To select the right power source for your applications, one of the first things that you must do is to figure out how much output power you need. For a DC supply, this is relatively straightforward. You first determine the highest output voltage you'll need and then the highest output current that you'll need.
The output power (in watts) is equal to the output voltage times the output current:
P (W) = Vout X Iout
In some applications, of course, you may not need the maximum output current and the maximum output voltage or vice versa. To be on the safe side, however, if you choose a power source that can supply the highest voltage and the highest current that you'll need, then you can be sure that the power source is not underpowered for your application.
DC power calculated with the formula above is sometimes called real power or true power. We call this real power because it is the amount of power that's actually available to do some work. This could include running DC motors or powering an electronic unit under test.
Apparently, not quite so real
For an AC power source, this calculation isn't quite so simple. The reason for this is that for some, if not most, AC loads, the voltage and current are out of phase with one another. If the load is capacitive, the current will lead the voltage. If the load is inductive, the voltage will lead the current.
Reactive loads make a power source work harder because they require a power source to supply power during a portion of an AC cycle, only to return a portion of that power later. The net effect is that a power source has to be able to supply more current than that calculated by the equation for calculating DC power.
Because this power doesn't do any real work, it is called apparent power or virtual power. To differentiate apparent power from real power, we use the unit volt-ampere, or var, instead of watts. The abbreviation for volt-ampere is VA. The equation used to calculate the apparent power is
P (VA) = Vrms x Irms
where Vrms is the root mean square value of the AC voltage and Irms is root mean square of the AC current.
The ratio of the real power to the apparent power is called the power factor (PF):
PF = real power (W) / apparent power (VA)
If you know the phase shift between the voltage and current, you can calculate the power factor using the equation:
PF = cos ø
where ø is the phase angle between the voltage and the current.
The power factor will always be between 0 and 1, and the greater the phase angle, the smaller the power factor. The smaller the power factor, the greater the apparent power, meaning that you'll need a supply with more output power to power a highly reactive load than you will need to power a load with only a very low reactance.