Do Power Factor Correction Devices Save Energy?
Today we're running a guest post by Chris Kaiser of Mapawatt.com. I met Chris recently because we're both blogging from the great city of Atlanta, and our blogs complement each other well. Here at Energy Vanguard, we focus mainly on building science and energy auditing/rating. Mapawatt concentrates on energy monitoring and energy efficiency products.
Chris's post covers the power factor correction device scam, and why you definitely don't want to throw your money away buying such a device. In addition to his post and the links he provides, another one on this topic you might want to check out is the very geeky NLCPR website, which, by the way, also has a page describing how to make your own Blower Door. ab3
Part of our goal with the Mapawatt Blog is to review the best products that can save energy and water in your home. Product developers know that as energy costs rise, consumer’s budgets get tightened, and people start to care more about their environment (the trifecta of sustainable drivers), those consumers are going to want products that help them save energy. But do all these products live up to their claims?
One of these products is a power factor correction device and can be seen here. This product claims:
Residential customers throughout North America could see a realized savings of 8% – 10% typically and as much as 25% on their electrical usage (and thus power bills).
However, I’m not buying it. There are two great resources on-line that address this same issue. One is Energy Star (page no longer available) and the other is a blogger I’ve been reading for 4 years and has a great section on electricity, Michael Bluejay. Both of these resources say power factor correction really won't help on your residential bill. It can make a difference for certain industrial users who may be billed by the Utility for peak demand, but this is another story (and it is addressed in the Bluejay article).
To go a little deeper, the formula for Power Factor (PF) is below:
PF = Real Power (Watts) ÷ Apparent Power (VA)
- or -
Watts = PF*Amps*Voltage = PF * Apparent Power
The power factor correction devices are said to improve the second half of the above equation, the Apparent Power. However you don’t pay your utility for Apparent Power. You pay them for Real Power (Watts). Apparent Power is defined as the total power in an AC circuit, both dissipated AND returned! (Scroll to the bottom of this link to view the power triangle and description of Apparent, Real and Reactive power). This means that if you currently have a poor power factor, your Apparent Power is higher, but all this means is that you are returning more unused electrons to the utility! But since they only charge you for used electrons (dissipated electrons = Real Power = Watts) you don’t give a hoot about your Apparent Power!
Let’s take an example of 2 completely identical motors sitting side by side. Both of these motors have the exact same efficiency and operate at 1.2 kW. The first motor doesn’t have a power correcting device. The second motors does have PF correcting device.
- Motor 1: 1.2 kW motor, connected to a 120 V circuit, PF = .7
- Motor 2: 1.2 kW motor, connected to a 120 V circuit, PF = .999 (this has the Power Factor correction device, thus the excellent PF!)
Using the equation above we can show the amps (current) that will be dissipated in motor 1:
1.2 kW = .7 *120V * A → A= 14.29
And we can do the same thing for motor 2:
1.2 kW = .999*120V*A → A=10.01
But this doesn’t mean you’ll pay less to the utility! All this shows as that your power factor increases (gets better) your amperage decreases, but the Real Power (Watts = what the utility charges you) stays the same! Therefore no matter your power factor, in residential settings the utility is still going to show that you took the same amount of Real Power off of the power lines, so that is what you pay.
I’m not the only site questioning the validity of Power Factor correction devices. Open4Energy has a great review of Open4Energy: Power factor correction Scam. I should note that it is in their “scam” section!