Nernst equation
Ecell = Reduction potential or voltage of the electrochemical cell. In SI units, the voltage counts the one kilogram blocks you have to push one meter for each 6.2x1018 electrons you move across the cell. In a perfect world, every time a 9 volt battery moves 6.2x1018 electrons, we could harness the slow-moving electrons to move 9 blocks one meter.