Watt's Law

Watt's Law, sometimes called the Power Law, describes the relationship between power ($P$), current ($I$), and voltage ($V$), so that power is defined as:

$P=IV$

Given tha values for any two variables, we solve for the third so that voltage, electrical potential energy, is defined as:

$V=P/I$

And current, which is the flow of electricity, is defined as:

$I=P/V$

Power and Watts

Power is measured in watts. One watt ($W$) is equal to one joule ($J$) per second ($s$), or $W = \frac{J}{s}$. Another way to put it is, a watt is the amount of electricity required to accelerate 1kg by 1 meter per second squared (a joule) over 1 second. In other words, a watt is "how much work can be done per second".

Video: Power, Work, and Energy

Full course on Khan Academy: Power)

History

The terms watt and Watt's Law are named after James Watt, best known for his work to improve the steam engine.