I have a window unit type air conditioner.  It's probably about 5
years old.  It's not a very small one or a big heavy duty one.  It's a
medium sized unit.  I'm not sure exactly how much BTU its rated at. 
But I dont' think it's one of those energy efficient ones (energy
star).  The question is:  is it cheaper to run it on low all the time
(24/7) or to just run it on high only when the room is occupied?  The
other question is:  what is the difference in electric consumption
between running on high or low?  (I'm thinking that it's not much) 
Here's some additional relevant information.  The room is in the
attic.  It's a full finished attic.  There are 2 ceiling fans that do
run 24/7.  3 people sleep there.  It is typically occupied only at
night.  So, there is at least 8 to 10 hours of the day when nobody is
in the room.  It does get pretty hot and humid up there with only the
ceiling fans running.  It does take at least 30 minutes to cool the
room once the a/c is turned on.  Basically I want to save money.  It
seems like a waste to have it on when nobody's there.  I think it
would be ideal if it could automatically be turned on 30 minutes prior
to the room being occupied.

Hi Adam,

If it's a 2000 model your a/c should be very efficient and no doubt
Y2K compliant! :) Fans are good. Reversible fans are even better to
counter the changing seasons when needed. Does your attic have
windows? If so, consider installing a "whirly-bird" on the roof. Leave
the windows slightly open, creating an updraft toward the
"whirly-bird" outlet loctated at the highest possible point upon the
roof. This will assist with your humidity problem also. Consider
natural ventilation as opposed to mechanical as best as you can until
things become uncomfortable. Then start using the mechanical bits and
pieces. Running anything 24/7 will eventually cost you more in
maintenance. In the hotter weather you may find that your a/c actually
freezes up due to the overload of work. A quick boost on high to drop
the temperature to a comfortable level may be all that is required. No
doubt there will be a/c mechanics here to further and properly explain
the best approach.

Then install a timer on both your fans and a/c to balance the comfort
factor. You could set it to come on say half an hour before the
regular usage start time of your attic.

Silver

Hi Adam, Silver is right.  Using natural ventilatiion is a great way
to combat energy consumption.  The use of the whirly-bird is a great
way to exhaust the warm air to the exterior.  An other item that may
help is a whole-house fan which is a powered fan that will replace the
inside air with outside air in just a few minutes.  Your local utility
company may be willing to offer rebates for such items as well.  With
regard to running your A/C unit on high or low, the only difference
between the two is the speed of the fan.  The compressor (which is
what really uses most of the power)inside the unit is going to run
regardless of whether it's on high or low.  I recommend turning the
unit off when you're not home and then running it on high, at least
initially, once you get home.  Once you've established some level of
comfort inside the space, then kick in back down to low.  Anyway, good
luck.  P.S.  The use of a timer will work great for starting it 1/2
hour before you get home.

See! .. Of course I'm right .. I'm always right!

:)