# Thread: A simple way to measure AC electric usage

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Originally posted by lynn rodenmayer
Can't you just clip your amp meter around L1 and do a little math to convert to watts, then to killawatts.
You can, but most homeowners don't have a ammeter. I posted on another thread that I measured the condenser electric usage using an ammeter and got about 2.94 KW. The electric meter method gave me 3.21 KW. The difference would be the blower motor in the air handler.

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Originally posted by dx
Go_redskins: You DID turn off all of your refrigerators and freezers during measurements to make sure their compressors don't kick in or out randomly and render the readings meaningless, right?
No, but I did say "Of course you have to make sure no other electric appliance is going on/off between the two measurements" in the original post.

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Originally posted by montr
I did some mesurement last weekend. I did not used the house meter but a DVM with a clamp type current measurement probe.

Here is the test conditions:
between 11:00 to 12:00 AM, 20 Aug 2005, Atlanta, GA
outside: 83 deg F, 60% RH, sunny
inside: 75 deg F, 48% RH

Unit 1 (4 Years old): condenser Trane XL1800 3 tons 2 stage TTZ036a100a, coil txc036s3, furnace TUY080R9V3A (blower set to normal 400 CFM/ton)

High stage: measured 2222 W for 35800 BTUh, SEER=16.11 (ARI rating for this configuration is 16.80)

Low stage: 935 W for 18000 BTUh, SEER=19.3 (I do not have the SEER rating for the low stage.)

Unit 2 (1 month old): condenser Trane XL14i 2 tons 1 stage (4TTX4024B1), coil RXC025S3, furnace TUD060R9V3 (blower set to low 350 CFM/ton)

High stage: 1723 W for 23600 BTUh, SEER = 13.7 (ARI rating is 13.50)
Someone correct me if I'm wrong, but I believe the SEER rating takes into account the electricity used by the indoor blower motor. That would have to be added to the electricity used by the condenser.

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Originally posted by jerrod6
But I just compare my bills in JUNE thru SEP with the bills in May and OCTOBER. During May and Oct no heat or AC are running.

Also my bills give me the average KWH used per day for each month along with a graph that shows it.
Right. You can use your electric bill to figure out roughly how much total electricity you're using for AC. But that doesn't tell you anything about how well your AC is operating. Your AC may be running inefficiently because:

1. it's output is less than expected and therefore running longer than it should
2. it may be consuming more electric than it should when it's running
3. both 1 and 2.

The method I described was a way to determine 2.

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I did not consider the blower power in my mesurement. I think the blower is around 100 W. The power usage for the whole house is 885 kWh for 07/18 to 08/16 (700 kWh for prior month). Power bill is \$83.15. There is 4500 sq. ft of conditionned space on 3 floor. Temperature inside is set to 76 for night and 77 for day. Outside was in the mid 90 for about 2 weeks.

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it seems easier for me to track btu/DD/sf

= 1.5- 1.7 for me, 1.54 last month. DD given by our utility ea billing -- I subtract out April05 from total monthly kwh, because used no a/c nor heating then = mild here.

I have 4y of data, utility site shows 15months

w/o DegreeDays, your data does not have much meaning -- like saying ave depth of Merrimac River is 2ft [but drop- off holes may be 25ft]

[Edited by cem-bsee on 08-25-2005 at 02:43 AM]

7. I figured mine out this morning.

Trane XR11 2ton w/ TXV

XV90 furnace at 400cfm/ton

I came at 11.05 SEER

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Btus/Watts = EER not SEER.

9. ## One person's results so far

Tried this measurement at my own house. I did *not* have the flexibility to turn off too many things, so there is the random activity of refrigerator, a couple fluorescent lamps, etc.

There are 2 AC systems in my situation, both are single speed ACs but VS air handlers. 3.5 tons each one is a Trane XE-1200 and the other Brand X. I believe the VS effect is pretty constant with a little ramping up and down due to Trane-Am.Std. "Comfort-R" being operative in one of the two systems. (I *can* observe which stage it is in by watching ESP, but that is getting complicated).

With both systems running, I have got measurements of 2.68 sec/rev and 3.28 sec/rev. I won't try to say either is the accurate reading, it just shows the wide degree of precision for me. I calculate the wattage is between 7.90 and 9.70 KW at this time.

With the NW (Northwest) AC system running and the SE one off, I have measured 5.15 KW and 4.75 KW. Have not captured a reading with SE on and NW off. It would take quite a bit more readings to narrow this down to a more precise number, this should be considered raw data. Maybe I will have some time to address this later.

Thanks for sharing this method with us, and for using pictures to explain it so simply.

Best wishes -- P.Student

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This does seem like a good way to figure this out, but I guess I cannot do this since I do not have a spinning wheel

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