It sorta depends on if I have my brain set to program mode or colloquial mode. I think that the literal answer is "down" if you want the temp to go up and the electric bill to go down, since (as already stated) the word "down" modifies "air conditioning".
This type of literalism gets me in trouble sometimes, as in "Do you not want this drink on the counter?" to which I say "no" if I want it, "yes" if I don't. Usually this results in the drink getting dumped. The Jargon File comments on this somewhere.