Window-mounted AC units represent the largest category of portable AC appliances. Other portable AC units are designed as floor models that can be placed inside a room. Window-mounted AC units are designed to fit in the bottom half of a double-hung window. This type of portable air conditioner is designed to cool the room where it's installed. Units are rated according to BTU output, from 5000-10,000BTUs. Easy to install, but not as convenient as central AC Installing a window-mounted AC unit is a fairly straightforward process that most homeowners can accomplish on their own. With the window's bottom sash lifted open, the AC unit is placed in the opening so that the unit's bottom edge rests on the window sill. The bottom sash is lowered against the top of the unit so that the mounting flange rests against the sash; then the adjustable side panels are extended to fit against the side jambs of the window frame. A window-mounted air conditioner offers an affordable and immediate solution to a room that gets too hot.