Here in Florida I am only seeing about a 3-4 mpg drop with 90's muggy weather. This car is mostly driven on the highway with a/c at medium speed/low inside temp.
I have been monitoring my mpg with ac use for the past week. I clearly see a difference in mpg when the AC is full (6-7 bars) vs. 2-3 bars vs. no AC. With full AC, I am in the high 30s; medum AC - low to mid 40s; no AC gives high 40s and low 50s and once as high as 60mpg. I will try to plot a graph and load it here.
G'day from down here, I haven't yet any info on summer fuel use affected by the air conditioner as my Prius C is only a few weeks old and we're only just moving into summer, but I think posters should mention if they are running on recycled (cool) air or with fresh (hot) air coming in, that is, continually drawing in hot air to the cabin. And if so, how fast is the fan running (drawing more or less hot air in)? Radiant heat through the glass windows is a relative constant but the demand to cool the air could be significant, both on how cool is the temperature setting and how much air is to be chilled. David S.