Well, we are all technically violating the warranty of our UTH when we put them on the bottom. Make sure you completely read the directions that come with the one you bought and don't have, for instance, the water bowl any where near it. Also you want to monitor your temps very closely for the first few days. (Ideally before you put in the gecko, but this may not always be possible.) You don't want the temperature AT THE SURFACE of the "floor" over the UTH to be higher than 90F to minimize the risk of burns. You may need to add extra substrate or buy something like a dimmer control to reduce the power output of the UTH. Also realize that it is very dangerous to have the UTH and a basking light on over the same spot both ON at the same time. (Timers are the best invention.)
OK, now that I've said all that... belly heat is really important to the digestion and you can't get that from the side mount. When the heat went out we had to run the light and the UTH just to keep the warm side in the low 80s, however I was home all day to watch it. So, we all do bad things, but you have to be really careful about it is all. Good luck.
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0.1 Albino Leo Gecko
0.1 Crested Gecko
1.0 Dear Boyfriend
Departed: Harvey and Spock