Don't use Zilla UVB bulbs!!! Especially the "slim Line" t5 bulbs! They kill other species of reptiles regularly (like anoles) by inducing photo-kerato-conjunctivitis! I've witnessed this personally and in laboratory settings, across five species (equestris, carolinensis, distichus, sagrei, and cristatellus). I would recommend using Arcadia or Exo Terra UVB bulbs.
I have not used this brand with varanids but the type of short wave UVB radiation emitted by the bulbs is not safe.
Here's a link
UV Lighting for Reptiles: A new problem with high UVB output fluorescent compact lamps and tubes?
This has remained an issue with Zilla products even recently as 2011.