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Electrical Power

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What is electrical power?

Power tells us how fast energy is being transferred or converted. Electrical power is measured in watts.

P = V × I

P and W

P is the usual variable for power. W is the unit symbol for watts. So P = 24 W reads: “the power is 24 watts.”

Reading the schematic: straight lines are wires; the unequal parallel lines are a cell/battery symbol; a zigzag is a resistor; a circle marked V is a voltmeter; a circle marked A is an ammeter. Labels beside the symbols tell us their values.
12 V 6 Ω 2 A 24 W

A useful first picture

Energy is like the amount of work available; power tells you how quickly that work is being done. Lifting the same load in one second requires more power than taking ten seconds to lift it.

Another way to think of this

A 100 watt device and a 10 watt device may both operate for one hour, but the 100 watt device transfers energy ten times as quickly while it is operating.

What is really happening?

One watt equals one joule per second. In an electrical circuit, multiplying volts by amperes gives watts because volts are joules per coulomb and amperes are coulombs per second:

(J/C) × (C/s) = J/s = W

Where did the name come from?

The watt is named for Scottish engineer James Watt (1736–1819), whose improvements to steam engines became important to the Industrial Revolution. Although Watt's famous work was mechanical rather than electrical, his name became the unit of power in both electrical and mechanical systems.

Worked example

LOAD 12 V 2 A P = ?

A 12 V load draws 2 A:

P = 12 × 2 = 24 W

Using Ohm's law, resistor power can also be written as P = I²R or P = V²/R.

Try these yourself

A 12 V load draws 2 A. How much power does it use? LOAD 12 V 2 A P = ?
P = V × I = 12 × 2 = 24 W.
A 10 V load draws 1 A. Its power is: LOAD 10 V 1 A P = ?
P = V × I = 10 × 1 = 10 W.
The watt is a unit of: LOAD V I W = power unit
The watt is the SI unit of power — the rate of energy transfer.

Worth memorizing for motors

1 hp = 746 W ≈ 0.75 kW

One horsepower is 746 watts, or roughly three-quarters of a kilowatt.

This is a useful mental reference when working with motors and on timed tests. For example, 4 hp is roughly 3 kW of mechanical output power.

A real motor must draw more electrical input power than its mechanical output because no motor is 100% efficient.

A little deeper

Power and energy are related but are not the same thing. Power is a rate. Energy is the total amount transferred over time. This is why electric utility bills use kilowatt-hours: a kilowatt-hour is a unit of energy, not power.

Our first sequence is now complete:

Charge → Current → Voltage → Resistance → Ohm's Law → Measurement → Series → Parallel → Power

< Parallel Circuits   |   Combination Circuits >