Most, if not all, manufacturers of window/portable air conditioners recommend that this appliance be plugged directly into a wall outlet. However, sometimes that's easier said than done. Some houses, especially older ones, don't have enough outlets to accommodate all of your appliances. That's when you need to find an extension cord that will safely and efficiently work, for example, with your air conditioner. You can't simply grab a household extension cord. It's not heavy enough to carry the power the appliance needs. Not only can the air conditioner be damaged, but the extension cord can overheat and become a fire hazard.
Stretch the electrical cord on the air conditioner out loosely towards the outlet; leave some slack. Measure the distance from the end of the cord to the nearest wall outlet. Round the measurement off to the next higher number, if need be. Heavy-duty air conditioner/major appliance cords typically come in lengths of 3, 6, 9 and 12 feet.
Visit a local hardware or home supply store and find the department that stocks heavy-duty extension cords. They may be in with the major appliances. Choose an extension cord that's specifically labeled for use with air conditioners. They are all "general use" cords that work with any portable air conditioner, including a 5,000- BTU model. Choose the right length you need.
Read and follow the manufacturer's instructions and warnings on the package. Plug the end of the air conditioner cord into the heavy-duty air conditioner/major appliance extension cord. Then, plug the extension cord into the nearest wall outlet.