Depends on the heater. You can have Electric heaters, propane heaters only to mention two.
Potential to kinetic. Before you turn the shower on, the water is above the ground, waiting in the pipes. Once you open the valve it falls, converting that potential energy into kinetic energy.
It converts potential energy (source) to kinetic energy (product)
No. Tornadoes are too rare and too hard to predict for us to use them as an energy source.
I think the kind of energy you are looking for is mechanical energy.
electricity
Potential to kinetic. Before you turn the shower on, the water is above the ground, waiting in the pipes. Once you open the valve it falls, converting that potential energy into kinetic energy.
Nuclear energy sometime uses uranium for its energy source.
It converts potential energy (source) to kinetic energy (product)
Electrical (alternating current)
Wind /Hydroelectric/geothermal/tidal
Geothermal, because it is one of the easiest to use and install.
any source
Carbohydrates
why do you think coal is the most energy source we are going to use in the future
light energy
Photoautotrophs use light as the energy source for producing proteins, lipids and other organic substances. Examples of photoautotrophs are plants.
The greates source of energy is the Sun