Jugami Environmental Science

High School · Environmental Science

Energy Resources

Where our energy comes from, and the trade-offs behind every light switch.

In 2025 the United States used about 96.5 quadrillion British thermal units (quads) of primary energy, according to the U.S. Energy Information Administration (EIA). That energy came from oil wells, gas fields, coal mines, uranium fuel, sunlight, wind, rivers, the heat inside Earth and plants.

No energy source is free of trade-offs. Each one has its own costs, risks, land and water needs and effects on air and climate, and each fits some jobs better than others. This lesson explains how the main sources work, how scientists compare them, and how the numbers are measured, so you can weigh the choices yourself.

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Drawing of a hilly landscape with several energy sources: a wind turbine, solar panels, and a steam power plant with a cooling tower beside a river. Power lines run from the plant to a small town. From energy source to light switch

On this page

  1. 1Energy basics
  2. 2How electricity is made
  3. 3Nonrenewable sources
  4. 4Renewable sources
  5. 5Capacity factor and intermittency
  6. 6The grid and energy storage
  7. 7The U.S. and world energy mix
  8. 8Comparing life-cycle emissions
  9. 9Conservation and efficiency
  10. 10Check your understanding
  11. 11Vocabulary

1. Energy basics

Scientists define energy as the ability to do work. It comes in many forms, such as heat, light, motion, electricity, chemical energy and gravitational energy, and EIA sorts all of them into two groups:

Stored energy

Potential energy

Energy of position or storage. Chemical energy is held in the bonds of molecules: coal, oil, natural gas, biomass and batteries. Nuclear energy holds the nucleus of an atom together. Gravitational energy is stored by height, like water behind a dam. Mechanical energy is stored by tension, as in a stretched rubber band.

Working energy

Kinetic energy

The motion of waves, electrons, atoms, molecules and objects. It includes radiant energy (light, including sunlight), thermal energy (heat), motion energy (wind, a moving car), sound and electrical energy, the movement of electrons.

The law of conservation of energy says energy is neither created nor destroyed; it only changes form. Burning wood turns chemical energy into thermal energy. A hydroelectric dam turns the gravitational energy of water into motion and then electricity.

But not every change is useful. Energy efficiency is the share of the energy put in that comes out as useful work. Many energy conversions are fairly inefficient: EIA notes that the human body is less than 5% efficient at turning food into useful work most of the time.

Energy versus power

Power is how fast energy is used or delivered. One watt (W) is one joule (J) of energy per second. So energy = power × time. A 1,000-watt (1 kilowatt) device running for one hour uses 1 kilowatt-hour (kWh) of energy, the unit on an electric bill.

energy (kWh) = power (kW) × time (hours)

UnitMeasuresHow big is it?
Joule (J)Energy (SI unit)1 watt for 1 second
Watt (W), kilowatt (kW), megawatt (MW)Power: energy per second1 kW = 1,000 W; 1 MW = 1,000 kW
Kilowatt-hour (kWh)Energy (electricity)1 kW for 1 hour = 3.6 million J = 3,412 Btu
British thermal unit (Btu)Energy (heat)About the energy of burning one wooden match. A gallon of gasoline holds about 120,214 Btu.
QuadEnergy (national totals)One quadrillion (1015) Btu

Lighting shows why efficiency matters. The OpenStax physics text notes that a 60-watt incandescent bulb turns only about 5 watts into light; the rest becomes heat. The U.S. Department of Energy (DOE) reports that LED bulbs use up to 90% less energy and last up to 25 times longer than incandescent bulbs.

Try it: how much energy does a device use?

Find the watts on a device's label or charger. Enter how long it runs, and see the energy it uses compared with an average U.S. home.

Example: a 1,000-watt microwave for half an hour a day for 30 days. EIA's average for a U.S. home in 2022 was 10,791 kWh a year, about 899 kWh a month.

  • Energy used: 15 kWh
  • Share of an average home's month (899 kWh): about 1.7%
  • Natural gas burned at an average U.S. gas power plant (2025 heat rate, 7,781 Btu per kWh): about 113 cubic feet

Talk about it

A phone charger is rated at a few watts, and an electric clothes dryer at several thousand. Which do you think uses more energy in a month? What else would you need to know to be sure?

2. How electricity is made

Electricity is a secondary energy source: it is made from primary sources such as coal, natural gas, nuclear fuel, sunlight or wind. In 1831 the scientist Michael Faraday discovered that moving a magnet inside a coil of wire makes an electric current flow. Nearly every power plant still uses that idea. In a generator, a spinning electromagnet (the rotor) turns inside a ring of insulated wire coils (the stator), and something has to turn the rotor.

Usually that something is a turbine, a wheel of blades pushed by a moving fluid. EIA reports that steam turbines generate most of the world's electricity. Steam plants differ mainly in where their heat comes from.

How a steam power plant makes electricity A heat source, which can be burning coal, natural gas or biomass, or nuclear fission in a reactor, boils water into high-pressure steam. The steam pushes the blades of a turbine and spins it. A shaft connects the turbine to a generator, where a spinning magnet inside coils of wire makes electricity. A transformer raises the voltage and sends the electricity onto the grid. After the turbine, the used steam goes to a condenser, where cooling water from a river, lake or cooling tower turns it back into liquid water. A pump sends that water back to be boiled again. Much of the fuel’s energy leaves the plant as waste heat in the cooling water. 1. Heat sourceburning coal, gasor biomass, ornuclear fission 2. Turbinesteam pushesthe blades 3. Generatormagnet spinsinside coils 4. Transformerraises voltagefor the grid 5. Condensersteam turns backinto water Steam Shaft Electricity To homes and businesses Used steam 6. Pump returns the water Cooling water infrom a river, lakeor cooling tower Warmer water out:waste heat leavesthe plant here Coal, natural gas, biomass, nuclear and some geothermal and solar thermal plants all work this way. Only the heat source is different. In 2025, U.S. coal and nuclear steam plants used about 10,400 to 10,700 Btu of fuel for each 1 kWh (3,412 Btu) of electricity: about 32 to 33% efficient (EIA). Hydro and wind turbines are pushed by water and air instead of steam. Dashed blue arrows: cooling water. Solid black arrows: steam, water and electricity moving through the plant.
How a steam power plant works. The same steps are used whether the heat comes from coal, natural gas, biomass, nuclear fission, underground geothermal steam or concentrated sunlight.

EIA divides the 3,412 Btu in a kilowatt-hour by a plant's heat rate, the Btu of fuel it burns per kWh, to get its efficiency. In 2025 U.S. coal plants averaged a heat rate of 10,647 Btu/kWh (about 32% efficient) and nuclear plants 10,443 (about 33%). Natural gas plants averaged 7,781 (about 44%), helped by combined-cycle plants, which use the hot combustion gases from one turbine to make more electricity in a second turbine.

Other plants spin the turbine without steam: falling water in a hydroelectric dam, moving air in a wind turbine and hot combustion gases in a gas turbine. Solar photovoltaic (PV) panels are the main exception. They have no turbine or generator at all (see section 4).

3. Nonrenewable sources

National Geographic Education explains the difference simply: renewable resources can naturally replenish themselves, while nonrenewable resources cannot. EIA calls coal, oil, natural gas and uranium nonrenewable because their supply is limited to what can be mined or extracted, and they take a very long time to form. Coal, oil and natural gas are fossil fuels. Coal holds energy stored by plants that lived hundreds of millions of years ago in swampy forests; oil formed from the remains of tiny marine plants and animals that lived millions of years ago.

SourceHow it is usedAdvantagesDrawbacks and impacts
CoalMined from surface or underground mines and burned, mostly to boil water for steam power plants. EIA ranks coal by its carbon and heat content: anthracite, bituminous, subbituminous and lignite.Large U.S. reserves (see below). DOE notes that coal plants can be run flexibly to meet changing grid demand.Burning releases sulfur dioxide, nitrogen oxides, particulates, mercury and other heavy metals, carbon dioxide (CO₂) and ash. Coal has the highest life-cycle greenhouse gas emissions of the sources in section 8. Plants use scrubbers and filters to cut some pollutants. Mined land must be reclaimed close to its original condition; abandoned underground mines can drain acidic water, and the ground above tunnels can collapse.
Oil (petroleum)Pumped from wells and refined into gasoline, diesel, jet fuel, heating oil and materials for plastics. A 42-gallon barrel yields about 45 gallons of products because of processing gain.A liquid that is refined into many fuels and products. EIA notes it is the main fuel for transportation.Burning releases CO₂ and air pollutants. Fracturing fluids can leak if wells are poorly built. Spills from wells, pipelines, ships, trains and trucks pollute soil and water; after the 1989 Exxon Valdez spill in Alaska, Congress passed the Oil Pollution Act of 1990.
Natural gasMostly methane, from wells drilled straight down or sideways. In shale and other tight rock it is released by hydraulic fracturing: pumping water, sand and chemicals down a well at high pressure to crack the rock. Burned for electricity, heating and industry.EIA: burning it gives fewer emissions of nearly all air pollutants, and less CO₂, than coal or oil for the same energy. DOE notes gas plants are used flexibly to meet changing grid demand.Methane is a strong greenhouse gas, and some leaks from wells, tanks, pipelines and processing plants. Fracturing uses large amounts of water, and production makes contaminated water that must be treated. Drilling can disturb people and wildlife.
Nuclear (uranium)In a reactor, uranium-235 atoms split (fission) in a chain reaction that releases heat to make steam. Natural uranium is just over 0.7% U-235, so it is enriched to 3–5% and formed into ceramic pellets. EIA: each pellet makes about as much energy as 150 gallons of oil.No air pollution or CO₂ while operating. Runs nearly all the time: since 1990, nuclear plants have made about 20% of U.S. electricity.Produces radioactive waste, such as mill tailings and spent fuel, that stays dangerous for thousands of years. The NRC says spent fuel can be stored in specially designed pools or in dry casks, and most is now in pools at reactor sites; the U.S. does not have a permanent disposal facility for high-level waste. Reactors need complex safety systems, and plants need cooling water.

The Nuclear Regulatory Commission (NRC) describes how a typical pressurized water reactor works: the core heats water kept under pressure in a primary loop; in a steam generator, that heat boils water in a separate secondary loop; the steam turns the turbine, and a condenser turns it back into water to be pumped back.

How do we know how much is left? Reserves are estimates

EIA estimates U.S. recoverable coal reserves at about 250 billion short tons. At 2022 production, that would last about 422 years, but EIA cautions that the real number depends on changes in production and in the reserve estimates themselves. For oil, EIA calls the popular "reserves divided by production" calculation "often cited, but misleading," because proved reserves count only known projects. EIA expects world oil reserves to likely grow over time as new technology increases production at existing fields and new projects are developed.

Talk about it

Every nonrenewable source in the table has an advantage that the others lack. If you were planning electricity for a region, which advantages and which drawbacks would matter most to you, and why might someone in another region decide differently?

4. Renewable sources

EIA lists five major renewable sources: solar energy from the sun, wind from uneven heating of Earth's surface by the sun, hydropower from flowing water, geothermal energy from heat inside Earth, and biomass from plants. Through most of human history, biomass from plants was the main source of energy. In the United States, fossil fuels began replacing it in the early 1800s. "Renewable" does not mean impact-free, and biomass is renewable only if it is regrown.

How a solar cell works

A photovoltaic (PV) cell is made of a semiconductor, usually silicon. When it absorbs sunlight, the energy frees electrons. The cell is built in two layers, n-type and p-type silicon, and the boundary between them (the junction) has a built-in electric field that pushes the freed electrons toward the top. Thin metal lines collect them, and they flow through a circuit as direct current (DC). An inverter changes DC to the alternating current (AC) used by the grid.

Cross-section of a solar photovoltaic cell Sunlight, shown as packets of light energy called photons, strikes the top of a solar cell. The cell is a sandwich of two thin layers of semiconductor, usually silicon: an n-type layer on top and a p-type layer underneath, with a junction between them that creates an electric field. Absorbed photons knock electrons loose. The electric field pushes the freed electrons toward the top, where thin metal contact lines collect them. The electrons flow through a wire as direct current, power a light bulb or other load, and return to the metal contact on the back of the cell. An inverter can change the direct current to alternating current for the grid. Sunlight (photons) n-type silicon p-type silicon − − − Metal contact lines (top) Anti-reflective coating n-type layer Junction: electric field p-type layer Metal contact (back) Load (a light bulb) Electrons flow as direct current White circles: electrons freedby sunlight and pushed upby the field. Cells → modules → arrays.
Cross-section of a silicon solar cell (not to scale). Electrons freed by sunlight flow out of the n-type layer, through the load and back into the p-type layer through the back contact.

Cells are wired into modules (panels), and modules into arrays. EIA reports that the efficiency of commercial PV panels rose from under 10% in the mid-1980s to about 15% by 2015, and state-of-the-art modules now approach 25%. Bell Telephone researchers made the first practical PV cell in 1954.

SourceHow it worksAdvantagesDrawbacks and impacts
Solar PVPanels on roofs or in large fields turn sunlight directly into electricity.No emissions while running; fits on a rooftop or fills a large field. EIA: a PV system makes back the energy used to build it within 1 to 4 years, and most last 30 years or more.Output depends on time of day, season, location and weather. Large plants clear land and can affect habitat. Making panels takes energy-intensive materials and involves hazardous chemicals; recycling programs are growing.
Solar thermalMirrors concentrate sunlight to heat a fluid and make steam. EIA describes three types: linear systems (parabolic troughs and Fresnel reflectors), power towers and dish/engine systems.Some plants can store heat to make electricity after sunset.Needs strong, direct sunlight and lots of land. Some plants use water for cleaning mirrors and cooling, which matters in deserts; some heat-transfer fluids are hazardous if they leak.
WindWind flowing over the blades creates lift, which spins the rotor. The rotor turns a generator directly or through a gearbox. Horizontal-axis turbines commonly have three blades; blade length is the biggest factor in how much electricity a turbine can make.No emissions while running. Each turbine has a fairly small physical footprint. Wind farms can be on open land, ridges or offshore.Output rises and falls with the wind. Blades make noise, and some people dislike the sound. Birds and bats can be killed by blades. Most turbine materials can be recycled, but current blades cannot; researchers are testing recyclable designs.
HydropowerWater flows from a height through a pipe called a penstock and spins a turbine. The energy depends on the head (how far the water falls) and the flow. Plants can be run-of-river, use a storage reservoir or use pumped storage.Water can be released to meet changing electricity needs, and hydro generators can start quickly. Plants can operate for 50 to 100 years. Most U.S. dams were built for flood control, water supply and irrigation rather than power.Dams can block fish migration, and turbines kill or injure some fish; fish ladders and elevators help. Dams change water temperature, chemistry, flow and silt. Reservoirs can cover natural areas, farmland or archaeological sites and force people to move, and they emit some greenhouse gases (amounts are uncertain). Droughts reduce output.
GeothermalWells reach hot water or steam underground, at about 300–700 °F for power plants. Dry steam, flash steam (the most common) and binary-cycle plants use it to spin turbines. Geothermal heat pumps use the steady temperature of shallow ground to heat and cool buildings.High capacity factor (section 5). EIA: plants emit about 97% less sulfur compounds and about 99% less CO₂ than fossil fuel plants of the same size; binary plants are closed systems with nearly no air emissions.Power plants work only where very hot water or steam can be reached. Plants use scrubbers to remove hydrogen sulfide, and most inject used water and steam back underground.
Biomass and biofuelsWood, crops, food and yard waste, garbage and manure are burned directly (the most common method) or turned into fuels: ethanol by fermentation, biogas by microbes, and biodiesel and renewable diesel from oils and fats. Ethanol made up 82% of U.S. biofuel production in 2022.Can use wastes such as garbage, wood waste and manure; burning garbage could mean less waste in landfills. EIA notes that plants capture almost as much CO₂ as biomass releases when burned, which is why biomass is called carbon-neutral.Wood smoke contains carbon monoxide and particulate matter. Harvesting wood faster than trees grow causes deforestation. Waste-to-energy plants must use air pollution controls.

Talk about it

Choose one renewable source and one nonrenewable source that could work where you live. Compare them on land use, water use, reliability and air quality. Which comparisons surprised you?

5. Capacity factor and intermittency

A plant's capacity is how much power it can make at one moment, measured in megawatts. Its capacity factor compares the energy it actually generated in a year with what it would have made running at full capacity every hour. DOE puts it simply: a plant with a capacity factor of 100% is producing power all of the time.

Capacity factors are low for different reasons. Solar and wind are intermittent: the sun sets and the wind changes. U.S. solar's monthly capacity factor in 2025 ranged from about 13% in December to about 32% in June and July. Other plants are low partly by choice: DOE notes that coal and natural gas plants are used more flexibly to meet changing grid demand. Natural gas combustion turbines averaged just 13.6% in 2025. Any plant also produces nothing while it is offline for maintenance or refueling.

Capacity factors of U.S. power plants, 2025 Bar chart of average capacity factors for U.S. utility-scale generators in 2025: Nuclear 91.1%; Geothermal 65.9%; Natural gas, combined cycle 58.7%; Coal 48.8%; Hydropower 34.4%; Wind 33.8%; Solar photovoltaic 24.0%; Natural gas, combustion turbine 13.6%. Data: EIA Electric Power Monthly, Tables 6.07.A and 6.07.B. Capacity factor, 2025: share of the year’s maximum possible output actually made 0% 25% 50% 75% 100% Nuclear 91.1% Geothermal 65.9% Natural gas, combined cycle 58.7% Coal 48.8% Hydropower 34.4% Wind 33.8% Solar photovoltaic 24.0% Natural gas, combustion turbine 13.6% Data: EIA, Electric Power Monthly, Tables 6.07.A and 6.07.B (utility-scale plants). Bar colors: brown, fossil; purple, nuclear; green, renewable. A low number can mean the fuel isn’t always available (sun, wind, water) or that operators run the plant only when it’s needed.
Average capacity factors of U.S. utility-scale power plants in 2025, from EIA's Electric Power Monthly. Each bar is labeled with its source and value.

Try it: capacity factor and a year of electricity

Enter a plant's capacity and its capacity factor to see how much electricity it makes in a year and about how many average U.S. homes that could supply.

2025 averages from the chart: nuclear 91.1, geothermal 65.9, gas combined cycle 58.7, coal 48.8, hydropower 34.4, wind 33.8, solar PV 24.0.

  • Electricity in a year: about 296,000 MWh (100 MW × 8,760 hours × 33.8%)
  • Average U.S. homes supplied (10,791 kWh each): about 27,400

Talk about it

A 100 MW solar farm and a 100 MW nuclear plant have the same capacity. Use the calculator to compare what each makes in a year. Why might a utility still choose to build either one?

6. The grid and energy storage

The grid connects power plants to users. High-voltage transmission lines carry electricity long distances, because higher voltage makes long-distance transmission more efficient and less expensive. Transformers at substations lower the voltage for distribution lines that deliver it to homes and businesses. The Lower 48 states are served by three large connected systems: the Eastern, Western and Texas (ERCOT) interconnections. EIA estimates that transmission and distribution lost about 5% of the electricity sent out from 2018 to 2022.

On a grid, supply has to match demand second by second. Organizations called balancing authorities manage the day-to-day operation of the grid in each region.

Energy storage helps by charging when electricity is plentiful and discharging when it's needed. Storage is not a source of energy: EIA notes that storage systems use more electricity to charge than they give back. Storage is measured two ways: power capacity in megawatts (how fast it can deliver) and energy capacity in megawatt-hours (how much it holds).

Water, uphill and down

Pumped-storage hydropower

Pumps water to an upper reservoir when demand is low and lets it flow back down through turbines when demand is high. It had the most U.S. storage capacity at the end of 2022, about 22,000 MW. Because it takes in more electricity than it returns, its net generation is negative.

Chemical storage

Batteries

Large battery systems have grown fast. EIA reports U.S. battery capacity of 43.6 gigawatts at the end of 2025 and nearly 52 GW by mid-2026, averaging 70% growth a year over three years. Many are paired with solar plants.

7. The U.S. and world energy mix

Primary energy counts energy in the form it is found: coal, crude oil, uranium, wind and so on. Here is how the United States used it in 2025.

U.S. primary energy consumption by source, 2025 Bar chart. Total U.S. primary energy consumption in 2025 was 96.5 quadrillion Btu (quads). Petroleum (oil): 36.1 quads, 37%; Natural gas: 34.6 quads, 36%; Renewable energy (all): 8.8 quads, 9%; Coal: 8.7 quads, 9%; Nuclear: 8.2 quads, 8%. Renewable energy broken down, in quads: Biomass 4.85; Wind 1.58; Solar 1.44; Hydropower 0.84; Geothermal 0.12. Data: EIA Monthly Energy Review, Table 1.3. U.S. primary energy use, 2025: 96.5 quads in all Bars show quadrillion Btu (quads) and percent of the total 0 10 20 30 40 quads Petroleum (oil)Fossil 36.1 quads · 37.4% Natural gasFossil 34.6 quads · 35.9% Renewable energy (all)Renewable 8.8 quads · 9.2% CoalFossil 8.7 quads · 9.0% NuclearNuclear 8.2 quads · 8.5% Inside “renewable energy” (quads): Biomass 4.9 Wind 1.6 Solar 1.4 Hydropower 0.84 Geothermal 0.12 Data: U.S. Energy Information Administration, Monthly Energy Review, Table 1.3 (downloaded October 30, 2026). Recent values may be revised. Petroleum excludes biofuels, which EIA counts under biomass. Parts may not add to the total because of rounding and electricity imports.
U.S. primary energy consumption by source in 2025, from EIA's Monthly Energy Review, Table 1.3. Fossil fuels supplied about 82% of the total, and petroleum about 37%. EIA's U.S. energy facts page gives petroleum as about 38% (35.9 quads); this page uses the Table 1.3 values throughout so the chart, text and quiz match.

The mix keeps changing. EIA reports that petroleum was about 49% of U.S. energy use in 1978. Natural gas grew from about 18% in 1950 to about 36% in 2025, while coal fell from 37% in 1950 to 9% in 2025. Renewable energy production and consumption both set records in 2025, and since 2019 the U.S. has produced more energy than it consumes.

Electricity is a different mix

Most petroleum goes to transportation, so the fuels that make electricity look different. U.S. utility-scale plants generated about 4.43 trillion kWh in 2025:

SourceShare of 2025 U.S. electricity
Natural gasabout 41%
Nuclearabout 18%
Coalabout 17%
Windabout 11%
Solarabout 7%
Hydropowerabout 6%
Biomass, geothermal and petroleumabout 1% or less each

Renewables together made about 24% of U.S. electricity in 2025, up from 12% in 1990. Wind passed hydropower as the largest renewable source of U.S. electricity in 2019.

The world

The International Energy Agency (IEA) reports that the world's total energy supply in 2024 was about 650 exajoules (an exajoule is 1018 joules). China used about 27% of it and the United States about 14%.

SourceWorld energy supply, 2024World electricity, 2024
Oil30.0%2.4%
Coal27.6%34.5%
Natural gas22.9%21.9%
Biofuels and waste8.7%2.6%
Nuclear4.7%8.9%
Hydropower2.5%14.7%
Wind, solar and other3.6%wind 8.0%, solar PV 6.6%, geothermal 0.3%

IEA World Energy Balances, 2024 data (CC BY 4.0); world electricity about 31,600 TWh. Percentages are rounded. EIA and the IEA count primary energy from nuclear, hydro, wind and solar in different ways, so their percentages are not exactly comparable.

Talk about it

Coal supplies over a third of the world's electricity but only about a sixth of U.S. electricity. What kinds of differences between countries (resources, costs, history, how fast demand is growing) could explain gaps like that?

8. Comparing life-cycle emissions

A wind turbine releases no greenhouse gases while it spins, but making its steel and concrete does. A life-cycle assessment adds up emissions from cradle to grave: mining and making materials, building the plant, producing the fuel, running the plant and taking it apart. The National Renewable Energy Laboratory (NREL), a DOE lab, considered about 3,000 published studies and summarized the results.

Life-cycle greenhouse gas emissions of electricity sources Bar chart of median published life-cycle greenhouse gas emissions, in grams of carbon dioxide equivalent per kilowatt-hour of electricity: Coal 1001; Oil 840; Natural gas 486; Biomass 52; Solar photovoltaic 43; Geothermal 37; Concentrating solar 28; Hydropower 21; Nuclear 13; Wind 13. Data: NREL, Life Cycle Greenhouse Gas Emissions from Electricity Generation: Update, 2021. Life-cycle greenhouse gases: grams of CO₂-equivalent per kWh (median of published studies) 0 250 500 750 1,000 Coal 1,001 Oil 840 Natural gas 486 Biomass 52 Solar photovoltaic 43 Geothermal 37 Concentrating solar 28 Hydropower 21 Nuclear 13 Wind 13 Data: NREL (now the National Laboratory of the Rockies), 2021 fact sheet, Table 1. Includes building, fuel, operation and disposal.
Median published life-cycle greenhouse gas emissions for electricity sources, in grams of CO₂-equivalent per kilowatt-hour, from NREL's 2021 update. Each bar is labeled with its source and value.

NREL found that coal-fired electricity releases about 20 times more greenhouse gases per kilowatt-hour than solar, wind or nuclear. For fossil fuels, burning fuel while the plant operates causes the vast majority of emissions. For nuclear and renewables, NREL says most emissions happen upstream of operation, in stages such as getting resources, making materials and building the plant. Battery storage adds a median of about 33 g per kWh it delivers, and pumped storage about 7.4.

How do we know? Comparing many studies

Single studies make different assumptions about things like plant lifetimes, locations and methods, so their results vary. Using the middle (median) value from many reviewed studies gives a fairer picture than any one study. NREL found that totals for renewables and nuclear are much lower and generally less variable than those for fossil fuels. Greenhouse gases are only one comparison; land, water, air pollution, waste, cost and reliability matter too.

9. Conservation and efficiency

EIA describes two ways to avoid or reduce energy use:

Better technology

Energy efficiency

About the technical performance of devices: doing the same job with less energy. Installing energy-efficient lights, buying efficient products and choosing vehicles with high fuel economy are efficiency.

Changing what we do

Energy conservation

Actions that reduce how much energy is used. Turning lights and appliances off when they're not needed, by hand or with timers or motion sensors, is conservation.

EIA notes that both can lower energy bills and may reduce greenhouse gas emissions. Using less electricity also reduces demand on the electric system, and high demand often raises the cost of generating and transmitting electricity. Many utilities offer efficiency and conservation programs to their customers. Remember the heat rates in section 2: when electricity comes from a steam plant that is about one-third efficient, each kilowatt-hour saved at home means about three kilowatt-hours' worth of fuel energy that plant didn't need to burn.

Talk about it

List three ways your school uses energy. For each, suggest one efficiency change and one conservation change. Which would be easiest to start, and which would save the most?

10. Check your understanding

Choose an answer for each question, then press its Check button (or Enter) to see if you're right. With JavaScript on, your best score is saved on this device only. The full answer key is below the quiz.

1. A 1,000-watt microwave oven runs for 30 minutes. How much electrical energy does it use?

2. A power plant burns fuel containing 10,500 Btu to make 1 kWh of electricity, which equals 3,412 Btu. About how efficient is the plant?

3. According to EIA, which energy source supplied the largest share of U.S. primary energy consumption in 2025?

4. In 2025, U.S. nuclear plants had a capacity factor of about 91% and U.S. solar panels about 24%. What does that tell you?

5. Which of these makes electricity without spinning a turbine?

6. Natural uranium is just over 0.7% uranium-235. What do most U.S. reactors need before the uranium can be made into fuel?

7. Why does EIA report negative net generation for pumped-storage hydropower plants?

8. NREL's 2021 summary of published life-cycle studies compared greenhouse gas emissions per kilowatt-hour. Which had the lowest median values?

9. Which of these is an example of energy conservation rather than energy efficiency?

10. Burning natural gas releases less CO₂ than burning coal for the same amount of energy. What is one environmental trade-off of natural gas?

Show the answer key
  1. C. 0.5 kWh. Energy equals power times time. 1,000 watts is 1 kilowatt, and 30 minutes is 0.5 hour, so 1 kW × 0.5 h = 0.5 kilowatt-hour.
  2. A. About 33%. EIA divides 3,412 by the heat rate: 3,412 ÷ 10,500 ≈ 0.33, or 33%. Energy isn't destroyed, but about two-thirds of it leaves the plant as waste heat instead of electricity.
  3. D. Petroleum (oil). Petroleum supplied about 37% of U.S. primary energy in 2025 (36.1 quads in EIA's Monthly Energy Review, Table 1.3), and EIA notes that petroleum is the main fuel for transportation. Natural gas was a close second at about 36%.
  4. B. Over the year, nuclear plants made about 91% of the electricity they could have made running at full power all the time. Capacity factor compares the electricity a plant actually made with what it would have made running at full power every hour. It is not the same as efficiency. Solar is lower mainly because the sun sets and clouds pass.
  5. B. Solar photovoltaic (PV) panels. PV cells are semiconductors: absorbed sunlight frees electrons and creates a voltage directly. Coal, gas, nuclear, hydro and wind plants all spin a turbine connected to a generator.
  6. D. It must be enriched to about 3–5% U-235. EIA explains that uranium is converted and enriched so that 3% to 5% of it is U-235, the isotope that splits easily in a chain reaction. Then it is made into ceramic pellets and fuel rods.
  7. A. Pumping water uphill uses more electricity than they generate. Every storage system loses some energy. Pumped storage moves electricity from times of low demand to times of high demand, but it gives back less than it took in, so its net generation is negative.
  8. C. Wind and nuclear. Wind and nuclear each had a median of about 13 grams of CO₂-equivalent per kWh. Other medians: 43 for solar PV, 486 for natural gas, 840 for oil and 1,001 for coal. For fossil fuels most emissions come from burning the fuel; for nuclear and renewables, NREL says most happen upstream of operation, before the plant starts running.
  9. D. Turning off the lights when you leave a room. EIA's example: installing efficient lights is efficiency (the device does the same job with less energy); turning lights off when they aren't needed is conservation (using the device less).
  10. B. Methane, its main ingredient, is a strong greenhouse gas that can leak. EIA notes that natural gas is mostly methane, a strong greenhouse gas. Some natural gas leaks into the air from wells, storage tanks, pipelines and processing plants, so the industry works to prevent leaks.

11. Vocabulary

Biofuel
A liquid or gas fuel made from biomass, such as ethanol from corn or biodiesel from vegetable oils.
Biomass
Organic material from plants and animals, such as wood, crops, food waste and manure, used for energy.
British thermal unit (Btu)
A unit of heat energy. One Btu is about the energy released by burning one match; 1 kWh equals 3,412 Btu.
Capacity
The most power a power plant or generator can produce at one moment, measured in watts, kilowatts or megawatts.
Capacity factor
The electricity a plant actually generated over a period, divided by what it would have made running at full capacity the whole time.
Combined cycle
A power plant that uses hot exhaust from a gas turbine to make steam for a second turbine, getting more electricity from the same fuel.
Conservation (energy)
Using less energy by changing what you do, such as turning off lights or appliances when they aren't needed.
Efficiency (energy)
Getting the same job done with less energy input, or the share of input energy that becomes useful output.
Energy
The ability to do work or cause change. It is measured in joules, kilowatt-hours, Btu and other units.
Enrichment
Raising the share of uranium-235 in uranium, from just over 0.7% in nature to about 3–5% for most reactor fuel.
Fission
Splitting the nucleus of a heavy atom, such as uranium-235, into smaller atoms, which releases a large amount of energy.
Fossil fuel
Coal, oil or natural gas: fuels formed over millions of years from the buried remains of ancient plants and animals.
Generator
A machine that turns motion into electricity, usually by spinning a magnet inside coils of wire.
Geothermal energy
Heat from inside Earth, used directly for heating or to make steam that drives a turbine.
Grid
The network of power plants, transmission and distribution lines, substations and transformers that delivers electricity.
Heat rate
How many Btu of fuel a power plant uses to make one kilowatt-hour of electricity. A lower heat rate means a more efficient plant.
Hydraulic fracturing
Pumping water, sand and chemicals at high pressure into rock to crack it and release oil or natural gas. Also called fracking.
Intermittent
Not available all the time. Solar and wind output rise and fall with sunlight and wind.
Inverter
A device that changes the direct current (DC) from solar panels or batteries into the alternating current (AC) used by the grid.
Joule (J)
The SI unit of energy. One watt is one joule per second.
Kilowatt-hour (kWh)
The energy used by a 1,000-watt device running for one hour, equal to 3.6 million joules. Electric bills are measured in kWh.
Life-cycle assessment
A study of the environmental effects of a product or power source from cradle to grave, including mining, building, running and taking it apart.
Nonrenewable resource
An energy source with a limited supply that takes a very long time to form, such as coal, oil, natural gas and uranium.
Photovoltaic (PV) cell
A semiconductor device that converts sunlight directly into electricity.
Power
The rate at which energy is used or transferred, measured in watts.
Primary energy
Energy in the form it is found in nature, such as coal, crude oil, sunlight or wind, before it is converted.
Proved reserves
The amount of a resource that can be produced from known deposits with today's technology and prices. The estimate changes over time.
Pumped storage
A hydropower system that pumps water uphill to a reservoir when electricity is cheap and lets it flow back down through turbines when it is needed.
Quad
One quadrillion (10¹⁵) Btu. EIA uses quads to measure national energy use.
Renewable resource
An energy source that is naturally replenished, such as sunlight, wind, flowing water, Earth's heat and biomass that is regrown.
Secondary energy source
A carrier, such as electricity or hydrogen, made from a primary energy source.
Transformer
A device that raises or lowers the voltage of electricity so it can be sent long distances and then used safely.
Turbine
A machine with blades that a moving fluid, such as steam, water, hot gas or wind, pushes around to spin a shaft.
Watt (W)
The SI unit of power, equal to one joule per second.

Sources

Facts and numbers in this lesson come from these sources, checked on October 30, 2026. The diagrams and charts are original drawings made for Jugami; the U.S. energy chart is plotted from EIA's data file, the capacity factor chart from EIA's tables and the life-cycle chart from NREL's Table 1. Recent EIA figures may be revised.

  1. U.S. Energy Information Administration (EIA), Energy Explained, “Forms of energy” and “Laws of energy.” www.eia.gov
  2. EIA, Energy Explained, “Sources of energy” (renewable and nonrenewable; primary and secondary sources). www.eia.gov
  3. EIA, Energy Explained, “British thermal units (Btu)” (Btu, quads and fuel conversions). www.eia.gov
  4. OpenStax, College Physics 2e, 7.7 “Power” (1 watt = 1 joule per second; energy = power × time; kilowatt-hours). openstax.org
  5. OpenStax, College Physics 2e, 7.9 “World Energy Use” (renewable and nonrenewable sources). openstax.org
  6. EIA, Frequently Asked Questions, “What is the efficiency of different types of power plants?” (heat rate and efficiency). www.eia.gov
  7. EIA, Electric Power Annual, Table 8.1, “Average Operating Heat Rate for Selected Energy Sources, 2015 through 2025.” www.eia.gov
  8. EIA, Monthly Energy Review, Table 1.3, “Primary Energy Consumption by Source” (2025 annual values; data file downloaded October 30, 2026). www.eia.gov
  9. EIA, Energy Explained, “U.S. energy facts explained” (2025 shares and history). www.eia.gov
  10. EIA, Energy Explained, “Electricity in the United States” (2025 utility-scale generation by source). www.eia.gov
  11. EIA, Energy Explained, “How electricity is generated” (turbines, generators and Faraday's discovery). www.eia.gov
  12. EIA, Electric Power Monthly, Tables 6.07.A and 6.07.B, capacity factors for utility-scale generators (2025 annual and monthly values). www.eia.gov
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  14. EIA, Energy Explained, “Coal,” “How much coal is left” and “Coal and the environment.” www.eia.gov
  15. EIA, Energy Explained, “Oil and petroleum products” and “Oil and the environment.” www.eia.gov
  16. EIA, Frequently Asked Questions, “Does the world have enough oil to meet our future needs?” (proved reserves and why reserves-to-production ratios can mislead). www.eia.gov
  17. EIA, Energy Explained, “Natural gas” and “Natural gas and the environment.” www.eia.gov
  18. EIA, Energy Explained, “Nuclear explained,” “The nuclear fuel cycle” and “Nuclear power and the environment.” www.eia.gov
  19. U.S. Nuclear Regulatory Commission (NRC), “Pressurized Water Reactors.” www.nrc.gov
  20. NRC, “Storage of Spent Nuclear Fuel” (spent fuel pools and dry casks). www.nrc.gov
  21. EIA, Energy Explained, “Photovoltaics and electricity,” “Solar thermal power plants” and “Solar energy and the environment.” www.eia.gov
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  24. U.S. Department of Energy (DOE), Wind Energy Technologies Office, “How Do Wind Turbines Work?” www.energy.gov
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  26. EIA, Energy Explained, “Hydropower explained” and “Hydropower and the environment.” www.eia.gov
  27. U.S. Geological Survey (USGS) Water Science School, “Hydroelectric Power: How it Works.” www.usgs.gov
  28. EIA, Energy Explained, “Geothermal power plants” and “Geothermal energy and the environment.” www.eia.gov
  29. DOE Geothermal Technologies Office, “Geothermal Basics” (heat pumps and direct use). www.energy.gov
  30. EIA, Energy Explained, “Biomass explained,” “Biofuels explained” and “Biomass and the environment.” www.eia.gov
  31. EIA, Energy Explained, “Energy storage for electricity generation.” www.eia.gov
  32. EIA, Today in Energy, “Battery storage capacity averaged 70% growth over the last three years” (August 7, 2026). www.eia.gov
  33. EIA, Energy Explained, “Delivery to consumers” (the grid, transformers and interconnections). www.eia.gov
  34. EIA, Frequently Asked Questions, “How much electricity is lost in electricity transmission and distribution in the United States?” www.eia.gov
  35. International Energy Agency (IEA), “World: Energy Mix” and electricity generation by source, World Energy Balances, 2024 data (CC BY 4.0). www.iea.org
  36. National Renewable Energy Laboratory (NREL), “Life Cycle Greenhouse Gas Emissions from Electricity Generation: Update,” fact sheet NREL/FS-6A50-80580, September 2021, Table 1. NREL was renamed the National Laboratory of the Rockies (NLR) in December 2025. docs.nlr.gov
  37. EIA, Energy Explained, “Energy efficiency and conservation.” www.eia.gov
  38. EIA, Frequently Asked Questions, “How much electricity does an American home use?” (2022 averages). www.eia.gov
  39. DOE Energy Saver, “Consumer Guide to Energy-Efficient Lighting” (2021). www.energy.gov
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