An energy source is a natural resource from which usable energy can be extracted. The list is long: sunlight, wind, water, coal, oil, gas, uranium, and heat from the Earth. Some are renewable. Others are finite. Some are clean at the point of use. Others emit pollutants.

Sources fall into two broad groups. Primary sources exist in nature: coal, crude oil, sunlight, wind, uranium. Secondary sources are produced by converting primary ones: electricity, refined gasoline, hydrogen. Most energy discussions mix the two, which can confuse the picture.

Choosing sources involves trade-offs. Fossil fuels are energy-dense and storable but emit carbon. Nuclear is low-carbon and steady but produces radioactive waste. Solar and wind are clean and abundant but variable. Hydro is reliable but disrupts ecosystems. No source is perfect. The mix depends on geography, economics, and politics.

Common sources

• Fossil fuels: coal, oil, gas
• Nuclear: uranium
• Renewables: solar, wind, hydro
• Geothermal
• Biomass