Undertank heaters don't really do well with dragons. It does give the floor extra warmth, but other than that, heat doesn't radiate upwards well from UTH's. If you need extra heat, use a ceramic heat emitter (produces no light, just heat) for extra heat at night or to warm the middle of the tank if you feel it drops in temp away from the basking light to fast.
What you need to keep your tank at the ideal temps during the day and night depends on the external temperatures of the tank as well as the kind of tank used. I find flood light style halogen bulbs work best for a heat source for basking. Alot of the normal household bulbs are being made more energy efficient so they do not put out as much heat as they used to. In my old cage for my first bearded dragon I used a 125-150Watt exterior flood light which kept the 5'x2'x2' tank heated up perfectly, though I did have to drop the wattage of the bulb (to 125W) during the summer and used the higher watt bulb during the winter. Because of the higher wattage of these bulbs, you definitely have to use a ceramic fixture. Plastic fixtures will melt from the heat put off by these bulbs. Fortunately it is easy to get ceramic fixtures. You can buy just the ceramic fixture and mount it to the top of your cage yourself (if the top is enclosed or partially enclosed) or you can buy the Exo Terra hooded/clamp light fixtures which are also ceramic (medium is the best to get, can manage up to 200Watt bulbs and holds the flood light bulbs nicely).
-----
PHLdyPayne