Why do tornadoes have so much energy and where does the energy come from?

A tornado gets it energy from its parent thunderstorm. The storm gets its energy from instability in the air, often resulting from a collision of warm moist air with cool and/or dry air. The warm moist air is the actual energy source. The water vapor in it holds a surprising amount of energy.

The majority of tornadoes form from supercells, the most powerful type of thunderstorms on earth. Structural differences between supercells and ordinary thunderstorms allow them to release energy more efficiently. The strongest part of a supercell is the mesocyclone or rotating updraft. This is powered by the buoyancy of warm, moist air that in some cases can rise at over 100 mph. A tornado forms when the rotation of a mesocyclone becomes more focused. So essentially the kinetic energy from a vortex several miles across becomes focused in an area just a few hundred feet across.