What is power factor correction, and why should I care about it when shopping for a power supply?

Many power supplies advertise 99% active or passive power factor correction in their supply ratings. I think that's supposed to sound impressive because it's 99% for some reason, but the little bits I can find on Wikipedia or elsewhere have more to do with what the power company sees on your lines than anything a typical consumer cares about. Plus, some manufacturers advertise an "active" power factor correction scheme as opposed to a "passive" scheme -- the word "active" usually means "better" when we're talking about PC components (see: Active Matrix LCDs, Active Noise Cancelling, etc.), but the little I've been able to find on this says there's no difference.

Is this all just marketing spin? How much/should I consider the value/type of power factor correction in a power supply?

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Your understanding isn't quite right -- it has very little to do with conversion from AC to DC, and it's not "electrical trash". – Daniel R Hicks Nov 15 '11 at 3:18
@DanH some reading for you...cpccorp.com/harmonic.htm – Moab Nov 15 '11 at 3:47
If you read my post you'll see that I mentioned the harmonic problem. As an electrical engineer I wouldn't regard the 3rd harmonic current waveform as "trash". (And for other readers, my first comment was directed to a comment that has since been deleted.) – Daniel R Hicks Nov 15 '11 at 12:42

2 Answers

Power factor correction means less power loss in your power lines, power that you pay for. When a device has a power factor less than 1, some power flows to the device and then back through the power line. While your meter counts only the power you used, resistance in the wiring means some of that power is lost as heat both ways. A power factor closer to 1 also helps your wires stay cooler.

As Wikipedia's article on Power factor explains it: "In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system, and require larger wires and other equipment."

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Ah, so it makes efficiency higher? – Billy ONeal Nov 15 '11 at 3:18
It makes efficiency higher if you include wiring losses in the calculations. That is, for the same output power, same internal power supply efficiency, and same wiring, a higher power factor will mean you will be billed for less power. – David Schwartz Nov 15 '11 at 3:19
In general you (as a residential consumer) pay for actual power delivered, so, with a bad power factor unit, you only pay extra for the miniscule additional power lost due to heating in your house's wiring. And since motors and fluorescent light ballasts in your house tend to counterbalance the computer PF, the net effect on the power company is likely minimal as well. It only gets to be a problem with large concentrations of computers. – Daniel R Hicks Nov 15 '11 at 3:24

It's probably not a big issue for individual consumers in individual residences. But you get a thousand or so boxes together in a large office and it's a problem.

"Power factor" has to do with the situation where the voltage and current do not rise and fall in sync, but, rather, the waveform of one "leads" or "lags" the waveform of the other.

With large inductive loads (electric motors, eg) current "lags" voltage, while with large capacitive loads (computers and some industrial processes) current "leads" voltage. Either is bad, from a power transmission standpoint, and sometimes from an actual electrical safety standpoint.

"Power" is voltage multiplied by current, but if the two are not in phase it takes more current to deliver the same power. This increases the current through transmission lines and in the major power cables through a large plant or building, causing excessive transmission losses and possibly causing the wires to overheat. And computer power supplies create an added problem because their current draw is "nonlinear" and contains a "third harmonic" component that increases "neutral current" in the 3-phase distribution systems used in plants and large buildings. (And high neutral current in a 3-phase system is not good.)

So this is not a big issue for you, but might be for your employer.

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Ah, that's a good point. Didn't think about lots of systems in a building or similar. +1 – Billy ONeal Nov 15 '11 at 3:09