Does the 7.5 have the info screen that shows how much fuel each of the cars systems is using, like it does on the 7 ??
Some things, its labelled "convenience consumers" and it gives a figure for gallons per hour.
I can't remember if AC is one of them, certainly heated screen, heated mirrors and heated seats appear on there.
But yes, of course AC does consume fuel, it can't appear from nowhere, that's basic physics but having climate control and the windows shut of course reduces the amount it consumes.
It will use more energy cooling or heating to a temperature than keeping it there.
Seriously Jeni, its the least of your problems and avoids other more expensive problems (like moisture build up in the car or needing your aircon cleaning out due to non-use)