Sticking to the question and ignoring my personal opinions:
This would take an electrical engineer doing calculations to get an
accurate answer. There are many variables, the most important of which is
the size of the house in question. The answer would be different for an 1800
sq ft ranch than it would be for a 4000 sq ft, 2 story house.
A typical central air compressor runs on a 30 amp circuit, a typical window
unit on a 15 amp. The majority of the electricity used occurs when either
unit kicks on. So in either case if the units run longer and start up less
often the efficiencies rise. Poorly sized central units "short cycle" and
that condition greatly decreases the cost efficiency. Same is true of the
window units. A larger house would require two or more compressors.
In addition, central air systems require an air handler. Once again, the
answer will be different if the house already has hot air heat and the A/C
coil is just going to be added to it rather than a new, separate air handle
installed just for the A/C. Air handlers typically run on a 15 amp circuit.
Once again, one or two depending on the size of the house.
So every time the central unit kicks on you've got juice running the
compressors and the air handlers with the total draw being a complex
calculation of the total wattage required, the initial startup surge, and
the steady run-time usage.
The window units do the same, but the combined wattage would need to meet or
exceed the central unit to use the same amount of juice.
You buy central air for convenience, comfort, and prestige. Forget about the
payback. That comes when it's time to sell the house.

"Nehmo" wrote in message
...
If a house currently has multiple window air-conditioners, are these
significantly less efficient than central air, if the house were
retrofitted with it?

I realize the comparison is complicated, so assume all other factors
equal, and assume: the vents and their position for central air would be
adequate; about half the rooms are normally in use; nobody's concerned
about the window-view obstruction, etc.; Chicago climate; poorly
insulated house.

A companion question is, does the efficiency of air conditioners
increase significantly with size?

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