How do I determine if adding an air conditioner will overload a circuit?
The only way to know what load a device draws is to put the device in to service and measure the load. The rating on the back is interesting, but many factors affect actual loads. An easy way to measure real loads is with a Kill-A-Watt.

Expect breakers to trip at 80% of rating, under continuous load. So 12A in your case.