A 65w is probably enough to heat the T8 enclosure in a room that is around 70F or warmer. If you buy your RHP from Reptile Basics, they have 40w, 80w, and 120w.... so since 40w isn't big enough for a whole T8, a lot of folks go with the 80w. Now if you keep the T8 in a room that is 65 degrees or cooler, you may want that extra wattage of the 80w. Then there are some like myself that have a T8 with a divider in it and if doing that, you need to get two 40w Rhp's as the 80w would interfere with the divider. Hope that helps.