Energy in general is defined as the capacity for doing work. Power is the rate of doing work or the rate of using energy:

P=Work/t=Energy/t , where

**t** is time.

Although casually the terms energy and power are often used interchangeably, technically they have different meanings. The SI unit of energy and work (which are numerically the same) is the

**joule (J)**. A joule is the work done by a force of one newton for a distance of one meter. This unit is usually used in physics. Energy comes in many forms, such as heat, motion, gravitational, radiated solar power, and electrical. For different types of energy other physical units are also utilized. For example, the British Thermal Unit (Btu) is often used to measure the heat energy or compare fuels. One Btu is what's needed to heat one pound of water one degree F. The SI derived unit of power is

**watt** (W). Watt is power required to produce or consume one joule of energy per second. This unit and its multiple kilowatt are usually utilized in ratings of various electric loads and sources of electricity, such as residential

generators.

How do we get an expression for electrical power from its general definition as work per unit time? By definition, work done by a constant force F when it moves an object by distance L in the direction of force is:

**Work=F×L**.

We know that in an uniform electric field with voltage

**V** over distance

**L**, the force acting on a charge

**Q** is equal to

**F=V/L×Q**.
(Particularly, in the field of 1 volt/meter, force of 1 newton is acting on one coulomb charge). Substituting this into the above general expression of work gives the equation for

**P** required to move a charge

**Q** in an electric field:

**P = F×L/t = V×Q/t**. The rate of charge flow

**Q/t**
is called electric current

**I**. Replacing

**Q/t** with

**I** in the above formula yields a familiar expression for instantaneous value of electrical power:

**P=V×I**. Note that in AC circuits voltage and current are often shifted in phase and are not sinusoidal. You can use the tools below to make instant online conversion between various SI, CGS, imperial and other energy and power units.

: types and classification.