Hi, I don't have an answer for you but if you do a search on this site it has been a much discussed topic. Basically whatever wattage you need to acquire the appropriate temp will work. Sorry I couldn't be more helpful.
Nancy
I ended up with 250's because those were the only red ones they had at the local hardware. For my set up it's way too hot and I had to find a way to get the lamps way up high to keep the perfect temp. I have one setup over the bathtub and one over a rubbermade tub.