There are two main types of energy that operate water heaters. Gas and Electric. There are also units which operate on solar power however most solar water heaters require some source of electrical or battery backup for uninterrupted operation. There are a lot of options to consider when it comes to the selection of water heaters and there appears to be pros and cons with each type of unit so when it comes to making a choice you certainly need to do some research and consult with professionals to determine what your needs are so that you make the appropriate selection.
Some of my recent work around town.