I would not go smaller than a 3000 watt, and would get a bigger one if you are planning to run the a/c all night! I ran a 6000 btu window unit with a 2000 watt Honda, but you could not run a lot of other stuff at the same time! It would run for 8 to 12 hours on a tank of gas,(just a little over a gal.) depending how hard the a/c was working! So, in a class A running the roof air, you will need 3k to 5k watts, if you want some cushion! the Honda's and Yamaha's are very quiet, around 59 dsb. so you can barely hear them running! JMHO! Rail!