# airconditioners and power usage

how much difference is there in power use with my car air conditioner between the low setting and medium and high is the compressor working the same at low as high and just fan speed increases or does the compressor work equally as hard to “keep up” with fan speed

I don’t know how much more strain on the compressor there is but I know there is more strain nad the compressor takes more power to turn. Also the fan itself will take more power.

That said the a/c might be more efficent at the high setting because you also move more heat.

The compressor is turning exactly the same, only the fan speed changes. Generally you would have it at high initially and shift down once the car is cool.

Sailor - what do you meen exactly the same. The rpm’s are the same - but is the quantity of gas compressed the same?

The compressor must do more work to move more heat.

Again, it depends on how you measure it. To bring the car’s temperature to a certain point involves transferring a certain amount of heat energy from one place to another. With the fan on low, fewer BTU per minute will be transferred, which means that the compressor will be “easier” to turn, but will have to work that much longer to reach the desired car temperature. The total energy expended by the compressor during the cool-down phase should be approximately equal when integrated over time.

This ignores the heat that is added back to the inside of the car due to the Sun, air leaks, and conductive transfer through the windows. The heat added by Sunlight will be fairly constant, but the heat added by conductive transfer will vary with the temperature difference. I don’t have the background to know exactly how to correct for those factors, but I suspect that when integrated over time, they will be the same in both cases. I.e. if the fan is on low and takes 10 minutes to reach desired temperature v.s. the fan on high and takes 5 minutes, and then spends 5 more minutes only working hard enough to maintain the temperature. When integrated, I suspect they will be very close to each other.

The only thing left is the power used by the higher fan speed. I’m guessing that this is probably fairly small compared to the energy required by the compressor to transfer the heat, but it would represent a small savings for using the lower fan speed.

Now that I think about it, the heat conducted through the windows back in the car would give an advantage to the lower fan speed. Since the rate of heat transfer depends on the temperature difference, more BTUs of heat will enter the car when the temperature drops quickly.

Don’t ask me to estimate the percentage difference tho.