# Thread: A simple way to measure AC electric usage

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You can easily measure how much electricity your AC uses per hour by looking at the spinning disk on your electric meter.

First you need to find the Kh factor on your meter. Here's an example circled in yellow. Mine says 7.2.

Now measure how fast the meter disk is spinning when your AC is not running.

I timed mine at 39 seconds per revolution.

The formula to calculate the amount of electricity being used is 3.6 x Kh_factor / seconds_per_revolution.

So it's 3.6 x 7.2 / 39 = 0.66 KWH without the AC running.

Now time the meter disk with the AC running.

I timed mine to be make 3 complete revolutions in 20 seconds. So it's taking 6.7 seconds per revolution.

That's 3.6 x 7.2 / 6.7 = 3.87 KWH with the AC running.

So the AC is using 3.87 - 0.66 = 3.21 KWH which is pretty good for a 13.5 SEER 4-ton system.

Of course you have to make sure no other electric appliance is going on/off between the two measurements.

I'd like to hear what kind electric usage other people are seeing with their systems.

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I think this is something every homeowner should check. Some people end up paying thousands of dollar more for higher SEER systems. But how do they know if their system is actually delivering the efficiency it's suppose to.

Here's a way to approximate the actual SEER of your system.

cooling_capacity / electric_usage = SEER

I have a 4-ton with total cooling capacity of 46,500 BTU/H. My system's electric usage was 3.21 KWH or 3210 watts/H.

46500/3210 = 14.5

This is better than the 13.5 SEER that my system is rated at. But it confirms what I've known annecdotally all along which was that my system has been running very well and my electric usage for cooling very low.

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## Evidence

Originally posted by go_redskins
Here's a way to approximate the actual SEER of your system.

cooling_capacity / electric_usage = SEER

I have a 4-ton with total cooling capacity of 46,500 BTU/H. My system's electric usage was 3.21 KWH or 3210 watts/H. 46500/3210 = 14.5
What is the ACTUAL Cooling provided?
What were the temperature conditions when you
counted the RPMs of the electric meter?
Mid-Afternoon?
Non-stop A/C ___

Measure air flow and Delta T along with moisture removal to obtain the Actual Total Cooling provided.

Perhaps, ~ 80% of nominal or maybe ~37,200 BTUh was provided at the time.
37,200/3,210= 11.6 EER

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That's why I emphasized (bolded) the word "approximate". I'm trying to provide a simple way for homeowners to have some idea of how well their system is running.

This exercise won't "prove" that a system is running as efficiently as it should but it will point out when it's not.

For example, my neighbor has a 4-ton Trane XL14i which was sold to him as a 14 SEER system. But it's actually consuming 5.7 KWH while my slightly lower SEER 4-ton uses 3.2 KWH.

The maximum cooling his 4-ton unit could possibly be delivering is 48000 BTU. Which would make the system's EER 48000/5700 = 8.4 at best. (And this is with outdoor temperature in the mid-70's.)

And even though we have similar size houses facing the same direction, his unit also runs considerly longer than mine. So the actual amount of cooling is probably significantly lower than 48000 BTU. His unit is probably delivering 5-6 SEER range even though it's rated at 14 SEER. And his unit is just 2 years old.

He's had his installing company come out, but they claim there's nothing wrong with it. Maybe they can blame the long run-times on bad duct work or lack of insulation, but I don't think there any way to explain their way out of the high electric usage.

My meter has 5 dials on it, I try to get a KWH reading every few days. Doesn't look like the meter in the picture is the type with those dials, thankfully I don't need a translation factor. My periodic reading is measuring something more related to my monthly electric bill, for what that's worth. As opposed to the post's method which appears to tell us spot KW.

[You can always tell someone with experience in the electric business, they don't confuse KW with KWH. Took me a few years working for the electric company before it started to make sense for me.]

I seem to be measuring about 80 KWH/day in the heat of the S.Texas summer, so I will expect a monthly bill perhaps in the 2400 KWH range. At .012/KWH electric rate it could be pushing \$300, but summer will soon be over. If we just had time-of-day electric rates things might get cheaper overall, but they would be a lot more complicated <g>.

Hope this helps -- P.Student

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Here's a full picture of the meter. Do you really not have a horizontal spinning disk in the center of your meter?

[Yea, yea, I'm intermixing KW and KWH. But I do know the difference. I think... ]

8. ## Now I see it

Oh yeah, the 2nd picture shows it clearly! The first picture shows part of the dials and I just did not see them at first. Yes I do have the spinning disk too. Was just using the dials on a time frame measured in days, not seconds. I'll have to check my own meter in daylight and try your method.

Best wishes -- P.Student

9. I got the day off tmrw.. Will have to see what mine takes..

It been lower 70's and had the A/C off for a few days now..

Will post of my results.

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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)

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I have a digital meter. It just has the numbers displayed and 4 arrows that flash on and off from left to right then right to left. They flash faster when appliances are being used.

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.

[Edited by jerrod6 on 08-24-2005 at 11:55 PM]

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Can't you just clip your amp meter around L1 and do a little math to convert to watts, then to killawatts.

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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?

Lynn Rodenmeyer: have you heard of power factor?

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