How do you calculate power factor?

I got started thinking about trying to monitor the electricity usage of some of my large power users - heat pump, electric water heater, air handler fan, electric drier, etc. I figured if I know about how much power each device draws when it's running and I use homeseer to keep track of their run times then I can determine how much power they're using.

I figured determining the amount of power they draw would be easy... I'd borrow a clamp on ammeter from work, clamp it on and measure the current draw in amps. But then I remembered that AC Watts = Volts times Amps times Power Factor. For resistive loads like the drier and the water heater I'm thinking the power factor is pretty close to 1, so I probably don't have to worry about it much for them, but I have no idea what it would be for the heat pump compressor or the air handler fan.

Is there an easy way for me to calculate power factor (or, for that matter, the power draw in watts) or am I over my head here?

Thanks,
Brett

Maybe you can cheat and use a Kill-A-Watt unit to check the Power Factor? :lol: Otherwise, you're at the mercy of the manufacturer to list it somewhere.

BrettS said:

Is there an easy way for me to calculate power factor (or, for that matter, the power draw in watts) or am I over my head here?

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Residental meters do not include the power factor, therefore you do not pay based upon power factor. So, if you are calculating your energy COSTS, you do not need it. If you truly want to know, some whole house meters include displaying the power factor. TED doesn't display the power factor. Brand Electronics OneMeter can display PF. Kill-A-Watt displays PF, but only for plug-in devices.

TonyNo said:

Maybe you can cheat and use a Kill-A-Watt unit to check the Power Factor? :lol: Otherwise, you're at the mercy of the manufacturer to list it somewhere.

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I have a Kill-A-Watt and plan to use it for my 120V appliances, but all of the ones listed above are 220V appliances and I'm reasonably certain the Kill-A-Watt would explode if I tried to use it at 220V

You do have a good point about checking with the manufacturer. I wonder if the power factor is listed on the rating plate or in the service manual for any of these things.

Thanks,
Brett

WayneW said:

Residental meters do not include the power factor, therefore you do not pay based upon power factor. So, if you are calculating your energy COSTS, you do not need it.

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Actually, residental meters essentially do include power factor. The whole thing is a bit confusing and can easily make one's head hurt, but I'll try to explain it briefly.

Residences are billed by the Kilowatt Hour, which is based on the number of watts used (kilowatt hour=(watts x hours) / 1000). As I said above, watts=volts x amps x power factor.

As an example, we'll assume my heat pump draws 17 amps at 220V. Therefore it draws 3740VA (VA=Volt Amps or Volts x Amps... in this case, 220 x 17). Now, we'll assume the power factor is .85. So then we know that it draws 3179 Watts (220V x 17A x .85pf). I'm billed based on the number of Watts it draws and not the number of VA, so in this case if I based my calculations on VA instead of watts I would be off by 15 percent.

Resistive loads have a power factor of 1, so my water heater is less of a problem since the heating element is little more than a really big resistor. In that case, if the water heater draws 15 amps at 220V it would draw 3300VA or 3300 Watts (220V x 15A x 1 pf). (If I actually measured it the result might be slightly less than 1 due to the other electronics in the water heater, but it would still be really really close to 1)

Now, some industrial companies are billed based on VA or are billed based on Watts, but they are also billed a penalty if their power factor is too low. This is where power factor correcting capacitors come into play (and also where it starts getting confusing).

The trick is to remember that the number of volts and the number of watts for a particular appliance always stay constant. And since we know that watts = volts x amps x pf then we know that if the power factor is raised then the number of amps will go down.

As an example, we'll take my heat pump again. Right now it draws 3179 watts, which is 220V x 17A x .85 pf. Rearranging the equation we can see that amps = watts / volts / pf (in this case, 17A = 3179W / 220V / .85pf).

Say we used a power factor correcting capacitor to raise the power factor to .97. Now we have 3179W / 220V / .97pf = 14.9 Amps. If you're a company that gets billed based on VA then you sure want to make sure that your power factor is as close to 1 as possible.

If you're a shady salesman trying to sell me a power factor correcting capacitor, then you use amps as an example. Put your ammeter on my air conditioner without the capacitor and show me that it's drawing 17 amps. Turn on your capacitor and watch it drop to 14.9 amps. It looks impressive, but the trick is... it won't make one bit of difference in my power bill because my power bill is based on watts and it's using 3179W both with and without the capacitor.

Hopefully that made sense (and didn't make your head explode) :lol:

Brett

BrettS said:

Actually, residental meters essentially do include power factor.

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The more I think about it, I think that statement is wrong. Residential meters do not include power factor (because the number of watts is the same no matter what the power factor is, and as I said above, residences are billed based on watts).

More accurately, however, if I know Volts and Amps then I need to know the power factor to be able to figure out how many watts an appliance draws... and that is what I'm trying to do. Perhaps there's another way to determine how many watts an appliance is using that doesn't require me to know the power factor. I guess that would have been a better question here - how do I figure out how many watts my heat pump or water heater or whatever draws?

Brett

wikipedia has a good description of PF here
http://en.wikipedia.org/wiki/Power_factor

BrettS said:

I got started thinking about trying to monitor the electricity usage of some of my large power users - heat pump, electric water heater, air handler fan, electric drier, etc. I figured if I know about how much power each device draws when it's running and I use homeseer to keep track of their run times then I can determine how much power they're using.

Is there an easy way for me to calculate power factor (or, for that matter, the power draw in watts) or am I over my head here?

Thanks,
Brett

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Brett, I am doing exactly what you are thinking about. I've been monitoring my water heater, air conditioner, dryer, refrig, freezer, and well pump for about 2 months. I have an excel spreadsheet that adds up all the ON time and gives me daily and monthly totals. I don't really care about the power factor issue because I am looking for ways to reduce my total electric usage. So, if I can reduce the ON time of my water heater by 5% then I know I will reduce the amount of electricity used by 5%. Right now I am trying to determine my baseline power consumption. Then I will begin experimenting to see if things like turning the water heater off at night will make any difference. One thing I have noticed is that my water heater turns on almost every time the hot water faucett is turned on. This is because our well water is very cold.

If you are interested I can send you the spreadsheet.

Steve Q

I suspect why a power company might wish their users to have reasonably high power factors, it that real low ones are a sign of high reactances generated by coils and capacitors. In some cases where they might be switching on and off the reactances could throw spikes and high loads onto the system. Sincerely, GDM