My home central air is about 25 years old and will not get the house below 78 degrees.  And seems to cost me a fortune in electricity.  So I bought a window AC unit.  A 12,000 BTU one.  Says it will do a 550 sq ft.  So have my house.  It does, it cools the living room well, but not the rooms so much, I kinda expected that.  And fans blowing air in doesn't seem to be doing all that well either.  Oh well.  I also saw a same rated unit for $50 cheaper at another store    Oh well again.  It was a major pain to put in too.  Maybe I should have just called someone to look at my old unit or replaced it.  But too late.