Energy is defined as the capacity of a physical system or a body to perform (useful) work. It is a property of objects. It cannot be created or destroyed. It can be converted from one form into another.
Energy exists initially in the form of potential energy (P.E): gravitational (mechanical) P.E., electrical PE, magnetic PE, chemical PE, elastic PE, nuclear PE etc. Other forms of energy are energies converted from potential energy. When work is done by a gravitational force (Newton's law), electrostatic force (coulomb's law), magnetic force etc., the PE is converted into kinetic energy or other forms.
This is the energy that depends upon the current physical position of the body in a field (that exerts force), like Earth's gravity, and also on the motion (speed) of the body. The former is called the potential energy and later is called Kinetic energy.
Mechanical energy = mechanical P.E + K.E.
This is the capacity to do electrical work. That is, ability to move a charge q across a potential difference of V. The energy and work involved are equal to qV.
A battery has a capacity to convert the chemical energy: it can move a charge q (or electrons) across potential difference V for some time duration, causing a current = dq/dt or V/R, where R is the resistance connected across battery.