Power vs Energy

Energy storage devices may be broadly characterized by their energy density (energy stored per unit volume or mass) and by their power (how fast the energy can be delivered).

Conventional capacitors have enormous power, but store only tiny amounts of energy.

Batteries can store lots of energy, but take a long time to charge and discharge, which means they have low power.

Supercapacitors sit between capacitors and batteries on the energy-power spectrum. They offer a unique combination of high power and high energy, bridging the gap between batteries and capacitors.

Fuel cells are very much an energy source, rapidly becoming inefficient under high power demands. They are most useful when used in tandem with supercapacitors, to provide a high-energy, high-power combination.