Power

James Watt did a lot of research into power and made some significant improvements to the steam engine, which is why the International System of Units unit is named after him. Another James, James Joule, did a lot of research into energy. We have to be careful, though as joules and watts do not measure the same thing. Watts are joules/sec, which is why we have time constraints such as watt-hours, which then cancels out that time factor in the denominator.

This difference between energy and power is important when looking at things like batteries and capacitors. Batteries have high power capacities but relatively low energy capabilities while batteries are inverted. What that means it that batteries can put out a lot of energy over a long period of time whereas capacitors can put out a lot of energy very quickly. This is why batteries are used as the main power source for electric cars but capacitors are used to store and release energy for braking and accelerating, when demand and supply peak.