UVB Fixture Length - 24 or 36?

Hi. Been reading up a lot on all of these topics and I seem to find conflicting info about the length of the UVB fixture for tank sizes.

Using a ReptiSun T5 10.0 bulb, I currently have a 24" fixture in a 36" L x 18" H tank. If I upgrade to a 48" L x 24" H tank, will that 24" fixture still work or should I upgrade to 36"? Would the height of the tank play into this equation, or would the 24" fixture be good for the longer/taller tank?

FWIW, I need a second fixture regardless of the tank size so I can mount it by the window for when our beardie perches there. So should I just buy another 24" or a 36"?

Thanks for your thoughts!

Either one will work. A 24'' is at least the length of most adult dragons so if you have that set over the basking area, they should be able to bask under a (relatively) even beam of UVB which is the goal. If you want to spread the beam even further since you'll have more space, or if it is easier to mount the bigger fixture in the enclosure, go for it. The 24'' will do the job though.

For window time, I don't bother with providing UVB. They enjoy the bright sunshine and warmth and that's OK. They don't need to be under UVB constantly. I do have several basking lamps set up around the house for free roaming. Some of them are mercury vapor bulbs to provide UVB and some are just halogen flood lights for heat. My guy tends to move between them and often seems to favor just warming up under the halogen in the afternoons.

Thanks so much for taking the time to reply.

This is really great information. After all my "research" these past few months, I guess I was under the impression that they ALWAYS needed a LOT of uvb, but from some recent comments, it seems that it's most important that they have the uvb correctly positioned/in place for basking and perhaps secondary exposure that doesn't need to be as direct as during the basking hours.

Much appreciated.