Unlike a window air conditioner or a portable unit which is "self-contained", a mini-split air conditioner or mini-split heat pump consists of two separate components: an outdoor condenser, and an indoor evaporator. A conduit, which houses the power cable, refrigerant tubing, suction tubing, and a condensate drain, links the outdoor and indoor units.
Ductless mini-split system make good retrofit add-ons to houses with "non-ducted" heating systems. They can also be a good choice for room additions where extending or installing distribution ductwork is not feasible, and very efficient new homes that require only a small space conditioning system. Since mini splits have no ducts, they avoid the energy losses associated with ductwork of central forced air systems. Duct losses can account for more than 30% of energy consumption for space conditioning, especially if the ducts are in an unconditioned space such as an attic. The US Department of Energy recommends ductless mini-splits as a way to reduce energy consumption and save money on your utility bill.