at the electrical specification label on the devices you wish run with the portable generator. Add all of these wattages together for the final total. Size the generator.

Step 5

Understand that all generators may be rated differently. Look for the "constant load" or "continuous load" specification on the generator. This is the amount of power in Watts that the generator can safely output for an extended period of time.