Hi, Welcome to Paw-Talk!
I think I might be able to help you ... the florescent wattage doesn't really have anything to do with the amount of UVB that is produced -- it generally describes the length of the florescent. The size of your tank will determine that more than anything.
What you need to look for is the actual percentage of UVB it produces. Most manufacturers make at least three percentages -- 3%, 5%, and 8%. Of course, more is better but I've always heard it recommended that you don't get anything less than the 5%. Make absolutely sure that you buy a UVB florescent... full spectrum doesn't necessarily mean UVB. The florescents you find at the home improvement store say full spectrum but they don't contain UVB. You'll know by the price. You won't find a bona fide UVB light for less than $15 for the smaller ones. ZooMed makes a great line.
T-Rex Solar Drops are vitamin D3 supplements ... not UVB. They may provide your turtle with the necessary vitamin ... but they don't allow for vitamin synthesis ... only UVB does that. And for that, you need either a florescent or actual sunlight.
Hey, if you can find them I'd look into the UVB coils. They are brand new on the market ... I haven't seen them around here yet but everything I've heard is positive. They are excellent because they fit in an actual lamp and the UVB lasts longer and is more concentrated because it is in a coil ... plus, once the UVB wears off you can actually use the bulb in other parts of the house ... cool, huh?
Hope I helped ...
I love turtles ...