A unit of measure of the electrical power delivered by an amplifier to a speaker. Very simply stated, the formula for power is:
P (power in watts) = V (voltage in volts) x I (current in amperes) = V2/R (resistance or impedance in ohms)
An amplifier’s maximum power output is determined by its power-supply voltage, the maximum current it can deliver, and the amount of heat it can tolerate without damaging its components. The current delivered by an amplifier is determined by the amount of electrical resistance or impedance presented by the speaker.
The maximum amount of power that a speaker can handle is called its power-handling capacity. This is also specified in watts. As a speaker’s impedance drops, the amount of power the amp tries to deliver increases. So, a low-impedance speaker is said to present a “difficult load” to the amplifier.