UV Watt Question

[Jenncure]

I'm about to order a Zoo Med ReptiSun 10 UVB tube flourescent bulb for my enclosure, but don't know what wattage to get. They come in 15, 18, 30 and 36 watt. It's for a 3' by 3' indoor enclosure, for my Russian tortoise. Any advice would be helpful!

 

[Yellow Turtle01]

I would try and get one to span half the enclosure, so your tort can have some shade if he wants.
I'm sorry to ask! but how big is your tort? 3x3 is too small for anyone but a hacthling!