yourethebuilder
Why not just use a 12,000 BTU. You'll get more then enough cooling with the cost of running three models

Peter W
You need to understand a fundamental issue with comfort refrigeration before you make any decision: a) Cooling is as much or more related to dehumidification as it is to refrigeration. b) An oversize unit will refrigerate a room before it dehumidifies it. c) Accordingly, you will be cold and clammy - not very comfortable. Put another way, you will be much more comfortable at 78F @ 20% humidity than at 65F @ 75% humidity. All that being written, standard window AC units come as small as 4,000 BTU and you could install two of them on a lead-lag system such that one works until the temperature reaches a certain point, then the other kicks in. The virtue of such a system is that with a constant low level of refrigeration you will get maximum dehumidification at a minimum of operating cost. But you will have a higher first-cost as two complete units will very nearly always cost more than a larger single unit. And the larger the unit (all other things being equal) the less energy it takes to make each BTU of cooling, so your operating cost will be slightly higher with both units running flat-out. If you do not anticipate that need very much, two smaller units _will_ lower your overall operating cost. Note also that AC units (as well as heating units) are at their most efficient when running flat-out. So at the very least the one lead-unit will be at its most efficient most of the time.

Dirkus Maximillius
If you are still uncertain after hearing other answers, you can always talk directly to a contractor. There are several services that will allow you to get estimates and advice: