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

[Bill Merchant]

The thing to remember is that the Prius has an automatic climate control system. If you set it manually, you get what you choose. If you set it to Auto, the car gives you what you want.

Let's say you have chosen 75° F, Medium fan speed, and the AC compressor disabled; if the ambient outside air temperature (OAT) is 45° the heater will come on to raise the interior cabin temperature (CT) to 75° and the fan will run at medium, unfortunately, the windows start fogging from the interior moisture. You drive into the desert and the OAT is 95°; the heater stops since there is no need for heat, the fan blows at medium, and the CT goes up to 105° (solar loading through the hatch and windshield). You're sweating and the HV battery starts to overheat so it cuts power and you use more gas to move.

Let's say you have chosen 75° F and Auto. The OAT is 45° so the fan comes on high along with the heater to quickly raise the CT to 75°; once it's there the fan slows and the heating is reduced to maintain the CT, the compressor runs slowly to remove the moisture and keep the windows clear, then shuts off. You drive into the desert, the sun is shining, and though the OAT is only 75°, the compressor starts to slowly run to keep the CT at 75°. As the sun climbs higher and the OAT goes to 95°, the compressor provides more cooling and the fan speeds up to keep the CT at 75°. Driving west, so the sun is pouring in the windshield, the air conditioning system switches the air vents to direct more cool air toward your face and the fan speeds up. The cabin air, flowing over the HV battery, keeps it running at peak efficiency.

[N3FOL]

Originally Posted by 9G-man

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.

At 77°, I'll sweat like a dog with the windows up . I never roll down my windows even when travelling at 30 mph. Vent is usually my first choice. Once it gets warm inside the cabin, I turn on the AC and set it at 68°.

The first time I used my AC I thought it was going to stop working once the ICE turns off...but it didn't to my amazement.

[berinker]

Originally Posted by Bill Merchant

The thing to remember is that the Prius has an automatic climate control system. If you set it manually, you get what you choose. If you set it to Auto, the car gives you what you want.

Let's say you have chosen 75° F, Medium fan speed, and the AC compressor disabled; if the ambient outside air temperature (OAT) is 45° the heater will come on to raise the interior cabin temperature (CT) to 75° and the fan will run at medium, unfortunately, the windows start fogging from the interior moisture. You drive into the desert and the OAT is 95°; the heater stops since there is no need for heat, the fan blows at medium, and the CT goes up to 105° (solar loading through the hatch and windshield). You're sweating and the HV battery starts to overheat so it cuts power and you use more gas to move.

Let's say you have chosen 75° F and Auto. The OAT is 45° so the fan comes on high along with the heater to quickly raise the CT to 75°; once it's there the fan slows and the heating is reduced to maintain the CT, the compressor runs slowly to remove the moisture and keep the windows clear, then shuts off. You drive into the desert, the sun is shining, and though the OAT is only 75°, the compressor starts to slowly run to keep the CT at 75°. As the sun climbs higher and the OAT goes to 95°, the compressor provides more cooling and the fan speeds up to keep the CT at 75°. Driving west, so the sun is pouring in the windshield, the air conditioning system switches the air vents to direct more cool air toward your face and the fan speeds up. The cabin air, flowing over the HV battery, keeps it running at peak efficiency.

Thanks for the detailed information but you
still didn't answer my question - I don't think?
If it is cool outside (lower than the temperature inside)
and I have the A/C turned on, you say the A/C air compressor
will be off. Does that mean that if I switched the A/C to off
would my mileage be the same as it was when it was on?
I know that on some other (maybe older) cars, when the
A/C switch is on the compressor is on all the time.
Thanks,
Bud

[JimboK]

The Prius MPG Simulator performs modeling using a variety of user-specified criteria, including speed, outside air temperature (OAT), automatic climate control (ACC), and the following combinations of window positions: all up, all down, two diagonally opposed halfway down, and two 1" down.

The attached chart shows results with 90F OAT, with and without ACC set to 75F (the temperature I find most comfortable in the summer), and all window position models. The models assume the following:

• Steady speeds
• Level terrain
• No wind
• 190' elevation
• 30.00 barometric pressure
• Smooth dry pavement
• Michelin Hydroedges inflated to 50/48
• E10 fuel
• 300# load, driver + cargo

The conclusions:

• All windows up and no ACC gives the best results across all speed ranges -- for the brave soul who can tolerate it.
• Two diagonally opposed windows 1" down is next best, and might be tolerated by some at high speeds.
• Two windows halfway down is superior to ACC at speeds below the low 50s.
• All windows down is superior to ACC below 40 MPH.

Specific values of course will change somewhat with different combinations of typical summertime OAT and ACC thermostat settings, but I doubt the overall relationships change much.

[Sheepdog]

here's what I noticed. I drive to work and the avg mpg is at say 50.0 I park at 215pm and sit in the car until 230pm with the ac still on and the car in park. it may cycle to recharge the batteries once.

By the time I turn off the car at 230pm to go in my avg mpg is at 49.8 !

Seems like the gauge is telling me it matters!!

[Bill Merchant]

As I previously said:

Originally Posted by Bill Merchant

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.

[abq sfr]

If you want to see your mpg go down fast, leave your better half in the car on a hot day while you run in to Worst Buy and leave the AC on so she is comfortable. In only 10 minutes or less your battery will be down to the minimum with the engine coming on frequently to keep it above 40% soc, and when you drive away you will be on 100% gas engine for a while (and probably 20 mpg when accelerating) until the soc comes back up.

Something else to know about the AC... the solar sensor on the left inside near the windshield. I was driving this weekend, there was blue sky and sun in between clouds, every time I went in the shadow of a cloud the AC blew slightly warmer, when in the sun, it blew cooler. I keep the thermostat at around 78 to 80, if you keep yours much cooler than that you might not notice. This is such an amazing car!

[maximus2112]

Well, the last couple of days (and the next couple of days) it has been
99-102 in Raleigh, NC. I rarely go above the 2nd fan setting, but I had
the AC blasting at MAX right after getting into the car. Needless to say
I have been struggling to maintain my MPG. I'm struggling to hold at 47mpg
right now. Its good to know it is not only the AC but the battery that is
affect by the heat, not to mention the driver!

[bbald123]

Originally Posted by berinker

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

My experience this winter showed to my satisfaction that turning the AC off on the climate screen, but leaving the system in automatic mode to heat the car as necessary, increase my mileage about 2%.

The negative was no drying of the cabin air so condensation was an issue in some conditions. In the long run, as mileage improved with increasing temperature, I decided that leaving the AC on was an acceptably small hit to mileage and I leave it on at all times now.