Every device that uses electricity relies on an energy sourceAny material or process that provides usable power to do work. Scientists classify energy sources into two categories: renewableEnergy that replenishes naturally on human timescales — sun, wind, water and nonrenewableEnergy stored over millions of years; once used it cannot be replaced quickly.

Nonrenewable Resources — Fossil Fuels & Nuclear
Fossil fuelsCoal, oil, and natural gas formed from ancient organisms over millions of years — coal, oil, and natural gas — formed when ancient plants and animals were buried and compressed over millions of years. Burning them releases carbon dioxide (CO₂)A greenhouse gas produced when carbon-based fuels burn; traps heat in the atmosphere and other greenhouse gasesGases that absorb infrared radiation and warm Earth's surface that drive climate change. Nuclear energy splits uranium atoms in a process called fissionSplitting a heavy atom to release enormous amounts of energy as heat, generating heat that powers turbinesSpinning machines that convert fluid motion energy into mechanical energy and generatorsDevices that convert mechanical energy into electrical energy using magnetism. Nuclear plants produce almost no CO₂ but create radioactive waste that must be stored safely for thousands of years.

Renewable Resources
Solar panels use photovoltaic (PV) cellsDevices that convert sunlight directly into electricity using the photoelectric effect made from silicon to convert sunlight directly into electricity — no burning required. Wind turbines capture kinetic energy from moving air. HydroelectricElectricity generated by harnessing the energy of flowing or falling water dams harness flowing water. Geothermal energyHeat energy from Earth's interior used to generate electricity taps heat from Earth's interior, while biomassOrganic material burned or converted chemically for energy burns plant and organic material.

How Electricity Is Produced
Almost all large-scale electricity generation follows the same path: a heat source spins a turbine, the turbine drives a generator, and the generator uses electromagnetic inductionGenerating an electric current by moving a conductor through a magnetic field to push electrons through wires, creating electrical current. Solar PV cells are the exception — they convert light directly to electricity without turbines. Electricity is measured in kilowatt-hours (kWh)The unit of electrical energy; 1 kWh = 1,000 watts used for 1 hour. A typical U.S. household uses about 10,500 kWh per year. The choice of energy source affects carbon footprintThe total greenhouse gases released by a person, product, or activity, air quality, water use, and long-term cost.

The Hoover Dam, completed in 1936 on the Colorado River between Nevada and Arizona, is one of the most famous hydroelectric dams in the United States. It stands 221 meters (726 feet) tall and can generate up to 2,080 megawatts (MW) of electricity — enough to power about 1.3 million homes in the American Southwest.

How it works: Water stored in Lake Mead (the reservoir behind the dam) sits at a much higher elevation than the river below. This height difference gives the water potential energyStored energy based on position or height; the higher the water, the more potential energy it has. When water is released through intake pipes called penstocks, gravity pulls it downward at high speed. This fast-moving water strikes large turbine blades inside the powerhouse, spinning them at up to 180 revolutions per minute. The spinning turbines drive generators that use electromagnetic induction to produce electricity, which is then sent through high-voltage transmission lines to homes and businesses across three states.

Environmental benefits and tradeoffs: Hydroelectric dams produce no carbon dioxide during operation, making them one of the cleanest electricity sources by carbon footprintTotal greenhouse gases released by a process or energy source. However, building a dam floods large areas of land, disrupts river ecosystems, and blocks fish migration. The Hoover Dam changed the entire ecology of the Colorado River delta in Mexico. Scientists and engineers must weigh these environmental tradeoffs when deciding where and whether to build new dams. Despite the challenges, hydroelectric power supplies about 6% of all U.S. electricity and over 15% of global electricity — making it the largest single source of renewable energy on Earth.

Comparing to fossil fuels: A coal plant producing the same 2,080 MW of power as Hoover Dam would release approximately 14 million metric tons of CO₂ per year. The dam releases essentially none. This difference represents a major advantage for hydroelectric power in the effort to reduce greenhouse gas emissions — though the best solution for any region depends on geography, river resources, and local environmental needs.