Watts are dimesionless unit of power. A 500-watt coffee maker is 500 watts, whether it's on for five minutes or 6 months - in the simplest of terms, it's essentially a measure of how fast energy is used. The electrical meter on the side of your house measures energy usuage, in kilowatt-hours (in the US. The UK uses kilojoules, instead, which is basically the same thing). One kilowatt-hour is the amount of energy consumed by a 1-kilowatt (1000 watt) load over a period of one hour.
Originally Posted by BCEasy
Some appliances are not labeled with power draw directly enumerated in watts. These items will have the expected load voltage listed, and the current draw, in amps or milliamps. To find out the (approximate) watts draw of such appliances, multiply the volts by the amps (convert milliamps to amps by dividing by 1000), For example, if the rating plate for an appliance reads 120 VAC @ 3 Amps, the watts draw is 120 x 3 or 360 watts. Depending on the nature of the applinace, this result may or may not accurately reflect the true power draw, as I will explain shortly, but it will be a good first-order approximation
Other items are listed with a figure expressed in VA, or volt-amps. Typically, appliances with large motors or other heavily inductive loads, such as high-power transformers, are labelled in VA. Volt-amps are slightly different than watts for circuits that are highly reactive, that is are composed primarily of inductors (coils of wire around an iron or air core, typically- which include motors and transformers), and/or of capacitors (electrostatic devices which are used to store electrical charge and are used in filters for power supplies and other devices, among other purposes). When an AC voltage is placed across a reactive load, the resulting current flow is out of phase with the voltage. if the load is purely inductive, the current is 90 degrees negative out of phase with the applied voltage. If the load is purely capacitive, the current is 90 degrees positive with respect to the voltage.
When this happens, you cannot simply multiply voltage and current to arrive at watts, since the two are not in phase. Instead, you will have a figure termed volt-amps. In order to calculate the watts draw for these types of loads, you need to know a figure called the power factor. In simplest terms, the power factor is the ratio of the voltage phase angle to the current phase angle (this is not exactly correct, but close enough for purposes of this discussion). For a purely resistive load with no reactive components, this figure will be exactly one. For all other loads, the result will be less than one. Unless you are dealing with highly reactive loads, however, you need not worry about power factor, and can simply multiply volts and amps and the resulting figure will reasonably closely approximate the actual power draw.
The power company likes loads to have power factors as close to one as possible, since this reduces their costs for various reasons too technical to go into here. If you're really interested in this sort of thing, you can read this of just Google "power factor".
Well, that'll teach you to ask a simple question!
"ATTENTION: Due to limits on my patience, I can only please one person per day. Today is not your day. Tomorrow's not looking too good, either."