Several things to consider before deciding. One is the size of the air cond. as some pull more power, second is the extension cord as they come in different size wire, some usable, some worthless. IF it seems close to working on a cord, the length of that cord is also a primary factor as longer means more resistance, meaning less likely to work.
Where and how is this test to be done? If it at your house and not too many others around, you may get away with a cord but if there are kids around or somebody steps on the cord while it is overheating, it can get BAD in a hurry. People can get hurt or killed so don't press it if all the small points are not well understood.