All forms of energy associated with a system is called internal energy or simply energy of the system (E).
This is expressed in Joule. This arises due to movement of molecules, arrangement of atoms in molecules, number of arrangement of electrons in atoms etc.
It is neither possible nor necessary to calculate the absolute value of internal energy of a system. It is a state function so depend only on the initial and final state of the system.
ΔE = Ef – Ei
In a reaction- R → P
ΔE = EP – ER
If Ef > Ei, ΔE is + ve
if Ef < Ei, ΔE is – ve