An air conditioner for a medium-sized room has a cooling capacity between 12,000 and 16,000 BTU. With these, you can cool down a room of up to 40m2. Think of a small living room or large bedroom. Are there many electric products in the room, or does the sun shine through the windows all day? The air conditioner won't cool as well, so it's better to choose an air conditioner with a higher cooling capacity.