The voltage of an electrochemical cell or battery is a familiar concept. It measures the potential for doing useful work with the cell.

The actual amount of work done is the product of this potential times the amount of chemical reaction carried out through the cell; just as the amount of work that can be harnessed from a waterfall depends on the height of the waterfall (analogous to potential, E), and the amount of water that flows over the falls (analogous to electrons, nF).

For both waterfalls and electrochemical cells, the product of potential drop and quantity of matter reacted is the free energy released in the process, D G.