Energy is the ability to change
Energy Efficiency
Heat Loss
A lot of energy is lost to heat. For the most part you can’t heat for anything practical.
Energy Density
Batteries do not store a lot of energy for their volume
Conservation of Energy
When an object hits the ground, the kinetic energy is high, but the gravitational potential energy is 0. This is caused because of the conservation of energy.
Non-Conservation of Energy
They say energy cannot be created nor destroyed. That is true for the entire universe, but if you define your system as a smaller part of the universe. For example, a cup is your system. If I applied work on the system, then energy of the system would be increased.