Side mounting a UTH in a tank is not very efficient. Heat rises, (not to be dumb but sometimes people forget the obvious) the open top added with the typical height most of the heat is lost to the top and height. More power would then be needed and the hotter the side has to be and that in turn could mean burns. I have encountered a system that used a primary UTH under the tank. The side mounted was used to increase the ambient temp and extended across the whole back and was controlled by an independent T-stat set to deliver 80ºF. This seemed to work well but I do not think it would be efficient alone.