Energy is the capacity or the ability of our to do work.It is a stuff required for the accomplishment of all the physical actions of our body.
It is required for all the systems to work.
It is variable as It is non-dispachable due to its fluctuating nature such as wind power and solar power. 
Energy is the ability and power to do active physical things and the feeling that you are full of physical power and is also determination and enthusiasm about doing things.
energy is an invisible power.there are many forms of energy such as sound energy,wind energy,heat energy is variable because of the different nature of every energy.these energies are being used by us daily to fulfil our requirements.