What gauge extension cord do I need for a 15-amp window AC?

Use a 12-gauge extension cord minimum. Don’t use 14-gauge—at 50 feet, it drops voltage, making the compressor work harder and creating a fire risk. 12-gauge handles the load safely, and 10-gauge is overkill unless the run is over 100 feet. Make sure the cord is rated for outdoor use and plug it directly into a wall outlet.