Electrical power is defined as the amount of work done in a given amount of time, or the rate at which work is done. The unit of electrical power is the watt, and its symbol is the letter P. One watt is used or expended when one volt causes one ampere of current to flow in a circuit. Generator sets are usually rated in watts. As mentioned before, watts signify the amount of electrical power that is generated or consumed.

Single-phase power (P) can be found by multiplying the circuit voltage (E) in volts by the circuit current (I) in amperes. OR;

P = I x E

EXAMPLE:

If a generator produced 20 amperes (I) at 240 volts (E), what is its power rating?

SOLUTION: P = I (20) X E (240) P = 4,800 watts

This is a 4,800-watt generator. The manufacturer may use this actual wattage as a model number or may use a rounded off number such as 5,000. In either case it is wise to check the specifications of the unit before purchasing to be sure of its capacity. In many cases the advertised wattage, whether it is used as a model number or not, is what is called the surge rating of the generator. This means that while the generator is capable of producing this wattage, it cannot sustain it for any length of time. The continuous rating is the most important because it tells us what the generator is capable of producing on a continual basis, and should always be used when sizing a generator.

The previous formula can be used to calculate power consumption also. For instance, if you have a circulating pump motor that has its voltage and amperage listed on its nameplate, you can calculate its power consumption.

Motor nameplate reads:

Amps 2.0, volts 120.

Solution: I (2.0) x E (120) = P (240) watts.

Another unit of electrical power that you will probably run across is called the kilowatt (kW). One kilowatt is equal to 1,000 watts. So to find kilowatts simply divide watts by 1,000. In the previous examples for instance, the generator at 4,800 watts would be 4,800/1,000 or 4.8 kilowatts. The pump motor would be 240/1,000 or 0.240 kilowatts. Sometimes you will also find wattage expressed in VA (volt-amps)when the wattage is under 1,000. So the above pump motor would be rated at 240 VA.

The following is a list of common household devices with their corresponding wattages. Use these values only if there is no readily available information for your device.