Electric power can be delivered over long distances by transmission lines and used for applications such as motion, light or heat with high efficiency.

If conventional current flows through the device in a direction from higher potential (voltage) to lower potential, so positive charge moves from the positive (+) terminal to the negative (−) terminal, work is done by the charges on the device.

These devices are called passive components or loads; they 'consume' electric power from the circuit, converting it to other forms of energy such as mechanical work, heat, light, etc.

In alternating current (AC) circuits the direction of the voltage periodically reverses, but the current always flows from the higher potential to the lower potential side.

Electric power flowing out of a circuit into a component is arbitrarily defined to have a positive sign, while power flowing into a circuit from a component is defined to have a negative sign.

In the case of resistive (Ohmic, or linear) loads, the power formula (P = I·V) and Joule's first law (P = I^2·R) can be combined with Ohm's law (V = I·R) to produce alternative expressions for the amount of power that is dissipated:

Using the Pythagorean Theorem, the relationship among real, reactive and apparent power is:

Where the currents and voltages have non-sinusoidal forms, power factor is generalized to include the effects of distortion.

The simplest example of this is in electrical circuits, as the preceding section showed.

In the general case, however, the simple equation P = IV may be replaced by a more complex calculation.

The closed surface integral of the cross-product of the electric field intensity and magnetic field intensity vectors gives the total instantaneous power (in watts) out of the volume: [5]

The fundamental principles of much electricity generation were discovered during the 1820s and early 1830s by the British scientist Michael Faraday.

His basic method is still used today: electric current is generated by the movement of a loop of wire, or disc of copper between the poles of a magnet.

Electricity is mostly generated at a power station by electromechanical generators, driven by heat engines heated by combustion, geothermal power or nuclear fission.

Other generators are driven by the kinetic energy of flowing water and wind.

[7] Since the invention of the first battery (or "voltaic pile") in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power source for many household and industrial applications.

According to a 2005 estimate, the worldwide battery industry generates US$48 billion in sales each year,[8] with 6% annual growth.

Batteries are available in many sizes; from miniature button cells used to power hearing aids and wristwatches to battery banks the size of rooms that provide standby power for telephone exchanges and computer data centers.

The electric power industry has gradually been trending towards deregulation – with emerging players offering consumers competition to the traditional public utility companies.

A country's per capita electric power consumption correlates with its industrial development.

[10] Electric motors power manufacturing machinery and propel subways and railway trains.

Electric power is used to provide air conditioning in hot climates, and in some places, electric power is an economically competitive energy source for building space heating.

The use of electric power for pumping water ranges from individual household wells to irrigation and energy storage projects.