This is a perfect exampe of the flaw of using a reciprocal unit to describe fuel efficiency, that is using miles/gallon instead of gallons/100-mile for overall economy or gallons/hour for cost to run accessories. This alone has led to much of the confusion I hear/read about fuel efficiency.

On a lot of websites I see people saying things like "expect this air conditioner to cost 1-2 mpg" which is absolute nonsense. The difference between going from 50 to 49mph vs going from 20 to 19mpg is almost a 3 to 1 in actual fuel consumption, but that does not mean a Hummer's air conditioner is more efficient than the one on the prius, just because it costs one .25 mpg and the other 4mpg. The most accurate way to describe cost to run air conditioning would be in horsepower or kW per hour, which could be approximated in gallons per hour* -- very easy to compute in dollars.

* Smaller engines may actually be more sensitive to load changes, depending on what RPM they are running at so even this isn't terribly accurate.