Under Tank Heaters (UTH)

Under tank heaters can be used as a primary or secondary heat source for many tanks or habitats of reptiles that need temperate or tropical heating, ranging from Hermit crabs and spiders, to large snakes and monitors. Under tank heaters, or UTH, as they will be referred to here, are a flat heating element, inside a durable rubber casing. They are great for continuous use of twenty four hour, round the clock heating of reptile habitats. For this purpose, UTH can be found in several physical sizes and wattages. Sizes range from mini at about four watts, up to large at about twenty four watts. Length and width, often vary by brand name and company. The size and wattage of the UTH needed, is usually gauged in tank or habitat size. Mini, being for about a five gallon enclosure and large being for a fifty so sixty gallon enclosure. The size and wattage actually needed will also depend on the ambient air temperature required for optimal condition specifications for each reptile.
The actual purpose of a under tank heater is to heat the ambient air temperature of a habitat or enclosure, not to heat the actual animal like a basking light or a ceramic heat emitter does. Ideally, an animal should never come into direct contact with an UTH. There should be several inches of substrate or bedding, in between where the animal actually lays or walks, and the UTH, if the UTH is actually used UNDER the enclosure.

I copied this from a website called everything reptile.