Cost to run central A/C

I want to compare what 1 or 2 8,000 BTU window units use compared to the central AC. I'm almost positive the window units are much more energy efficient than the central air.

Reply to
Tony
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That's exactly what I getting at. I'm almost positive it costs less to run 1 or 2 8,000 BTu window units than the central ac, even if I close off the rooms not being used.

I suppose if I compared oranges to oranges and converted both central and window rattler to a basic BTU's/watt that would be very helpful, but I still need the specs on my compressor.

Reply to
Tony

Have you sought help from your electric company?

Reply to
LouB

Have you looked on your bill and read the $/kwh? Compared the chart for month to month, and this year vs. last year comparisons?

AC, in my estimation, has to do with keeping things cool. Once a room is cool, it takes less power to keep it cool than to cool it down from warm. With central units, they will do this better than room units that have to cool a warm room. Up. Down. Up. Down.

But, then, the only way you find out is to install central, and do a post mortem analysis.

Sometimes a large central AC won't have to work as hard as several smaller ones. But then, there's the thing of cooling unused space ......... It's hard to say.

Steve

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A fool shows his annoyance at once, but a prudent man overlooks an insult.

Reply to
Steve B

I don't see how they can help, what kind of help would I be seeking?

Reply to
Tony

Part of their biz need is estimating electric usage. Its not going to cost anything to ask so why not?

Also, tried google? Here is a start:

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got About 396,000 results (0.35 seconds)

Reply to
LouB

Yes. But comparing last years to this years has too many variables.

I'm with you there.

No up down up down here. In this 93F weather the little 8000 BTU unit runs on high 24/7 unless it gets too cool, then I turn it down. Cycling it makes it less efficient.

I have central. I believe it costs much more to operate than a window unit or two, but it's hard to say without the right numbers.

Right now I have one 8000 BTU unit and one fan to circulate the air. Running it 24/7 30 days would cost me less than $60. It never runs all night so it's maybe $45-$50 a month to cool 960 sq ft. Forgot the fan to circulate the air, add about $4. per month.

I always considered "work as hard" an odd expression in this context. I had a Fedders, maybe 7000 btu run pracically 24/7 each summer for 15 years and it never stopped working. I left it at my old house because it wasn't nearly as efficient as the newer ones.

Reply to
Tony

What I did for the zone room (this is the room where I spend 99% of my time) stick a plain Jane window unit through the wall, make that a quiet Plain Jane unit. Get a thermostat, relay and transformer. Wire them up to feed the outlet serving the window unit. Set the thermostat and forget it.

Reply to
FatterDumber& Happier Moe

But as you say, there are so many variables ...... insulation, reflectivity of roof, roof venting, single or dual pane windows, airtightness of the structure, even natural shade tree placement. You have asked a complicated question with no answers, and are shooting down people who are answering you. If you know all these things, just go ahead and do whatever it is you were going to do from the get go. Unless it is new construction where you can control the variables, you're pretty much stuck with a WYSIWYG situation.

Steve

visit my blog at

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watch for the book

A fool shows his annoyance at once, but a prudent man overlooks an insult.

Reply to
Steve B

The real factors are how clean the units are, and what type of compressor. Rotary scroll is far more efficient than an old piston compressor.

I've seen outdoor AC unit drawing about 4.5 amps at 220 volt. (2 or

2.5 ton, can't remember). Rotary scroll compressor.

Dirty units draw a lot more power.

Reply to
Stormin Mormon

Yes I like remote temperature controls on window units. I can't find the one I made. It was an electric heat thermostat with the generic snap action switch replaced with one that was the opposite as new, meaning I don't remember if new it had a NO or NC switch, but I changed it to be used for cooling instead of heating.

Reply to
Tony

No wonder you say it's complicated! I'm talking about using the same house for both *tests*, factoring in the weathers "cooling degree days".

I didn't think my question was so complicated. What is the rated current draw on the compressor I listed? That's all.

It seemed like somewhere there would be an easy answer, another web page maybe? I guess not.

and are shooting down people who are answering

I'm trying to use the abilities I have to do a simple comparison. Using my electric companies meter isn't very accurate as I explained. It's presision is +/- 19Kwh.

Reply to
Tony

The answer as to how many amps the compressor is rated out may not be complicated, but it also doesn't answer the question you are seeking, which is how much it cost to run your AC.

If you want to know how many amps the compressor is actually drawing, just buy a cheap clamp-on amp meter from HarborFreight or similar. They have them for around $12. But that leaves out power factor and how much the rest of the system, eg blower is using.

As I and others suggested, why can't you turn off any loads that might come on, eg electric water heater, attic fan. refrigerator, etc, then run the house for a couple hours of continuos AC and a couple hours without AC and record the electric meter readings. That will give you how many Kwh it takes to run and you'll be way ahead of where you would be with the amp rating of the compressor.

Reply to
trader4

Like I said before... A couple hours run time using a kwh meter with an accuracy of +/-

19Kwh??? That may show my AC runs for free.
Reply to
Tony

Have you even taken a look at your electric meter? When that AC is running, the meter is going to be flying and in a couple of hours you will indeed have enough usage to increase the least significant digit or two digits, substantially. Certainly enough to get a good enough reading to determine the approx cost of running the AC continuosly for those two hours. There is no way you could falsely conclude that it's "running for free." Assuming you did what I suggested, which is to turn off any other large loads that might come on, ie electric water heater, pool pumps, etc. just taking the entire usage during that test period and counting it as AC usage isn't going to be too far off. Think about how big your ENTIRE bill is during months with no large loads, ie no heating, no AC. Divide that down to 2 hours, and it isn't much compared to what the AC will use.

If you want to get a better read on the low usage from the rest of the house when the AC isn't running, measure it for 8 hours, or whatever it takes to move the meter enough to get a good reading. Or better yet, just open all the other freaking breakers so there is no other load other than the AC for two hours.

This ain't rocket science.

Reply to
trader4

Yes many times.

When that AC is

Sorry, my meter doesn't do anything fast. It's not mechanical and the little fake LCD "spinner" thing doesn't move fast either. To read my meter you must multiply by 20. If I stand in front of it watching, the number changes 1kWh for every 20kwh's used.

There is no way you could falsely conclude that

If I read it only an hour apart it may very well have the same reading. It may have the same reading for hours! If the reading just turned to

0003777, it takes the use of 20 kwh's to change to 0003778. So if it's been a few hours, it may very well have the same reading it did when first checked. That is why I said it's accuracy is +- 19kWh. It all averages out in the long run, but for less than a few days or more, it's not very accurate at all.

Assuming you did what I suggested, which is

Two hours isn't nearly long enough and it would be tough to turn off everything else for a couple of days.

It seems to be, since no one seems to understand how my electric meter works.

All I asked was how much power does my compressor draw. I already figured in the fan and blower motors, I just needed to know what the compressor draws.

As someone suggested, I didn't buy it yet, but I put an "amp clamp" meter on my Harbor Freight shopping list. So thank you to everyone who replied and I will take the advice of getting an ammeter (and an hour meter to keep track of how long the AC is actually running).

Reply to
Tony

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?

Reply to
Tony

Thanks, that is what I'll be doing. Harbor Freight is about 60 miles away so it will be a little while until I can get there again. (I was a Harbor Freight virgin until last week. I see what people meant about not leaving the store with only the item I intended to buy!)

Thanks again, amp clamp is the way to go.

Reply to
Tony

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.

:-)

Reply to
cjt

^^^^^ 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

Reply to
cjt

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