if a generator like (see description) runs at full rpm but the electrical load is less than wat its making wat happens to the extra energy?

Not drawing full load, means you aren't drawing the full amount of horsepower from the engine. Most generators can be calculated at approximately 400 Watts/Horsepower from an engine unless the alternator has permanent magnets than figure around 450 Watts/Horsepower. I know there is 746 Watts in a horsepower, but there are losses from friction, exciter windings, and internal resistance. And don't forget that most engine manufacturers lye about the actual horsepower of their engines.

Generators usually have a governor. This is a throttle (analogous to the gas pedal in a car) that will attempt to keep it at a constant rpm as the load varies. No load= just enough gas to keep it running at operating speed. Excess energy is wasted as heat and noise. Full load= throttle wide open. (Excess energy is still wasted as heat. There isn't a 100% efficient way of converting the energy in gasoline or deisel to electrical energy. There will always be losses.) The governor is either mechanical, based on a set of weights spinning against the force of a spring, or electronic.