Prevent Portable AC from tripping breaker
A gadget that shuts power off at the outlet before it hits 15A load, is not really the solution for your problem.  For instance a healthy air conditioner will ordinarily spike above 15A, but return to spec limits soon enough.

The real problem is elsewhere.  Either


Your air conditioner has a problem, and is drawing more than it's supposed to.  This often happens to air conditioners that are near their end-of-life, i.e. that have a slow leak.  In any case, 11.4 amps is a lot for a window air conditioner, I suspect a new, efficient one may pay its cost in electricity savings.  
There are other loads on that circuit that you aren't telling us about, or that you don't know about.  


For the first one, you can test with a common power meter such as a $20 "Kill-a-watt".  Don't sweat a momentary startup spike, watch for continuous loads higher than advertised.  If you see that, window air conditioners are generally not repairable (or worth repairing) and it's better to upgrade to a modern, efficient one anyway.  For the same amount of cooling, look at BTU, not watts or amps.