Cost to run central A/C

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.

Reply to
Tony
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OK, so your meter moves slower than the meters I have had.

But, it would seem that fake LCD spinner thing must move at some rate substantially faster than the speed with which the lowest digit on the meter increases. Mine moves very fast, easily 20X or more for one increase of the lowest digit. You say the lowest digit on yours equals 20Kwh. If it takes just 10 rotations of the spinner to make the lowest digit increase by 1, that means one rotation is 2Kwh. An AC unit is probably using on the order of 6KW. So, in an hour the spinner should be rotating about 3 times. If it's 20 rotations to increase the digit by 1, which I would think is more likely, then the spinner will rotate 6 times in an hour.

Given that, I don;t see why this procedure wouldn't work:

Turn on the AC and as many other large loads as you can, eg ovens, dryer, stove, water heater, etc. That will get you an increase in the lowest digit in under an hour. Keep an eye on the meter or pay your kid to watch it until the lowest digit changes once. Then count the rotations of the spinner until it changes again, which will be

20kwh. You probably don't need to watch it the whole time. Just record the time the meter digit changed and how long it takes for one spinner revolution. Then record the time when the lowest digit increases by one again. Based on how long it takes for one rev and how long it takes to increase one full digit, you can calculate the multiplier. As an alternative, you could call up the electric company and maybe they can tell you if you find the right person.

Knowing that, just turn on the AC with all the other loads turned off and record how long it takes the spinner to make a full revolution and you can calculate the cost to run the AC per hour of use.

I don't know why you need to know, nor how exactly you want it, but for any purposes I would have, I could just look at my electric bill for April and May, compare it to July and August and know close enough without doing anything else.

Next you'll be worrying that the amp meter only records amps and without knowing the power factor of the motors involved, you still won't have the true energy used.

Reply to
trader4

So take your readings several days apart.

Reply to
cjt

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.

Reply to
Tony Hwang

I think I wrote that a bit misleading. I was talking about the kwh meter from the electric company. I do however have a clamp around ammeter on my shopping list. I'd like to get one that reads DC current also, of course they are more expensive.

Reply to
Tony

If ever a thread needed a summary....

No clamp-on meter will do what you want. You need to know the phase angle between the current and voltage 'cause you're billed by the watt-hour. Amps times volts is volt-amps. Different by the power factor.

I've been logging my AC consumption for several days to determine how much it costs to run. It's not rocket science. You use the power company meter. Old meters have a wheel that goes around. They count the number of times it goes around and send you a bill. This is 100% accurate. Any uncertainty in your measurement is due to your ability to operate a stop-watch. Newer meters have a row of numbers with an underline that moves left to right to simulate the wheel. Mine also emits an IR pulse for every movement of the underline. Calibration varies. It's written on the face of the meter in terms you probably don't understand. Mine is one watt-hour per blink of the IR pulse..You can call the power company and get the factor to multiply the readings to get kwh.

I programmed my palm pilot to read the IR pulses and readout in watts then graph that vs time. But that's another story.

Put your house in a stable power configuration with the air conditioner on. Go measure how long it takes for the wheel to go around. Timing multiple rotations can increase the precision by spreading your timing error over several rotations. Turn off the AC and repeat the measurement. A little math later, you have your answer.

Go repeat the entire measurement with some load you KNOW...like the water heater. If you get the right answer, your math is probably correct.

Now you know the power consumption of the part of the system you turned on/off.

Might want to repeat the measurement to make sure you get the same answer...just in case the fridge or water heater or something else came on.

You still need to time the on/off cycles.

As previously suggested, you can take a battery operated clock. My first endeavor used a cardboard flap on a microswitch placed over an air outlet. Air comes on, switch closes, clock runs till the air goes off. Second generation uses a palm pilot to sense the swtich and graph the data. I can tell you exactly how much it costs to run my air conditioner.

But if you run the fan continuously, you need a different system. BlueLine Innovations makes a wireless sensor that straps on your power company meter and reads out on a remote unit. Resolution is not so good, but close enough. YOu can buy 'em on ebay for about $35 shipped. I got mine at a garage sale.

If you can find a place to clamp it, a clamp-on meter into a DVM into a computer can provide the on/off logging function.

Knowing is only the first step. Now, what are you gonna do about it? By watching the percent-on-time over the day, I'm indentifying situations where solar heat gain is a significant factor and taking steps to reduce the cost significantly.

mike

Reply to
mike

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