AC sitting effect on MPG

I had the need to leave the car in ready state with AC on 74, outside air temp at 80, for two hours yesterday. I was wondering what it would do to my MPG average and how I might measure the effect.

Turned out it was not as bad as I expected and very measureable. I had driven 36 miles and was showing 50.x at the start. At the end, the MFD showed 40.x MPG. That works out to less than 0.1 gallons per hour.
(36 mi @ 50mpg = .72 gal, 36 mi @ 40mpg = .88 gal,
so I burned 0.16 gal for 2 hours of comfort, or roughly 0.1 gal/hr)

If I continue driving with an average MPG of 50 the lost 0.2 gallons will have cost me 10 miles. Around 390 miles I should be 8 gallons down giving 48.8 MPG instead of 50 MPG.

After a year of driving 12K miles that two hours sitting will have cost (10 mi lost) or 0.04 MPG (49.96 MPG instead of 50 MPG). It looks like I can sit for another 23 hours this year and still end up at 50 MPG.

Florida is so "cool".

Tovli