Physics tells us energy cannot be created or destroyed, only changed from one form to another. This is known as the conservation of energy. A highly useful form of energy is electrical energy. Electrical energy is produced by transforming another more malleable energy source. For example, the chemical energy contained in coal, natural gas, and oil, the kinetic energy of free falling water, or from nuclear energy.

Classical Energy
Energy is generically defined as the work one system does or can do on another system. It can be measured in a variety of ways, depending of the energy form. In each case, energy measurements are expressed in Joules (J), a SI unit. Classical physics describe one Joule as the work done by a force of one Newton moving an object one meter along the direction of the force.

Force is an entity that can cause mass to accelerate. A Newton is a specific force measurement, defined as that required to accelerate a kilogram of mass at a rate of one meter per second squared.

Electrical Energy
Because electrical energy is an energy form, it also is expressed in Joules. Each energy form has a unique set of formulas describing how it is quantified. In the case of electrical energy, one Joule is the work required to move an electric charge of one Coulomb (C) through an electrical potential difference of one Volt (V).

A Coulomb measures the amount of charge moving through a cross section of wire. Charges are present in subatomic particles, commonly expressed as electrons and protons. Electrons have a charge of -1 while protons have a +1 charge. 1 Coulomb contains 6.241506×1018 electrons. Conversely, 1 charge equals 1.602 x 10-19 Coulomb.

Electrical current is Coulomb movement over time. Current can be thought of as the electron flow rate.

Energy & Power
Now, with a basic understanding of electrical energy, voltage, and current, let's discuss Power. Power specifies the constant rate any energy form can be delivered. Power is energy over time and measured in Watts (W). Relating Power to electrical energy we calculate 1 Watt by dividing 1 Joule by 1 second and then substituting electrical energy's definition of work, i.e., Coulomb multiplied by Volts.

Noting that Amps (A) is Coulombs (C) divided by time (s), we arrive with 1 Watt = 1 Volt x 1 Amp, which naturally corresponds to the equation for electrical power.

A computer power supply is another energy transformation device, converting from one type of electrical energy called Alternating Current (AC) into Direct Current (DC), the form required by digital circuitry. Power supplies are rated at a maximum sustained output measured in Watts. For example, 300W power supply means the device is capable of continuously delivering 300 Joules of energy in 1 second periods. Because computer power supplies have multiple output voltages and DC power is additive, calculating total rated power is an exercise is applying P = V·I to each output and adding the results.