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 used. 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 use of one joule of energy per second. This unit and its multiple kilowatt are usually used in ratings of various sources of electricity, such as
generators for homes.
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. In AC circuits voltage and current are often shifted in phase and are not sinusoidal. For basic relationships between various AC power characteristics (watts, VA, power factor) see
power factor calculator and formulas. You can use the tools below to make instant online conversion between various SI, CGS, imperial and other energy and power units.