# Cost to run central A/C

I'm trying to figure the cost of running the central AC in my house. Inside was fairly easy, a 1/2hp fan motor. Outside the numbers on the plate from the factory are long gone. I found on the fan motor that it is 1/8hp. Now the compressor.... On it is listed the voltage but not the amps or watts but it does list the LRA (locked rotor amps) of 105.0 It is a Copeland CR38K6-PFV-370.
I found this site but even this isn't very useful to me only giving me the RLA of 19. I read the RLA is "rated load amps" and does not mean "Running Load Amps". RLA being the most current it should draw under the harshest conditions.
http://www.nordyne.com/literature/8569%20Compr.pdf
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Tony wrote:

None of those numbers are of much use in calculating operating costs since none of those items runs 24x7. All those numbers will tell you is how big a circuit the items require.
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Pete C. wrote:

I've kept track a few times and during a hot 90F days with lows of 70F it runs between 8 to 10 hours/day. That may be a low estimate, I'm going to put a time meter on it for a much more accurate figure.
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Tony wrote:

Depending on the system, even a run timer may not quite get you accurate numbers. On my HVAC the air handler has a 90 second post run after the compressor turns off in order to increase efficiency.
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Pete C. wrote:

No delays here. Anyway that doesn't sound like it would increase efficiency much, if any. I wouldn't like it blowing all that moisture from the evaporator back into the house again as it warms up. If it's cold when it turns off as it warms up it is still taking heat away from what ever is around it. And if it doesn't cool off much in between cycles, then it blows cooler air faster when it kicks on again.
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Time wont do it nor will ratings on the fan or compressor take into account the controll board, other components in the condensor and how age and condition of unit play a big part in total useage. Use a clamp on amp meter at the circuit panel, an old fan or compressor near failure can use alot more power, air restrictions you are unaware of will also affect everything.
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In article

Depends on the weeks. This week, we had most days high 80s to mid-90s. Next week the forecast is calling for at least 3 days with highs in the mid 70s to low 80s. So just turning or now tells you next nothing by itself.
--
I want to find a voracious, small-minded predator
and name it after the IRS.
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On 6/26/2010 1:31 PM, Tony wrote:

The best would be to compare the electric meter with the unit running and not running. This way you will have actual watt-hours and you don't have to think of things like power factor, etc. But, it will change depending on temperature. And the run time will vary when the temperatures are cooler than when it is just plain hot outside.
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Many electric bills give a 13 month use history. Mine even gives the average temperature for the month. Look at months like April and October when neither heat or AC is used much and compare to July and August and you get a pretty good idea. Aside from that, the only accurate method is a recording ammeter over time.
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Ed Pawlowski wrote:

I think you mean a recording watt-meter. Amps don't account for power factor.
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Correct. I was first thinking Amprobe and doing the math yourself but that does not take time run into consideration.
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A couple of years ago, I hooked up an amprobe to my 3 ton carrier unit outside and it read 16 amps (220V). My inside fan blower I think used 5 amps (120V).
So that's about 4,120 watts total full load. Keep in mind I usually keep my inside fan on continuosly, and my outside unit cycles on and off throughout the day.
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Find an old mechnical clock that has time and date. Those ones with the litte flip numbers work well. Set it to 1/1/2000 midnight and hook it into the blower circuit. At the end of a month you'll have the number of days and hours the unit ran.
You could also buy an old meter and run a circuit through it. You'll need a subpanel though.
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cjt wrote:

Another question maybe you can help? If I buy a clamp on type ammeter that plugs into my DMM to display the current... if my DMM measures AC with true RMS, will that fix the problem of the power factor?
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Tony wrote:

No. Power factor arises from the phase relationship between the voltage and current. For purely resistive loads, they are in phase and the power factor is one. For reactive loads, they are not in phase. It's not a question of RMS vs something else.
BTW, if you're willing to try something a bit more interesting, you can measure total daily power use and minutes your A/C is on each day over a period of time. Plot the two against each other and fit a line to the data using a least-squares fit. Done correctly (including scaling minutes to hours, making the correct choice of axes, etc.), the slope will be A/C watts and the intercept will be your background use (i.e. the average energy consumed by the rest of your appliances) within a margin of error that you might even be able to quantify statistically (although if your background use varies too dramatically or is correlated to A/C use, the analysis might fall apart or at least become difficult). The longer the time period you incorporate in your analysis (within reason -- you don't want seasonal changes in efficiency, etc. to get involved), the more accurate you can be.
:-)
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cjt wrote:

^^^^^ make that energy ;-)

or if the background use is too large relative to the A/C use
the analysis might fall apart or at least become

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cjt wrote:

Not sure I understand all that. But if you mean to use the power companies kwh meter, keep in mind that it only changes one kilowatt hour for each 20 KWh used. This makes it difficult to estimate usage since it can display the same number for hours even if I am using electric. It wont add 1 kwh at a time, only 20 kwh at a time.
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Tony wrote:

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Art Todesco wrote:

That is very difficult with the newer digital electronic meter I have and the way it's set up. With it on a current transformer the meter hardly moves at all. The readings have to be multiplied by 20 for the actual usage.
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Tony wrote:

Hmmm, I have an Amprobe DMM with built in true RMS reading. IUt has a clamp. Just set the switch to lo or hi response and clamp, it reads the current at the moment.
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