A power supply is a device that supplies and modifies the output of energy to meet the energy demands of a device that needs electrical power. The power that is generated through various methods must be adapted to meet the requirements of the output; often the input amount of power is too large for everyday use.

It helps to think of electricity as water, and the wires along which electricity travels as hoses of various sizes. The power that is generated at a facility is like hooking a large hose up to a river. The power used to charge a phone, run a toaster, and even switch on the lights requires a significantly smaller hose size. A power supply is much like a hose adapter and changes the amount of power that can come through.

There are several different units of measurement used for measuring electricity, and it is important to understand the variations when discussing how electricity powers the devices of the world. Electricity is simply a flow of electrons along with a conductive current. Three units are commonly used to describe electricity. Amplitude, or amps (A), references the base unit of measurement that describes the amount of electricity present. Volts (V) describes the speed of the electricity as it travels through conductive material, usually in the form of copper wire. Watts describes the rate at which the electricity flows. When one watt flows through a conductive material at the speed of one volt, it equals one amp.

Power supplies need a source of power to function, like a garden hose needing a source of water. The definition of a power source, or energy source, is a method of producing electricity. Power sources convert either mechanical or chemical energy into electrical energy which is then used by the circuitry of a device to power that device. Today, there are several ways electricity is produced, categorized by how sustainable the resource used in each method is.

This diagram illustrates how mechanical energy is converted to electrical energy in a power plant.

Non-renewable resources use resources that are not naturally replenished in an average human lifetime and include the use of fossil fuels. Fossil fuels include crude oil, natural gas, and coal, and are burned through a variety of methods to produce electricity. Fossil fuels are considered non-renewable because the process that creates fossil fuels takes place over millions of years. Fossil fuels are made from the decomposed and chemically changed remains of ancient plants and animals that lived even before dinosaurs roamed the Earth. After death, the remains of these organisms were buried under millions of years of sediment and water, compressed, and chemically changed into oil, natural gas, and coal. Because it will take millions of years to produce more fossil fuels, their use of them is a finite resource and they will eventually run out.

Renewable resources use resources that are replenished naturally very quickly and include hydroelectric power, wind power, and solar power. These energy resources use water, wind, and the sun's energy respectively to generate electricity. Bio-mechanical power is a relatively new renewable resource that uses human movement (by walking or by bicycle riding) to mechanically generate electricity when walking and has promising applications that can replace battery power. Nuclear power is another power source that uses nuclear reactions to make electricity and is more sustainable than the use of fossil fuels. However, nuclear power still produces toxic waste that must be disposed of properly, and uses uranium as a fuel source which is a finite resource.

Water turbine schematics showing how mechanical energy is converted to electrical energy.

Batteries can also be a type of power source. Batteries rely on a steady rate of chemical reactions that cause a flow of electrons to go from one end of the battery to the next through a circuit. This flow of electrons powers the device as the electricity flows through the circuit. The amount of power, how long the battery lasts, and its changeability depend on the specific materials used in the chemical reaction. Generally, highly acidic material is used within the battery, and the electricity is generated by the chemical reactions taking place within the material. Batteries power devices that are not hooked up to an electrical power source connected to the grid, such as cars, boats, and devices such as phones, flashlights, and laptops.

Typically, power supplies that are connected to the grid source power from an electricity-generating facility. There are many resources used to generate electricity, such as the above-mentioned renewable and non-renewable resources. Resources that are burned to generate electricity do so by heating water into steam, which is piped to a turbine and causes the turbine to spin. This turbine is connected to a shaft that spins a magnet inside a coil of copper wires. The mechanical, or kinetic, energy of the turning shaft is converted to electrical energy when electrons jump from the magnet to the copper wires, which produce a steady flow of electricity that is then transported onto the gird.

Renewable resources, such as water power and wind power, do not need steam to turn the turbine because the source itself provides the mechanical energy to turn the turbine. Solar power is a bit different in producing electricity and uses solar panels to collect light energy which is converted to electrical energy within each cell of the panel.

Once the electricity is produced, it must filter through a series of components that modify the voltage of the electricity to make it compatible with household outlets. The power that is generated is described as AC (alternating current), which means that the electricity flows in two directions as in a wave, and alternates between positive and negative currents. Once processed, the power will be in a DC (direct current) mode, which means it is either positive or negative and flows at a steady rate that is ideal for electrical circuits. The following components are included in this modification process: