Best way I can tell you is to look at the starting amps needed to cycle the AC on (So say it takes 18 AMP) and multiple that by 120. That will give you the amount of watts you will need..... Then factor in other items that will be running at that time.
So 18 AMPS x 120 Volts = 2160 Watts at Start Up...
Most AC units will have a bigger hit at start up.
I have heard smaller campers with AC getting away with 3000 watts, but that was on a 13,500 BTU system.