If I run my AC will my MPG go down?

yes, electric motor. there have been estimates of 1-2 mpg hit depending on how much it has to run. also, on the highway, probably no hit at all, and some say it's less of a hit to keep your windows closed over a certain speed for aerodynamics and run the a/c.

 

Your HV (and 12v but that is not as important) battery is cooled by your A/C. There is a vent by the right rear passenger seat, so do not skimp on A/C.

Aerodynamics is better with the windows closed, so above city speeds A/C is cheaper than driving with the windows down.

And yes, it costs me 2 MPG in Mississippi compared to spring and fall.

 

The hit will be determined by your local climate and the setting you choose for your AC.

If it is very hot outside and you choose a low temperature setting then the mpg hit can be quite large until the cabin cools down and the compressor winds down. After that point the mpg hit is very small at 1-2mpg. Unfortunately it can take 30 miles or more if the temperatures are at extremes. i.e. it's 110F out and you have your AC set at 65F! Last summer I observed a 10mpg hit frequently when temps were very hot. Those hits generally lasted until the cabin cooled down.

Here is a thread on how to minimize your mpg hit.

http://priuschat.com/forums/gen-ii-prius-fuel-economy/97814-how-reduce-mpg-loss-because-c-use.html

 

Note that F8L is talking instantaneous MPG while I was talking seasonal MPG, we do not disagree, we just measure differently.

 

I completely agree.

Thanks for clarifying that. I should have been more clear.

 

There are no belts on the gen III design. The a/c uses an electric compressor so you don't feel any "hit" or "kick" like on a conventional ICE design.

I have an ultra-gauge in our Honda civic and can see a 2-3mpg difference on the instant mpg readout when pushing the a/c button on and off.

 

I see the same 1-2mpg hit in my GenII when the temperature outside is similar to what I have my AC set at. It's weird how it works in the Prius.

 

Noobie Q that I'm sure has been covered: What about using the A/C while parked? How does that work? Will it run off of the electric battery until it dies, then the ICE turns on to recharge it?

 

Yes, sort of. Sometimes the engine may not even shut down depending on the draw from the AC system.

 

As the a/c is electric you might hear a loud electrical humming/buzzing noise coming from the engine bay when you first turn the a/c on after the car has been sat in the sun. It's normal, alarming the first time you hear it, but it's normal.

 

I do it quite often. Yes, the a/c is powered off the HV battery until the battery reaches its usual low point and then the ICE comes on to recharge the HV battery. I find the ratio is about 10-1 to 8-1 (minutes on battery-minutes recharge) depending on how hot the ambient temps are compared to the interior setting.

Some folks even "camp" this way using the Prius as a cheap motel. They report it will consume about 3/4 gallon per hour. Of course, the cooler the night, the better you "mpg" will about.