I havent heard anything about bulb wattage. I just bought one from the reptile section of Petco and called it good.

An UTH alone isn't going to be able to warm the whole tank. It's only there to be a hot spot really. Especially in a 20 gal tank. My set up was a 20gal tank as well, and I found that I needed the heat lamp. I switched it for a ceramic heat emitter after dealing with the hassle of switching bulbs from night to day and shutting down lights at night. So if there's any recommendation of mine that would be it, swap the lamp (because they dont really need the light anyway) for a CHE and you should be good.