Powering Your PC: Truths and Misconceptions
A Guide To Power Supplies

Power Factor:

Power Factor Correction (PFC) has become a hot topic, especially in European countries which PFC is required for power supplies above a certain power rating. Without getting all technical, Power Factor (PF) simply has to do with Actual Power vs. Apparent Power in an AC circuit. As we all know AC stands for "Alternating Current", which is the type of power that we all get from the power company, though it may differ in voltage and frequency. Here in the USA, 120V / 60Hz single phase is the normal plug power, though for our examples it doesn't matter, we are referring to single phase AC power in the generic sense.

A sine wave is the most common waveform that AC power is transmitted as (because it yields the best efficiency). The voltage actually swings from positive to negative over and over many times per second. This is where the measurement Hertz (Hz) comes in, Hz is the number of cycles per second. To know where we are along this sine wave for each cycle we use simple degree measurements. One complete cycle would equal 360°, half would be 180°, and one quarter would be 90° (you get the idea).

The term "phase angle" comes about to represent the angle by which the sine curve of the voltage leads or lags the sine curve of the current.

In a purely resistive load, then the phase angle between voltage and current would be 0° (voltage and current in sync).

Purely Resistive Circuit

In a purely inductive load, the phase angle is 90° (voltage leading).

Purely Inductive Circuit

In a purely capacitive load, the phase angle is -90° (current leading).

Purely Capacitive Circuit

Because of the complexity of most devices (loads) we plug into the wall these days, they end up being either capacitive or inductive (or could vary based on operation), and we call these loads "reactive". The current draw from a reactive load doesn't follow the voltage, it's out of sync (as illustrated in the 2nd & 3rd picture above). The illustrations above also show perfect waveforms for the current, however in real life it could look extremely different. In simple terms, the more out of sync (or out of phase) the current gets with the voltage the worse the "Power Factor" will be and the more apparent power will be required.

• Actual Power
  • Also known as "true power"
  • Is dissipated (used) by the load
  • Expressed in Watts (W).
• Reactive Power
  • Also known as "phantom power"
  • Is power absorbed and returned (not used),
  • Expressed in Volt-Amps-Reactive (VAR).
• Apparent Power
  • Also known as "total power",
  • The sum of both Actual Power and Reactive Power,
  • Expressed in Volt-Amps (VA).
• Power Factor
  • Is simply the Actual Power divided by the Apparent Power.
  • It is expressed as a decimal with a range between 0 and 1.
  • There is no unit of measure.

In the purely resistive circuit illustration above, the power factor is one (1), meaning it is perfect because the reactive power equals zero. In the purely inductive or capacitive circuit illustrations, since there is no resistive component it would be considered purely reactive, and thus have a power factor of zero (0). Any power factor (PF) that is less than 1 means that the circuit's wiring has to carry more current (and thus require thicker wires) than is truly necessary.

Because this reactive power is required along with the true power it can put undue stress on the transmission lines of the power company. They are having to carry extra power through the lines to your home, only to get it dumped right back into the power grid, this is basically wasted capacity. Also, the wiring in your home only accommodates so many amps per circuit, so you wouldn't be able to power as many devices with a poor power factor.

I know the question on everyone's mind right now is, "Will I get billed for this reactive / phantom power?", and the answer is, "Maybe". Most people will say that only commercial / industrial facilities have their bill adjusted based on PF, and that residential homes are not charged. After doing some research, the truth of the matter is in some areas (even in the US) the power company can (and does) bill residential homes an additional fee if they have a poor power factor rating (I saw one power company requiring a PF of .9 or better). So the only real way to know is to call up your local power company if you are unsure.