Electrical costs for my pool

[mas985]

Wow. Glad I posted a popular topic. I wasn't aware of the Power Factor. And it sounds like I need to do some more research on calculating motor costs.

I've found several articles on the web to read, but alas, my work is calling me... In addition to the PF number, there also seems to be a .00173 number that is also being thrown in. This was used to calculate kVA. Siting from http://www.cop.com/SEUhtmDOCS/SEU17.htm, the formulas are:
kVA = 0.00173 * V * amps

This is for a 3 phase motor. Standard motors are kVA = 0.001 * V * amps

kw = kVA * PF * .01 (PF being % between 1-100)
running for one hour would be kwh and then multiply by number of hours times cost per kwh.

Now, looking at the calculations above and the linked site, they are using the stated amps on the nameplate, where I am using the measured amps on my amp meter. It looks like the only place they're using the measured amps is to help calculate the PF.

So what is the 0.00173 number? If I throw in the .00173 number and a 90% power factor, I'm actually increasing my kwh because the .00173 increases more than the .9 decreases it.

Robert

See comment above.

Also, to make it simple, kwh = Volts * Amps / 1000 * (PF%/100)

[cleancloths]

As others have said, the best way is to use the house electric meter. You don't have to turn everything else off, just make sure it stays constant when you run the test. You might want to unplug your refridgerator, as you cannot control when it will cycle. The power factor can vary alot. I have an old fridge that has a power factor of about 0.6 rather than the 0.9 mentioned.

Volts times amps only works for DC not for AC.

[JohnInSoCal]

>just make sure it stays constant when you run the test

Yes, that is my issue. I have 5 refridgerators of varying sizes, some compact, under counter, etc. 2 AC units, freezer in the garage etc. I would need to shut them all down. And then monitor the usage over some time like a half hour correct and see what the difference is in KWhr ? My electric meter has a little wheel that spins like crazy, then 5 dials that directly show the KWhr used. If I monitor that over a half hour or hours time it could tell me the difference roughly as it's hard to tell half dial increments, etc.

[mas985]

>just make sure it stays constant when you run the test

Yes, that is my issue. I have 5 refridgerators of varying sizes, some compact, under counter, etc. 2 AC units, freezer in the garage etc. I would need to shut them all down. And then monitor the usage over some time like a half hour correct and see what the difference is in KWhr ? My electric meter has a little wheel that spins like crazy, then 5 dials that directly show the KWhr used. If I monitor that over a half hour or hours time it could tell me the difference roughly as it's hard to tell half dial increments, etc.

If you use the horizontal wheel that spins very quickly as I suggested in my earlier post, you should not need to run the test more than five minutes. The pump uses enough power probably to give you 10 rotations in less than 2 miniutes. Without the pump, the rotations are so slow that one rotation should suffice. Time with and without the pump and use the formula I posted earlier. It is very easy to do. The other dials are not fine enough resolution to use effectively in a short period of time.

[mas985]

As others have said, the best way is to use the house electric meter. You don't have to turn everything else off, just make sure it stays constant when you run the test. You might want to unplug your refridgerator, as you cannot control when it will cycle. The power factor can vary alot. I have an old fridge that has a power factor of about 0.6 rather than the 0.9 mentioned.

Volts times amps only works for DC not for AC.

While that is technically true for motors, most other devices (e.g. light bulbs, electronic equipment, etc) watts = volts * amps AC or DC. The exact formula for AC is RMS Watts = RMS Volts * RMS Amps * Cosine (theta) where theta is the power factor angle and cosine (theta) is the power factor. For most circuits, theta is 0 so yes watts = volts * amps. However, for most motors, theta is small and the power factor about 90%.