bob haller wrote:

but energy efficency wise your far better off with a central AC. more
comfy, much more efficent, no window tied up with a AC, unusable for
anything else, no winter air leaks, even if you seal the unit great
the cold from outdoors will be conducted indoors.

I disagree.

Consider a typical usage pattern:

A. 6:00 am - 7:30 am | Usually no need for a/c.
B. 7:30 am - 6:00 pm | a/c off while everybody's at work
C. 6:00 pm -11:00 pm | a/c on
D. 11:00 pm - 6:00 am | Need for a/c while sleeping

If you use only central air, it will be on 12 hours a day (C + D). If you
have a window unit in the bedroom, the central air will be running only five
hours a day (C) and the window unit running seven (D).

The cost difference is between the central air and the window unit for seven
hours a day. Even if the window unit is not particularily efficient, it will
still cost less than cooling the whole house for those seven hours.

There are obvious exceptions: For example, four bedrooms each with its own
window unit may be more expensive than the central unit. But the main point
remains: why cool the whole house when only part is occupied?