Even if they have a central HVAC (which is a given in Phoenix) running a home sized AC for 24 hours a day would result in about 240 a month in Phoenix rate electrical costs

You're off on this a bit. I have a house significantly smaller than 4,000 sqft but it actually requires 2 AC units to keep cool. When they are both on, they pull about 8 KW combined. They are also on during the day, when our electricity is more expensive. It might be 10 c/kWh during the night (or averaged down to that combined), but it's more like 15 c/kWh during the day when everyone is running their AC and straining the grid. They don't pay $800 every month, just during the hottest months. For an older home of 4,000 sqft in July in Phoenix, that sounds about right to me.