I doubt that AC is anywhere close to 20 amps. The start up may spike up to a big number but running, these things are not that big a load. A 144 sq/ft building should be fine with half a ton. A 6k BTU window shaker pulls about 5a.
The playhouse we set up for the kids has a big old CRT TV in it, lights and a 1/2 ton AC. It runs fine on a 15a circuit.
I think the TV draws as much as the AC.
