Window AC units are generally inexpensive, easy to install and preferable for renters as many San Diego landlords do not invest in central HVAC systems. For small spaces, window AC units work well, but for large homes, it is incredibly inefficient. Window AC units can be cumbersome - make sure the window frame is strong enough to support the extra weight. Once they install the unit, natural light and the ability to open and close the window is blocked. Window AC units might also make your home more vulnerable to intruders who can rip it out and use the window as a point of entry.