We're running 36000 btu to cool 1200 sq. ft and it's struggling right now. Really gets bad in the afternoon when the sun hits the roll up door. I made a curtain out of some roll out attic radiant barrier to cover the door and that helped a lot. It was like standing next to a radiator. When it gets to 100 outside we still need to run the auxiliary window unit. That 15 foot ceiling makes a lot of cubic feet to cool.