Ive done both 24, 20, and 18 on, havent noticed a huge difference. However, at least where i live, my power company offers a "dynamic peak pricing plan" where from 7am-3pm i pay a regular rate, from 3pm-7pm i pay like 10x the regular, 7pm-11pm regular, and 11pm-7am i pay a out 25% the regular rate. So i keep all my lights off from 3-7pm and run the budroom 7pm-7am. I notices a 200 dollar drop in my bill when i switched over, now i pay "only" 1100 a month.