Power supplies Understanding PSU Power Factor Correction
Darren Yates continues his short series on PC power supply units, this month looking at the concept of real power versus apparent power, plus a strawberry milkshake.
One feature you’ll see advertised on many PSUs is ‘Power Factor Correction’ or ‘PFC’. It’s long been mandatory on PC PSUs in the EU and U.S. and for the sake of the planet, PFC is actually important. However, you might also read it can save you money. Like most things, it’s not that simple, so this month, we’re diving into some electrical theory to understand just what PFC is, why it’s in your PSU and what difference it makes.
Real power vs apparent power
We’ve said it before, but for those late to class, a PSU converts the AC mains voltage into DC voltage sources or ‘rails’ that power your PC’s components. Each rail supplies electrical current and if you multiply the current for a rail (measured in amps) with its rail voltage (volts), you get the electrical power (watts) that rail can supply. Add up this power rating for each rail in the PSU and you get the total power rating of the PSU in watts you see stamped on the box, whether its 450-watts, 650 or whatever.
This power is also known as ‘real’ or ‘true’ power, as its power that does actual work – in this case, powering your PC. However, there’s also another type of power known as ‘apparent power’. This is the power the power generation companies have to supply to your mains socket in order for your PSU to power your PC.
But hey, isn’t that the same thing? No, not quite. A PSU is what’s called an ‘inductive’ electrical load. When converting AC power into DC power, some of the AC power is temporarily stored within a component called a ‘transformer’. This is essentially two or more windings of wire wound around a small block of thin laminated steel plates.
This stored power quickly returns to the AC mains socket, but it changes the relationship or ‘phase’ between the voltage and current. This change creates ‘reactive power’. Add together the ‘real power’ your PC uses with this additional ‘reactive power’ and you get the ‘apparent power’ the power generation companies have to supply you.
Don’t laugh, but an easier way to explain it is usually with a glass of a certain alcoholic amber fluid, but as this is a family show, we’ve come up with our deluxe ‘electrical power milkshake’ diagram instead. The real power in your milkshake is the actual milk – that’s what you’re paying for. The reactive power is the froth on top that, while a bit of fun, isn’t really useful to you. Finally, the apparent power is the whole milkshake glass.
If you can stand just a little more theory, you can look at real, reactive and apparent power as a right-angle triangle – with real power, the voltage and current are ‘in-phase’, whereas reactive power will have the voltage and current 90-degrees ‘out-ofphase’. Apparent power takes into account this phase change created by the reactive power.
So what’s ‘power factor’?
If you divide the real power the PSU delivers to your PC by the
apparent power the PSU takes out of the AC wall socket, you get what’s called the ‘power factor’ (PF) of that PSU. The ideal power factor is ‘1.0’ – this is good for everyone, since the power station only has to generate what you’ll actually use. However, if your PSU also needs a certain amount of reactive power, let’s say 20%, you may use 400-watts of true power, but the power station has to supply you 500 AC-watts or VA (‘volts-amps’) to overcome the 20% reactive power loading.
Given there are now minimum PF levels required for all PC PSUs, they all include what’s called ‘Power Factor Correction’ (PFC). There are two kinds – passive-PFC and active-PFC. Passive-PFC uses ‘passive’ components (capacitors) to negate the reactive-power
component, while active-PFC uses ‘active’ components (transistors) to do a better job. Active-PFC gets nearer the magic 1.0 mark (usually better than 0.9), but also costs more.
How to measure Power Factor
If you need to measure power factor, the simplest way is to use a decent AC mains power meter. You’ll find good home models under $30 should do the job. For more serious work, heaviergrade meters, such as the ‘Power-Mate’ units, sell from $130. You’ll typically find PF varies with load, with higher loads giving better PF.
Does having PFC save you money?
The short answer to this is ‘go find your last electricity bill’. If you pay your electricity in cents-per-kilowatt-hour (c/ kWh), then no, PFC won’t help you much. However, if you pay a portion in cents-per-kilo-voltsamps (c/kVA), then yes, PFC could save you money.
As far as we can tell, Australian electricity retailers only charge domestic users for the real power (c/kWh) they use, not apparent power (c/kVA). However, many commercial businesses are charged for apparent power. So, if you run a business in commercial premises and you pay for apparent power, a PSU with a higher power factor should help lower your electricity bill for that PSU power, by the proportion its PF is improved.
If your next question is ‘how much can I save’, that’s too complex for us to answer. Essentially, it depends on the change in power factor and the real power your PCs consume. The more real power you consume and the greater the PF using an active-PFC PSU, then the more you will likely save. But remember – you must be currently paying for apparent power for PFC to be of any benefit.
Actually, that’s not strictly true. In an altruistic sense, as we showed before, PFC helps everyone, especially the electricity generators. The more we can reduce reactive power, the more we can reduce the total power generation capacity. That could lead to smaller, cheaper
CoolerMaster’s MWE 750 Gold has Active Power Factor Correction (AFPC). generators, less power wastage through the transmission grid system and less greenhouse emissions. Electricity suppliers are also known to offer PFC units to big industrial and commercial users to improve power factor and reduce kVA costs.
PFC is important
One PSU with low PF won’t sound like much, but if you have 10 million PCs with a low-PF PSU, it can add up to a sizeable amount of extra power generation required. Whether it makes a direct difference to your hip pocket depends on whether or not you pay for apparent power. Most good PSUs will offer active-PFC, but now you should have a better idea of what it means, what it does and why it’s not bad to have it on your next PSU.
“If your next question is ‘how much can I save’, that’s too complex for us to answer. Essentially, it depends on the change in power factor and the real power your PCs consume.”