When selecting a room air conditioner, the most important measure of efficiency is the Energy Star label, which tells you a unit uses at least 10 percent less energy than conventional models.
Energy Star qualified room air conditioners come in a range of sizes (you can browse the program’s list of qualified room air conditioners
for the right match), and often include timers for even better control over room temperature and the energy you’re using.
Before going shopping, follow the Energy Star room AC sizing guidelines
to accurately calculate the unit size you’ll need. When selecting an energy efficient air conditioner, choosing a until that is too big or too small can lead to an uncomfortable indoor climate and reduce the energy savings you were after in the first place.