Weight
The lightest weight window units are commonly referred to as miniature or mini window air conditioners. The lightest of the lightweight mini window units weighs around 40 lbs. For example, Frigidaire manufactures a mini unit that produces 5,000 BTU and weighs in at approximately 42 lbs.
Physical Size
The lightest weight units are also the smallest air conditioning units. These units are designed to fit into small window openings that will not accommodate larger units. The smallest units are barely more than a foot tall and less than a 18 inches across.
BTU
The term BTU stands for British Thermal Units. BTU describes the cooling capacities of window air conditioning units. The smallest, lightest window units also have the smallest BTU ratings, with 5,000 BTU being the smallest size commonly sold. A 5,000 BTU air conditioner can only comfortably cool approximately 150 square feet of space.
Energy Efficiency Ratio
If you want to get the most efficient small window unit for your money, look specifically for a unit with a high energy efficiency ratio. The energy efficiency ratio is commonly called the EER, and window units that receive an Energy Star rating use up to 10 percent less energy to cool the room, keeping your cooling costs at a minimum. You should always look for a unit with an EER over 10 or an Energy Star label.