I have been leaving the AC on all year here in San Diego. I keep my windows closed leave the AC On even when the system is heating (it drops into the high 30's at night in the winter). It keeps the inside dryer.

My question is twofold:

1. Does leaving the AC ON reduce the life of the system?

2. Does leaving the AC on reduce mileage even when the compressor is not actually working?

(Paraphrased from online sources)

Does it reduce the life of the system? No, this will help keep the system active and fluids flowing through the a/c compressor. In point of fact the AC should be run for 30 minutes once a week during the winter to keep everything in good working order. This helps reduce the loss of refrigerant which occurs when the air conditioning unit is not used for extended periods of time. If the air conditioner is not run during the winter, a problem with the system may not be noticed until the summer. Which means the you could wind up driving around without air conditioning when the hotter months come around.

A 2004 report from SAE and General Motors reached the same conclusions as the MythBusters. In their test conditions, they found that running AC was less fuel efficient than having the windows down. The test studied an SUV and a sedan at low, medium and high speeds. Not surprisingly, both cars were the most fuel-efficient with the AC off and windows rolled up. When the windows were rolled down, fuel economy dropped, especially for the sedan, due to the increase in drag mentioned above. For both cars, fuel efficiency was at its worse with the AC on, windows up.

Thanks for the response ... My AC is on auto (SL) and I like to keep the windows up as it's quieter inside. As for mileage, it was much better on my Altima SL but "it is whet it is".