My 4K recently went south so I replaced it with a 5K... I only have 1 roof air unit and the reason I went larger instead of smaller is that even with the 4K you would have to turn the roof air off if you wanted to run the microwave or make coffee as it would overtax the generator.. I would think a 2.8K would be working very hard to power 1 roof air unit alone.. If you want to figure your power output, Volts X Amps = Watts.. A coffeepot uses about 1200 watts alone and that is about half of your generator output already..