Cost to run central A/C

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.

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.

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.

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.

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.

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.

Best method. After they read the meter start using it for a month. Next time they read the meter, stop using it for a month until the next reading. They usually read it the same day of the month. Take it your self to make sure they are not estimating your bill and the reading is actual. You could also take your own reading daily and run it one day and not the next to get an idea of daily usage. Using stamped values and ideal math is not accurate and won't count load, wear, age and your comfortable temperature setting. You can also get an amp clamp and check one leg of the supply power from the box which will vary when the fan/ compressor cycle on/off. (Is the inside fan on the same circuit?)

ultimately is the comfort you gain worth the larger electric bill?

"Art Todesco" wrote

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.

I think you mean a recording watt-meter. Amps don't account for power factor.

If your heating is other than electric, then the cost is the difference in your electric bill from the winter to the summer.

Or if there is a month in the spring when you don't use heating or A/C, then the difference in the electric bill from that month and a summer month.

Hint: If you get a couple of window air conditioners and just cool the room(s) you are using, the cost will be lower. So just cool the living room during the day and just cool the bedroom at night. If you have 6 kids and 7 bedrooms, then not much savings of course!

"cjt" wrote

Correct. I was first thinking Amprobe and doing the math yourself but that does not take time run into consideration.

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.

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.

Run it one month and then look at your electric bill.

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.

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.

I see two approaches. For the macro view, as per the above, just comparing electric bills for summer with months in Spring or Fall with no AC or electric heat use would give a decent approximation. My utility bills show usage by month over the last year. I can easily see the effects of running the AC. Of course, if you had some other seasonal load, like pool pumps, etc, that could complicate things.

For the micro view, as someone else suggested, record the electric meter for a time period on a day with no usage and then do the same with the AC on.

I can guess the date they read the meter give or take a day or two. They read it from their office, no more meter man. Besides, I think I'd die without AC for a day. Also, with my meter I could be off almost 20 kwh's a day without knowing it. My meter reading must be multiplied by