In this graph you can see the amount of free energy released during the reaction progress
Photosystem II vs. Photosystem I
In Photosystem II the electron (e-) comes from the break down of water (H2O), however in Photosystem I the electron (e-) comes from the Electron Transport Chain. Also, Photosystem II best absorbs the wavelength 700 nm, thus why its called P700, but Photosystem I is best at absorbing the wavelength 680 nm, creating the nickname P680.