Watts are a measure of power. The symbol is W (never w). Electrically Watts = Amps x Volts. The name derives from Scottish engineer and inventor James Watt (1736 – 1819) in honour of his work that transformed the primitive steam engine into a practical power unit.

Watts are obviously useful for indicating the power output of an amplifier or a transmitter but they are also convenient for power consumption purposes. Although the battery voltages applied to a piece of portable equipment may have quite a wide range the power requirement, quoted in watts, will usually vary very little. For any nominal battery voltage an approximate current consumption can be calculated

A 12 W device will draw 1.0 A from a 12 V battery, 0.9 A from a 13.2 V one and 0.83 A from a 14.4 V one.

Watts are a measure of any power, so a 40 W device will radiate something approaching 40 W of heat as well as consuming that amount of electrical power.


Analogue audio circuits, with the exception of power amplifier outputs, carry insignificant power – <1 mW.

Radio Microphones have an RF output 10 – 50 mW.

Phantom Power can draw 100 – 200 mW

Headphones are usually rated at about 200 mW maximum.

A 100 W power amplifier will only deliver that level for peaks or very loud, low dynamic range programme – more normally the speaker drive will be in the range of 10 – 100 mW.

See also Volts, Amps, Ohms.

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