Air conditioners typically need a special outlet, and the three most common require 115 voltage, 125 voltage and 220 voltage. A common rule of thumb is air conditioners that provide less than 15,000 BTUs use the lower voltages, but units over 15,000 BTUs require the 220 voltage.
The manufacturer lists the type of the plug needed on the box, as there are five commonly used versions that don't fit with normal outlets. When purchasing a window unit, the buyer needs to confirm voltage and the plug setup to ensure it works with the available outlet. The BTUs are factored by the room size and the cooling capacity of the individual unit.