ENERGY, WORK AND POWER UNITS CONVERSION

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

Power=Work/t=Energy/t

, where t is time.
Although casually the terms energy and power are often used interchangeably, technically they have different meanings. Energy comes in many forms, such as heat, motion, gravitational, and electrical. 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

.
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. For different types of energy other physical units are also used. For example, the British Thermal Unit (Btu) is often used to measure the heat energy or compare fuels. One Btu is the energy 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 use of one joule of energy per second.

How do get an expression for electric power? In an uniform electric field with voltage V over distance L, the force acting on a change 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 power 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. In AC circuits voltage and current are often shifted in phase and are not sinusoidal. In such cases the quantity volt-amps describes the apparent power. For basic relationships between various AC characteristics see Watts, VA and power factor: calculator and formulas. For the mathematics of these electric quantities also see Electric power primer. Use the tools below to make instant online conversion between various SI, CGS, imperial and other units of measure of power and energy.