BTU
BTU, or British Thermal Unit, is a thermal heat measurement. It takes one BTU to lower the temperature of 1 lb. of water in one minute. A 120-volt air conditioner typically has between 5,000 and 14,000 BTU, while a 220-volt air conditioner has 15,000 to 23,000 BTU. The BTU you need depends on the size of the room you want to cool. A very powerful 15,000 BTU air conditioning unit will not cool a 100-square-foot room better than a 5,000 BTU unit -- it will cool the room unevenly and run overly short cycles. If a 5,000 BTU air conditioner is placed in a 1,000 square foot room, it will struggle to keep it cool. Always measure the room you're cooling and use the size to determine the BTU needed.
Circuit Compatibility
A 120-volt air conditioner plugs into standard wall outlets, making them a convenient choice for most homes. A 220-volt air conditioner requires a larger three-pronged outlet for heavy-duty appliances. If you have a very large space that requires more than 15,000 BTU to cool it, you'll need to have an electrician install a 220-volt outlet. It's not possible to force a 220-volt plug into a 120-volt outlet, and it's dangerous to try.
Efficiency
When running cycles that are equally long, a 120-volt air conditioner and a 220-volt air conditioner are equally efficient, using roughly the same amount of energy. Both types run inefficiently if improperly placed -- 120-volt air conditioners are inefficient and use excess energy when they are placed in a room that's too large for it, and 220-volt air conditioners lose efficiency when placed in small rooms. To ensure the maximum efficiency of your air conditioner, match the BTU to the room size.
Portability
Typically, 220-volt air conditioners are larger and heavier than 120-volt air conditioners. 220-volt air conditioners for home use are either window or portable floor units, while 120-volt are available in more compact versions as well as portable versions. If you plan to move the air conditioner from room to room, a 120-volt unit will run connected to any standard outlet and is light enough to move easily once drained.