How much horsepower does it take to run A/C?

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I rarely use the A/C in my 3 when driving by myself. I drive with all 4 of the windows down traveling in the city at 0-55mph. My trip computer for this tank of gas has been in the mid 29mpgs. It is at 29.3 now. A few days ago, when I had a passenger in the car I used the A/C and the mpg dropped down to 28.3. This was only about 30 minutes of driving! The temps have been in the 90s all while this was done. I have noticed that higher temps result in better gas mileage. I assume it is because of faster engine warm ups. I do not know how low it would be if I used the A/C all the time like most people do, but it wouldn't be pretty. So, how much power does running the A/C typically use?
 
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If you figure it's not too different in capacity from an average window air conditioning unit in your house, and one of those typically takes about 13-15 amps at 120 volts, then that would be about 1500-1800 watts, or about 2 horsepower.
 
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Mind you though: even though home-based A/C units (by the horsepower) typically runs you over 12amps for a 11,000btu units (for approx. 1hp equivalent), automobile A/C units are way more powerful than that (it all depends on the engine's capability, the cooling capacity and requirements within the compartment itself, etc. In other words: in order to cool a small subcompact car, A/C may have to be capable of at least 2~3hps for such small confined space so as to be able to cool sufficiently (you have to factor in all the glass and windshields that let the sun in). So, typically, A/C units would run you about 3~6hps off of your engine while running. Q.
 

CharlieJ

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 Originally Posted By: Dave Sherman
or about 2 horsepower.
2 horsepower! It sure feels like a lot more than that when it is on and loading the engine. I've heard sayings that it can take up to 20 horsepower.
 
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I doubt it is 20 HP, a 3 ton home central air unit runs about a 5 HP compressor, and that will cool way more volume than an auto A/C a modern car A/C uses 3 HP on average. As far as A/C saving gas, depends on what speed you are driving at, since drag draws progressively more power at highway speeds. For me, a loss of 1 MPG makes it worth always running the A/C, for a penny a mile at most.....
 
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BTW, peak power usage on an A/C system, when the compressor first kicks on, can be much higher until the system gets to normal operating pressures, which doesn't take very long...
 

CharlieJ

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 Originally Posted By: KilgoreBass
For me, a loss of 1 MPG makes it worth always running the A/C, for a penny a mile at most.....
That was only about 30 minutes of driving, I am not sure how much fuel economy drops while driving with the A/C on in the city. I only use it on the highway, when raining, or if the temp is about 100F.
 
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 Originally Posted By: CharlieJ
 Originally Posted By: Dave Sherman
or about 2 horsepower.
2 horsepower! It sure feels like a lot more than that when it is on and loading the engine. I've heard sayings that it can take up to 20 horsepower.
No way in heck it would ever take 20 horsepower. Car AC units, especially newer ones, are actually pretty efficient. The only time I ever remember reading a direct comparison to home AC units was back in '93 when the Chrysler LH series came out, and they claimed that its AC cooling capacity was comparable to a 5-ton home unit because of the large cabin and lots of glass area. Having owned one of those cars, I'd say that was a fair claim- it had one of the most powerful car AC systems I've ever seen. And it certainly didn't draw any more than 5 horsepower at most. The reason it "feels" like a lot is that most cars only need about 15-20 horsepower to cruise at a steady 60 mph. Some take far less. If you're cruising along using 15 horsepower and a 2-horsepower load kicks in, you WILL feel it and it will make a significant difference in mileage.
 

CharlieJ

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I guess when driving a 4-banger the power decrease is much more noticeable compared to a 8 or even a 6 cylinder car with twice the power. My next car needs a bigger engine.
 
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 Originally Posted By: CharlieJ
I guess when driving a 4-banger the power decrease is much more noticeable compared to a 8 or even a 6 cylinder car with twice the power. My next car needs a bigger engine.
If you get a throttle by wire car, even a 4 cylinder, they usually program them so you can't feel the drag... until you want WOT. Something with a CCOT compressor will feel smoother too.
 
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I keep wondering if something is wrong with my TL. With the AC running it will hardly get out of it's own way. During acceleration from a stop to 65mph I can feel every time the comperssor cycles. It's so bad I keep thinking it's trying to freeze up and I'm worried about breaking the belt. I turn it off sometimes until I'm up to speed.
 
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On my old Cavalier I would not set it to "Max" so I could hit the A/C button to shut it off if I actually wanted to, you know, get going sometime today. It had no WOT cutout. This was when I lived in Vegas. That [censored] never cycled either. If you were idling and turned it on the engine would rev to about 1000 in preparation for the load. I like the V8 in my Vic. It just sits there at 600RPM and cycles the compressor. Just a smooth engine interrupted with the occasional *click*. You have to also factor in additional load on the alternator from electric fans also. My Vic has PWM variable fan. It makes kind of a spooky noise. OOOOOOOOooooooooooooooooOOOOOOOOOOOOOOOO..... :P
 
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Since most car A/Cs are driven off the accessory belt, I think it really depends on a lot of factors. The number one factor has to be engine speed. What it uses (and how powerful it is) must have a lot to do with whether it's at idle or at cruising speed. Essentially it's just a parasitic load on the engine. Has there every been a successful car A/C run off an electric motor? That could be run independent of the engine speed, but I'd wonder how fast it might drain the battery without the engine on.
 
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Prius has electric AC. Remember if your engine is running at idle instead of speed, that's 1/3 the RPM... but AC outputted is per minute, so it takes three times the torque. Add that to being below the engine's torque curve and most cars are real dogs until they get going.
 
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When they sit idling the high side pressures rise as well due to lower airflow across the condenser, only compounding the sluggishness felt with a lower torque engine. The sluggishness factor and fuel economy are likely why some mfrs. have lately used relatively expensive scroll compresors even on some lower priced cars.
 
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 Originally Posted By: eljefino
Prius has electric AC. Remember if your engine is running at idle instead of speed, that's 1/3 the RPM... but AC outputted is per minute, so it takes three times the torque. Add that to being below the engine's torque curve and most cars are real dogs until they get going.
All the Toyota/Lex hybrids have electric AC. This is one of the things I like the best about the Camry. EAC is fantastic. When you want to maximize FE, you punch the ECO button, and it works like any other car. Turn ECO off (normal) and you can get meat locker cold (black car, southern summer) very quickly. The best part is, though, is how if you're on a long trip, and want to stop and nap in a rest area, the EAC will run for as much as 20 min (depending upon how you set it) strictly off the traction battery (basically, the kinetic energy converted as you slowed into the rest area cools the car for that long), with the ICE totally OFF. ICE will then run for ~5 min recharging the TB. Then it'll cool, electric only for another 10-15 min. Very cool, literally and figuratively. The second coolest thing is that, even more than the DBW cars, there is simply ZERO AC sag as the compressor kicks in, like you get in so many smaller-engined cars. None. Also, these cars have true variable speed compressors. The electric system turns them only as fast as is needed. EAC WILL be the car AC of the future.
 
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Well the A/C in my cavalier is excellent. It is always arctic cold, and I can't imagine it draws more than 3HP. It is a dog when accelerating anyway, so the A/C really doesn't make much difference.
 
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The compressor in your cavailer (V-5) also was one of the first types to automatically destroke the compressor as cooling demand tapers off or the engine goes into the upper RPM range.
 
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Really? I had almost the exact same car and it always felt like I lost a cylinder running the A/C. Upper RPM was non-existant. More so with the A/C on.
 
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