Why Power is the rate of energy transfer, and in a DC circuit it equals the current through the device multiplied by the voltage across it: P = I x E, giving watts when I is in amperes and E is in volts. This is the basic power law that pairs with Ohm's law (E = I x R). Substituting Ohm's law gives the useful variants P = I squared x R and P = E squared / R.
Watch out The choices that divide voltage by current or current by voltage are mixing up power with resistance or conductance: E / I is actually Ohm's law solved for resistance, not power. The current-squared form only works when multiplied by resistance, not voltage.
Watts = Volts x Amps. Think "PIE": P = I x E.