In some cases, you may agree that split AC is a better option than window AC in energy efficiency. But a 5-star rated window AC is as power-efficient as a 4-star split AC. Also, if you are looking for AC with a built-in inverter, it will consume very less power. Although more expensive. An older 1.5-ton AC uses about 1.5 units per hour, while an inverter air conditioner uses only 0.91 units per hour.