Greek UVB lighting/bulb

[RaphsMama]

Hey all I have an adult greek tort and I live in Michigan. I'm currently using the 100w powersun bulb for her but it is time to change it again and I'm wanting to use something else. The bulbs cost too much for how frequently they need changed. I was looking into the Zilla T8. What I'm wondering is if the or how the difference from 100w to 15w matters. Will she get enough of the UV from the 15w? Clearly I'm no light expert lol I just want her to get what she needs. Thank you very much in advance