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.
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