Can improving Power Factor help your Energy Bill?

***Update - 4/21/09: An Electrical Engineer friend of mine just send me the most detailed technical and economic analysis I've seen on the topic of residential power factor correction devices.  Check it out after you read the blog below if you are really interested:  NLCPR: Power Factor correction

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 from the National Institute of Standards 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 wont 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 would like to see more info from the manufacturers of these devices on how improving PF helps save you Watts!  Basically, your utility doesn't really care what your Power Factor is, so I want to see some evidence on how this device impacts your energy bill.

One more thing!  On the product's website there are many customer testimonials.  NEVER believe customer testimonials.  Especially on things like energy saving devices or products that claim they can improve your gas mileage.  The reason people believe these testimonials is because they can't see electricity flowing through their home, so they just take the product's statements at face value.  Even if the "customer" really said the things about the product, how do you know they aren't mentally insane and or compulsive liars?

Also, if your energy bill goes down the month after you install this, how do you know its not just because you didnt have your AC or lights on as much?  You dont! Only believe data and analysis from trusted web sources (Mapawatt, Rocky Mountain Institute, Energy Star, Michael Bluejay, etc.).

I'm not saying this doesn't work, but I am saying that I need more evidence, and until I see more evidence, I think your and my money can be spent on better energy savings

I'm not the only site questioning the validity of Power Factor correction devices.  Open4Energy has a great review of Power Factor correction devices and another post on Energy Saving Scams.  I should note that it is in their "scam" section!

enjoyed our post? let others know: 


I have a Masters Degree in Electrical Engineering and I have read with fascination the conflicting convictions of the theorists and the practitioners on the subject of power factor correction and energy saving. The theorists are definitely right, so why do the practitioners report lower readings on their power meter after the power factor is corrected? A plausible answer is that their power meter is defective and recording reactive power. If this is the case then the power meter should be replaced and the power factor correction device sent back to the supplier. Has anyone out there had any experience calibrating power meters in situ, by impressing various resistive, inductive and capacitive loads to measure the results. Surely identifying and replacing faulty power meters would be a better service for the electricity consumer.
ckmapawatt's picture
James, excellent point. Don't ask me why, but I was actually thinking about this issue this AM in bed. I realized a great way you can test one of these devices: Install it in a home with a switch that routes power to the device, or direct to the panel box. Install a TED 5000 or some device that you can monitor your energy in real time. I was checking my TED 5000 yesterday to see how much electricity my furnace blower fan used, and I realized on the TED dashboard that it displays power factor. Anyway, you unplug your fridge (the only thing in your house that really comes on when it wants to) and only turn on one furnace and any other appliances that run at constant power. You then monitor energy consumption with the the PFC device turned on and off and see if there is a difference! The key is you are monitoring ENERGY, and not AMPS! I've seen these jokers do the test where they only monitor amps, and that means nothing (because you have to multiply amps by power factor to get true power).
Gday, I'm a little curious about the original example of the 1.2kw motor. From the small amounts of electrical theory ive had, I always believed that power was a "byproduct" as such of votage and current. In your example, the 1.2kw motor. The motor without the PFC drew 14.29 amps. now using simple ohms law - P=VxI = that motor should use 1714.8W With the power factor correction - drawing close to 10A - the power figure matches up. I'd be curious to take some real world power readings of this example. Like I said, I've had limited exposure to the electrical trade. Saying that, my impression is that in your original example, due to the low power factor, your motor is drawing 1.7kw to perform a 1.2kw job. So in theory, by using the PFC, you are saving close to 500w alone just on this motor? I had a quick squiz through this page, and seems to agree with me. Can someone please clarify how, if power is a factor of voltage and current, that by reducing current useage, you don't reduce power useage? cheers
ckmapawatt's picture
Tom, Great question and it's one that I get confused by sometimes, but the mistake you're making (and it's an easy one to make) is that Power does not equal V x I for your motor. Ohm's law only applies to purely resistive circuits! Your motor is not purely resistive, as there is induction going on in the motor's copper wires. For your motor, power equals: Power (Watts) = Power Factor (PF)*Amps*Voltage = PF * Apparent Power. The motor is never drawing 1.7 kW to perform a 1.2 kW job. The motor will use 1.2 kW. Now, you might see an amperage rise that if you were only using Ohm's law would correspond to 1.7 kW, but you are forgetting to multiply by power factor! That's the key. A low power factor may mean a high amp draw, but since the power factor is low, all those amps aren't being put to use. The motor is only using enough amps to perform the 1.2 kW of work. The rest of the amps are just hanging out on the electrical lines, taking up space, and creating heat (more friction because there are more electrons on the power line). So in closing, when calculating the power consumption of motors, you have to know your voltage, your current, AND the power factor of the circuit.
Thanks for clearing that up Chris. I was getting pretty confused. Cheers mate, -Tom
I got this from Wikipedia: The metallic disc is acted upon by two coils. One coil is connected in such a way that it produces a magnetic flux in proportion to the voltage and the other produces a magnetic flux in proportion to the current. The field of the voltage coil is delayed by 90 degrees using a lag coil. I have one question, if the meter uses flux from volts and amps, how does power factor effect the KWH recorded by the meter. Real case of a refrigirator: 240volt,1amp, power factor .33 VxI=240watt, but my VIcosPhi= 79watt. Which of these two is my utility meter recording?
ckmapawatt's picture
79 watts would be your utility reading. You can hook a kill-a-watt or a <a href="" rel="nofollow">belkin conserve energy monitor</a> up to it to verify.
I only intend to find a solution not a debate. I understand most of your explanations for and against such a residential device. I too a skeptic, decided to install the device on my home in November 10'. I live in eastern NC. And our weather, like most of yours, has been significantly colder than normal and for a longer period of time. I have a TOU meter (time of use, I get a lower rate if used during off peak hours). I have to declare that I have noticed a difference so far (haven't calculated exactly but will post back as soon as spring peaks its head out). A friend of mine demonstrated the same scenario with the electric meter, Kvar unit and a 1/4hp motor. If the dial turned slower on the meter with the Kvar online, than offline, then shouldn't your bill also reflect that? Also if we are being forced to use cfl lamps in the future, then wouldn't our inductive loads be increasing also? Just asking. Thanks for any input.
Chris, since you have a TED 5000 have you actually tried using a PFC device and see if there was a difference? I am curious about the scenario you described with the furnace blower fan as I know mine is an old beast and sucks up tons of power when it runs. If you have any feedback it would be appreciated.
ckmapawatt's picture
That's not a bad idea, but I'm not going to spend the money to get a PFC device installed when I don't believe they work. I'm surprised there haven't been any non-profits take on the challenge of doing a detailed analysis on these products.


Post new comment

Subscribe to Comments for "Can improving Power Factor help your Energy Bill?"