In a galvanic cell the existence of overpotential means less energy is recovered than thermodynamics predicts.
Overpotential is experimentally determined by measuring the potential at which a given current density (typically small) is achieved.
Energy can be expressed as the product of potential, current and time (joule = volt × Ampere × second).
A likely reason for the lack of strict definitions is that it is difficult to determine how much of a measured overpotential is derived from a specific source.
The electrochemical reaction rate and related current density is dictated by the kinetics of the electrocatalyst and substrate concentration.
Concentration overpotential spans a variety of phenomena that involve the depletion of charge-carriers at the electrode surface.
It occurs when electrochemical reaction is sufficiently rapid to lower the surface concentration of the charge-carriers below that of bulk solution.
An example is the electrolysis of an aqueous sodium chloride solution—although oxygen should be produced at the anode based on its potential, bubble overpotential causes chlorine to be produced instead, which allows the easy industrial production of chlorine and sodium hydroxide by electrolysis.
They can also include aspects of electrolyte diffusion, surface polarization (capacitance) and other sources of counter electromotive forces.