In a few weeks I'll be buying and installing my long-delayed air conditioner, and I am finding it not so easy to determine just what capacity I need.

Too small and it runs all the time and costs a lot to operate. Too big and it doesn't run enough to dehumidify the air (although I don't think that would be a big problem here, this is high desert with low humidity.)

The best source I have come across is this one where they take into account a lot of variables that the simple rule-of-thumb charts seem to ignore. Things like ceiling height, hallway widths, window area relative to wall area, wall construction, home location, SEER rating, etc.

I am distrustful of the results, because the annual operating expenses seem unreasonably low. Attached is a rough layout of the area that I am trying to air condition. The calculator linked above said I would need 26,300 BTU/hour. Does this seem reasonable? There is only about six weeks a year that the A/C would operate, daytime temperatures seldom exceed 90 degrees (F), evening temperatures usually drop down into the high 60's. We would be cooling the house to about 75-80 degrees. All walls (interior and exterior) are 8" masonry, as is the ceiling, all uninsulated.

I know some of you have a lot of experience with air conditioned homes. Does 26K BTU seem right? How many hours a day do you think it might need to operate?

tanstaafl.