Energy

Energy is defined as work that can be performed by a force.
It can have many forms, like, kinetic, potential, thermal or electromagnetic.
Leibniz defined it as the product of mass by velocity squared.
Any form of energy can be transformed into another form, but the total remains the same.

Primary energy is the energy storage in raw fuels that has not been transformed yet.

Electricity is considered a secondary energy form simply because it is generated by conversion of primary energy.