So...does using the A/C affect my mileage?

[andreax716]

Sorry if this is a dumb question I am a new owner of an '08 magnetic grey, and one thing I am always wondering is if using the A/C drains gas in my prius the same way it drained gas in my gas vehicles?

My first tank avg. 42 mpg and I ran the A/C about 40% of the time (never on max)...I'm wondering if I will get better mpg if I don't use it?

[Bill Merchant]

Not a dumb question, but not a simple answer either. Using the AC in your Prius will not hit your fuel efficiency as hard as using AC in a conventional car, but all the power in a Prius comes from gasoline so it will affect your milage. The Prius compressor is electric and runs off the HV battery, so you can get cooling with the ICE off. Also, it's a scroll compressor, so if you only need a little cooling, the compressor only works a little, rather than the full on/full off of a regular car's AC system

NOT using your AC can also adversely affect your milage. Cabin air passes over the HV battery to cool it (or warm it in winter). The battery works best at temperatures you would be comfortable in. So if you think you'll get better milage by turning off the AC on that 90° F day, you won't. The battery will get hot and not put out as much electricity so the ICE will have to run more.

The third thing to consider is drag. If it's 70° F and you're driving at 30 mph, turn off your AC and roll down the windows. But when you get on the interstate, turn on the climate control and close the windows for better fuel efficiency. The Prius has a very low coefficient of drag, with the windows closed.

And welcome to PriusChat!

[patsparks]

Hey Bill, a top answer but I have a little correction.
Scrolling compressors are used in automotive air conditioners a lot but the electric compressor in the Prius is supplied through the inverter with just enough power to maintain the cabin air temperature at the desired temperature all the time so when the air conditioner is on the compressor doesn't cycle like a conventional automotive air conditioner (where the compressor speed is determined by the engine speed) but the speed varies to maintain a stable temperature using the minimum amount of energy. This allows a Prius full cooling when stopped (normal systems don't) or at highway speed and with more consistent temperature control.

This diagram is for a home air conditioner but it gives you the idea. No peaks and troughs.

[9G-man]

Originally Posted by patsparks

Hey Bill, a top answer but I have a little correction.
Scrolling compressors are used in automotive air conditioners a lot but the electric compressor in the Prius is supplied through the inverter with just enough power to maintain the cabin air temperature at the desired temperature all the time so when the air conditioner is on the compressor doesn't cycle like a conventional automotive air conditioner (where the compressor speed is determined by the engine speed) but the speed varies to maintain a stable temperature using the minimum amount of energy. This allows a Prius full cooling when stopped (normal systems don't) or at highway speed and with more consistent temperature control.

This diagram is for a home air conditioner but it gives you the idea. No peaks and troughs.
<IMG style="MAX-WIDTH: 800px; ; WIDTH: expression(this.width > 800 ? 800: true)" alt="" src="http://www.airmarkairconditioning.com.au/img/cool_inverter.jpg" border=0>

Very, very interesting.
I've never seen that.

[9G-man]

Originally Posted by andreax716

Sorry if this is a dumb question I am a new owner of an '08 magnetic grey, and one thing I am always wondering is if using the A/C drains gas in my prius the same way it drained gas in my gas vehicles?

My first tank avg. 42 mpg and I ran the A/C about 40% of the time (never on max)...I'm wondering if I will get better mpg if I don't use it?

As Bill pointed out, use of a/c has less negative impact on gas mileage in the Prius, than other cars. For steady speed highway driving it's use has little impact. An ideal set temp for the cabin is 77 deg. for a best balance of comfort and fuel economy.

Use of a/c at lower speed inhibits you ability to attain and maxize glides, and stealth, if you are into that.

But by the some token, when it is really hot, not using a/c can have a negative impact, because the batteries need to be kept cool, to allow extended stealth (electric operation.)

Moral: to maximize your MPG, if it's not hot, don't use it. Just use vent air. If it is hot, use it, but set it to 77 deg for an ideal balance of comfort and economy.

[andreax716]

Thanks for all the great info!!

[dwdean]

Originally Posted by andreax716

Sorry if this is a dumb question I am a new owner of an '08 magnetic grey, and one thing I am always wondering is if using the A/C drains gas in my prius the same way it drained gas in my gas vehicles?

My first tank avg. 42 mpg and I ran the A/C about 40% of the time (never on max)...I'm wondering if I will get better mpg if I don't use it?

Andreax,

See the previous answers above to the "whys and wherefores", I want to put a little real-world practicality around this one.

I live in south Florida. Today is a moderately hot day; the high will be around 85 F. Do I use the A/C? You bet (I've got it set to the mid-70's)!

If doing that has put a dent in my mileage, I haven't seen it, my average right now is 53 mpg. OK, I know that's not anything to brag about compared to what some of the really serious FE guys are getting, but it's more than double what I was getting before (with our without the AC.)

I haven't gotten brave and tried driving without the AC yet (I really don't want the car to start smelling like a gym.) So I can't say how big the AC "hit" is, but I'm happy with the FE that I'm getting with it running. Be comfortable and enjoy the new car.

As an ex-pat Bay Area resident I know you're going to have less call for AC than I do now, so one of the AC "tricks" I've read about may help you somewhat. The trick is to set the AC about two degrees lower than the outside temp, that'll cause the exterior vents to open and you'll use the fan less. I tried that here, and well, setting it at 83 didn't do much for making the car comfortable. In your more temperate climate, it'd probably be OK. Can't really say if that impacts your FE any, but I think that there are threads on here talking about that relationship.

Good luck!

*****Edit added 05/19/08*****
Experience with weather so hot that the AC couldn't keep up (i.e., the thing was running at Max for protracted periods of time) indicates that you can suffer a real FE hit. With outside temps in the 95-100 this weekend and the AC set to the standard 78, I was seeing about a 10% FE hit.

Simply adjusting the set temperature up some took care of that and kept me comfortable at the same time, the AC just didn't "blow icicles". I keep forgetting that the AC on this car isn’t “set it and forget it” like other cars, you can actually change how hard the compressor works by adjusting the set temperature.
*****End Edit 05/19/08*****

[morpheusx]

If you are travelling at a steady speed with little to no stops you will notice practically no difference in your mileage. At stop and go driving you will take a good bite if traffic isn't flowing.

[mheinle]

What a great question! I too am a new owner and like a driver of a gas guzzler assumed the A/C would hit the MPG pretty hard... It is amazing how much info can come out of one not so stupid question.
~thanks

[berinker]

Originally Posted by mheinle

What a great question! I too am a new owner and like a driver of a gas guzzler assumed the A/C would hit the MPG pretty hard... It is amazing how much info can come out of one not so stupid question.
~thanks

OK, here is another stupid one. If I leave the A/C on in the manual mode
(not auto) but it is set at a higher temperature than the outside air - such
as start-ups in the morning so it is obviously not cooling - will it cut down on my mileage as if I manually switched it off?
Bud