I've ended up buying three of these UTHs.  When I bought the first one, the directions recommended installing it on the side of the tank, which left me wondering why the call it an an "under" tank heater.  As it turns out, if you install the heater on the bottom of the tank, you need to worry about two things.  First, you need to worry about water spilling on the bottom of the tank and then the glass cracking.  Second, you need to worry about the heat it produces possibly ruining the furniture on which the tank sits (if, like me, you have the tank on a piece of furniture, such as a dresser, rather than on a tank stand).  As to the second of these issues, the instructions are also very clear that you need to raise the tank a little bit above the furniture supporting the tank to ensure that the heat does not get trapped under the tank.  Due to all these issues, I installed the first one of these that I bought on the side of a 40 gallon glass tank that houses one of my ball pythons.  However, I quickly discovered that side-mounting did not have a significant impact on the ambient air temperature in the tank.  in fact, it only raised the air temperature by 1-2 degrees.  After calling Zoo Med, I learned that the UTH is more efficient when installed on the bottom of the tank.  Consequently, I bought a second UTH, and this time installed it on the bottom of the tank.  Though it still doesn't have a significant impact on the air temperature, it helps give the ball python "belly" heat.  Ultimately, I ended up buying a third, which I installed on the bottom of another 40 gallon glass tank that houses a second ball python.

One additional note:  The UTH comes with four little rubber bumpers that you can attach to the bottom corners; however, I ended up using some larger rubber pieces (the ones that you would put under furniture to prevent the furniture's feet from leaving marks in the carpet) that I bought at Home Depot.  I found that the little rubber bumpers that came with the UTH did not stay adhered to the bottom of the UTH.  In addition, they did not raise the tank enough for my comfort level (one of our tanks sits on my son's dresser and he discovered that the UTH was warming the clothes inside the dresser, which gave me concern as to whether the heat of the UTH might be discoloring the dresser's wood top).
I've ended up buying three of these UTHs. When I bought the first one, the directions recommended installing it on the side of the tank, which left me wondering why the call it an an "under" tank heater. As it turns out, if you install the heater on the bottom of the tank, you need to worry about two things. First, you need to worry about water spilling on the bottom of the tank and then the glass cracking. Second, you need to worry about the heat it produces possibly ruining the furniture on which the tank sits (if, like me, you have the tank on a piece of furniture, such as a dresser, rather than on a tank stand). As to the second of these issues, the instructions are also very clear that you need to raise the tank a little bit above the furniture supporting the tank to ensure that the heat does not get trapped under the tank. Due to all these issues, I installed the first one of these that I bought on the side of a 40 gallon glass tank that houses one of my ball pythons. However, I quickly discovered that side-mounting did not have a significant impact on the ambient air temperature in the tank. in fact, it only raised the air temperature by 1-2 degrees. After calling Zoo Med, I learned that the UTH is more efficient when installed on the bottom of the tank. Consequently, I bought a second UTH, and this time installed it on the bottom of the tank. Though it still doesn't have a significant impact on the air temperature, it helps give the ball python "belly" heat. Ultimately, I ended up buying a third, which I installed on the bottom of another 40 gallon glass tank that houses a second ball python. One additional note:  The UTH comes with four little rubber bumpers that you can attach to the bottom corners; however, I ended up using some larger rubber pieces (the ones that you would put under furniture to prevent the furniture's feet from leaving marks in the carpet) that I bought at Home Depot. I found that the little rubber bumpers that came with the UTH did not stay adhered to the bottom of the UTH. In addition, they did not raise the tank enough for my comfort level (one of our tanks sits on my son's dresser and he discovered that the UTH was warming the clothes inside the dresser, which gave me concern as to whether the heat of the UTH might be discoloring the dresser's wood top).