In reading about electronic cars in the new edition of The Economist, I was reminded about an important difference between capacitors and batteries. Since I only touched on this topic in coursework in college and grad school, my background in this area is rather weak. The following reinforced for me the difference in the amount of energy created by both:
"Unlike batteries, which store energy chemically in the material of their electrodes, a capacitor stores energy physically, on the electrode's surfaces. One electrode has a surplus of electrons and the other a deficit. If the electrodes are then connected through an external circuit a current flows until the surplus has neutralised the deficit and both have the same electrical potential. Electrodes surfaces are easy to get to, so a capacitor can be charged and discharged quickly, giving it a high power-density. But surfaces cannot hold as much energy as entire volumes, so capacitors have lower energy-densities than batteries."