Just got my electricity bill today, AC is not what I expected

I suffered through last summer, desperately trying to save every last yen. This summer I said forget it, and kept the AC at 27 or 28 most of the time when I was home, unless it was cooler at night.

Just got the bill for July, and it was only about 2500 yen higher than average. I'm kicking myself for the hell I went through needlessly last summer.

I think the trick is, just cool one small room and basically live there, except to cook or use the washroom.

EDIT: One thing I found is use the dehumidify function. Dry air is a lot easier to keep cool, and it'll help keep mold away.