Internal energy (U) is defined as the energy which is related to atomic, sub atomic and molecular energy of the system. Internal energy comprises of the kinetic and potential energy related with transitional , rotational and vibrational motion of the molecules, it also includes the energy of electromagnetic interactions of molecules , atomic and sub atomic constituents of molecules.
Mathematically,
Internal energy (U) = Energy.
Dimensional formula of Energy = M1L2T-2
So Dimensional Formula of Internal Energy (U) = M1L2T-2
SI unit of Internal Energy (U) is Joule (J)