A/C effect on mpg

The effect is largely dependent on speed. In short, the slower you go, the greater the impact. That's because without AC the engine runs less at lower speeds. Though the AC runs off the battery, the engine needs to run to recharge it. Stop and go is probably the worst-case scenario.

See this for more discussion and a chart showing the effect of the AC and windows down at various speeds.

 

AC impact can be significant, especially when wives are involved

Here in Phoenix, I can still get my average back up over 50mpg at 110F with a few tricks so only a couple mpg off my average. Wife gets about 42-44, instead of her normal 48-50.

I haven't gotten to drive the Prius a lot this summer but here are my observations;

1. Keep AC on auto, and as warm as comfortable. In our dry climate I find 82-84 fine. The Prius cranks back the electric compressor not just the fan, so it makes a big difference vs. just setting it on medium or higher all the time.

2. While AC is strong, avoid stops and gliding. AC will burn down battery and kill mileage. I generally do better on highway than in town under this condition. Once the AC dials itself back to low you can pretty much resume driving as normal, pulse and glide, momentum driving, whatever.

3. Park in shade. Makes a big difference how quickly car cools down. The longer the AC blasts, the more your FE will suffer. Anything you can do to speed up the cooldown will help alot (shades, tint, crackwindows, lower windows for a minute after starting up etc). Short trips are a killer for the same reason. You never get down to low AC where you can make up some of the damage.

4. If you have to make stops or glide while AC is blasting, turn off the compressor and make sure you are on recirculate. The AC system pressure takes a while to bleed off, so you will still get cold air for 10-20 seconds. After that the fan will keep you fairly comfortable (as long as its not pulling in hot air from outside) for a minute or two. Don;t do this if the car is still really hot and the battery fan in the back seat is running.

5. Keep acceleration gentle for the first few minutes. When the car is really hot the battery controller will prevent the battery from charging or discharging by setting its current limits to more or less zero to protect the battery. Since the AC is electric, and the battery can only really be cooled by using the AC to cool the cabin and then blowing cabin air across the batteries, the AC has to be powered directly by the ICE via MG1. The combination of this extra load and the lack of electric assistance from MG2 means the ICE is much more likely to rev up into its very inefficient ranges, really killing your FE.

6. Avoid sitting in the car with AC running and the car not moving at all costs. Its amazing how fast that pulls you down. This is my wifes biggest impact for sure. In fairness she is usually nursing our 3 month old, but still! Where are her priorities?

Good luck!
Rob

 

Or you can move up north.....

 

In the summer Set the A/C for Auto 77deg. and forget about it.
That will keep you, your wife, and your hybrid battery cool and comfy.
Turning off the A/C in hot weather will worsen your MPG because the battery gets hot.
Now devote your attention to driving the car efficiently. Learn how to glide. That's where infinite MPG is. Increase your tire pressure. Don't follow other cars to closely, thereby avoiding stop and go, and giving youself more room & opportunity to glide.
Exploit down-hills to glide. Learn how to shut down that ICE with you right foot, anytime you please.

 

What 9G-man said.

Oh, and ventilate the car for a couple of minutes before starting the AC

 

Here in AZ I've found driving with the windows down for a few minutes makes a huge difference in how hard the A/C is working. This pulls all the 150+ degree air out of the car and replaces it with hot, but much "cooler" 110 degree air. When sitting in the sun, the A/C used to be at it's highest fan setting (auto A/C setting) entire way home from work (20 miles). Now if I roll down all 4 windows for the first 2-3 miles, my auto A/C has my fan speed on around medium within several minutes (temp usually set to around 77-79)

 

What 9G-man wrote. Really hot weather and short trips hurt MPGs, and *not* using the A/C hurts them even more. I see this every summer in Dallas.