What watt amount should I use? (Ball Python)?

I have a 6 month old ball python in a 20 gallon long tank, and he really does not like his "day light" and I honestly think it's just too bright. I want to get a ceramic heater, but I don't know what watt to get. I have a 100W "night light" which is just a black light, but I wanted the ceramic heater because, as I'm sure someone will point out, glass/plexiglass tanks do not retain heat all that well. What watt ceramic bulb should I get?