Clean Energy: What It Is, Types and How It Differs from Green Energy

Clean energy is one of the most used — and most loosely used — terms in the power business. Utilities sell “green” plans, governments set “clean electricity” targets, and the word “renewable” is often swapped in as if all three meant the same thing. They overlap, but they are not identical. This guide defines clean energy precisely, separates it from green and renewable energy, answers whether nuclear and natural gas count, and backs every comparison with lifecycle emissions data from the IPCC and NREL and generation data from the EIA and Ember.

What is clean energy?

Clean energy is energy produced with very low greenhouse gas emissions and little or no air pollution, across its full life cycle. In practice that means wind, solar, hydropower, geothermal, ocean energy and, in most definitions, nuclear power.

The definition rests on two tests:

1. Low carbon. The source emits little carbon dioxide (CO2) and other greenhouse gases per kilowatt-hour (kWh) produced, counting mining, manufacturing, construction, operation and decommissioning.
2. Low pollution. It does not release significant sulfur dioxide, nitrogen oxides, particulates or mercury into the air where people live.

A kilowatt-hour (kWh) is the energy used by a 1,000-watt appliance running for one hour; our kilowatt-hour guide explains the unit. Emissions are usually reported as grams of CO2-equivalent per kWh (g CO2e/kWh), which converts other gases such as methane into the CO2 amount with the same warming effect.

Clean energy vs green energy vs renewable energy: what is the difference?

Renewable energy is about whether the source refills, clean energy is about emissions, and green energy is about overall environmental impact — so a source can be one without being the others.

Term	Core question	Includes	Usually excludes
Renewable energy	Does the source naturally replenish?	Solar, wind, hydro, geothermal, biomass, tidal, wave	Coal, oil, gas, nuclear
Clean energy	Is it low-carbon and low-pollution?	Solar, wind, hydro, geothermal, nuclear; sometimes carbon-captured fossil	Unabated coal, oil, gas; often biomass combustion
Green energy	Does it come from nature with minimal environmental harm?	Solar, wind, geothermal, low-impact hydro	Nuclear, large dams (in strict definitions), fossil fuels
Low-carbon energy	Are lifecycle greenhouse emissions low?	Renewables plus nuclear	Fossil fuels without capture

Some practical examples of where the labels split:

• Nuclear is clean and low-carbon but not renewable, and many environmental groups do not call it green because of mining and radioactive waste.
• Biomass (burning wood, crop residues or energy crops) is renewable, but its IPCC median lifecycle emissions of about 230 g CO2e/kWh and its smokestack pollution mean many people do not consider it clean.
• Large hydropower is renewable and low-carbon, but big reservoirs can flood ecosystems and, in tropical regions, release methane, so strict green definitions sometimes exclude it.
• Wind and solar pass all three tests, which is why they anchor almost every clean energy plan.

For the stock-versus-flow side of the story, read renewable vs nonrenewable energy.

History of the terms “clean energy” and “green energy”

“Clean” began as an air-pollution word and became a carbon word; “green” began as a marketing and environmental label for renewable electricity. Policy has repeatedly redrawn where each line falls, especially for nuclear and natural gas.

In US policy, “clean” first meant clean air. The Clean Air Act of 1970 targeted smog, soot and sulfur from smokestacks and tailpipes, not carbon dioxide. A cleaner fuel was one with less local pollution, which is why gas was long sold as “clean-burning”. The Public Utility Regulatory Policies Act of 1978 (PURPA) then opened the grid to small renewable and cogeneration plants called qualifying facilities (FERC), creating the first market for independent renewable power.

“Green power” took off in the late 1990s, when several states opened retail electricity to competition and suppliers began selling renewable plans. To separate genuine products from empty marketing, the Center for Resource Solutions launched the Green-e certification program in 1997 (Green-e). In that retail sense, “green” meant renewable, usually excluding nuclear.

From the 2000s, as climate change moved to the center of energy policy, “clean energy” shifted toward meaning low-carbon. In his 2011 State of the Union, President Obama set the goal that “by 2035, 80 percent of America’s electricity will come from clean energy sources” (White House); the accompanying fact sheet counted renewables, nuclear, efficient natural gas and coal with carbon capture (fact sheet). Science then supplied a yardstick: NREL’s 2012 harmonization studies and the IPCC’s 2014 lifecycle tables (below) let policymakers rank sources by grams of CO2e per kWh. In 2015, the UN made “affordable, reliable, sustainable and modern energy for all” Sustainable Development Goal 7 (UN).

The newest definitions are emissions-based rather than technology lists. The EU’s Complementary Climate Delegated Act of 2022 classed certain nuclear and gas activities as transitional under strict conditions (European Commission). In the US, the technology-neutral Clean Electricity Production Credit covers facilities placed in service after December 31, 2024 whose greenhouse gas emissions rate is not greater than zero (IRS).

Year	Milestone	Source
1970	US Clean Air Act: “clean” defined by local air pollution	EPA
1978	PURPA opens the grid to small renewable “qualifying facilities”	FERC
1997	Green-e certification launched for “green power” products	Green-e
2011	US goal of 80% “clean energy” electricity by 2035, including nuclear and efficient gas	White House
2012	NREL LCA Harmonization papers in the Journal of Industrial Ecology	doi:10.1111/j.1530-9290.2012.00465.x
2014	IPCC AR5 Annex III publishes lifecycle emission medians by technology	IPCC
2015	UN adopts Sustainable Development Goal 7 on energy	UN
2022	EU taxonomy adds nuclear and gas as transitional activities	European Commission
2025	US technology-neutral clean electricity credit applies to new zero-emission plants	IRS

What are the main types of clean energy?

The main types of clean energy are solar, wind, hydropower, geothermal, nuclear and ocean energy, plus energy efficiency and storage as enabling technologies.

Clean energy source	How it makes electricity	US share of utility-scale generation, 2025 (EIA)	US capacity factor, 2025 (EIA)
Nuclear	Uranium fission heats water into steam	about 18%	91.1%
Wind	Moving air spins turbine blades	about 11%	33.8%
Solar (PV and thermal)	Photovoltaic cells convert sunlight; CSP uses mirrors	about 7%	24.0% (PV)
Hydropower	Falling water turns turbines	about 6%	34.4%
Geothermal	Underground heat produces steam	less than 1%	65.9%
Ocean (tidal, wave)	Moving seawater drives turbines	negligible	n/a

Source: EIA Electricity in the United States and Electric Power Monthly Table 6.07.B, 2025 data. Each source has its own deep-dive: solar energy, wind energy, hydroelectric power and geothermal energy.

A photovoltaic (PV) cell is a semiconductor device, usually made of silicon, that converts sunlight directly into electricity. A capacity factor is the share of its maximum possible output that a plant actually produces in a year.

How clean is each energy source? Lifecycle emissions compared

Over their full life cycle, wind, nuclear, hydro, geothermal and solar emit roughly 11 to 48 g CO2e/kWh, while natural gas emits about 490 g and coal about 820 g (IPCC AR5, 2014 medians).

No energy source is truly zero-emission: steel, concrete, glass and silicon all take energy to make. Life cycle assessment (LCA) captures this. The IPCC’s Fifth Assessment Report compiled hundreds of LCA studies, and NREL’s LCA Harmonization project re-ran many of them on common assumptions, which narrowed the ranges without significantly changing the medians.

The coal study shows how that works. Whitaker et al. (2012) screened 270 references and kept 53 that reported 164 estimates for coal power, ranging from 675 to 1,689 g CO2e/kWh with a median of 1,001. Adjusting them to common system boundaries and consistent inputs cut the interquartile range by 53% while moving the median by only 2.2% (Journal of Industrial Ecology). Parallel harmonizations covered wind (Dolan & Heath, 2012), nuclear (Warner & Heath, 2012) and crystalline silicon PV (Hsu et al., 2012). Later work integrating LCA with energy-system models projected 2050 lifecycle emissions of just 3.5–12 g CO2e/kWh for nuclear, wind and solar in a 2°C scenario, against 78–110 g for fossil plants with carbon capture (Pehl et al., 2017).

Why is this coal median (1,001 g) higher than the IPCC’s 820 g? The two figures summarise different sets of studies with different assumptions about plant type, efficiency and fuel, so they are not directly comparable. Both tell the same story: coal emits roughly 75 to 90 times more per kWh than wind’s median of about 11 g.

Technology	Median lifecycle emissions (g CO2e/kWh)	Times cleaner than coal
Wind (onshore)	11	about 75x
Nuclear	12	about 68x
Hydropower	24	about 34x
Geothermal	38	about 22x
Solar PV (rooftop)	41	about 20x
Solar PV (utility-scale)	48	about 17x
Biomass (dedicated)	230	about 3.6x
Natural gas (combined cycle)	490	about 1.7x
Coal (pulverized)	820	1x

Source: IPCC AR5 WGIII Annex III (2014). The “times cleaner” column is our arithmetic: 820 divided by each median.

Is nuclear clean energy?

Yes, by the emissions test: nuclear’s median lifecycle footprint is about 12 g CO2e/kWh (IPCC, 2014), and plants emit no air pollutants while running. It is not renewable, and its waste and fuel supply raise separate issues.

Arguments that nuclear belongs in clean energy:

• Lifecycle emissions on par with wind.
• No combustion, so no smog-forming pollutants.
• Very high output per plant: US reactors ran at a 91.1% capacity factor in 2025 (EIA), so they deliver clean power around the clock.
• Small land footprint for the energy produced.

Arguments against calling it clean or green:

• Spent fuel stays radioactive for thousands of years and the US still has no permanent disposal site.
• Uranium must be mined, and US operators import most of it; EIA data show Canada (27%), Kazakhstan (25%) and Russia (12%) were the top sources of the 40.5 million pounds bought in 2022.
• Accident risk, though rare, carries high consequences.

Many government clean electricity standards in US states and the EU’s sustainable finance taxonomy include nuclear under conditions, while most “green power” retail products do not. Our non-renewable resources guide covers uranium supply in more detail.

How much of our electricity is clean energy?

Clean sources produced 40.9% of global electricity in 2024 (Ember) and about 42% of US utility-scale electricity in 2025 (EIA: renewables about 24% plus nuclear about 18%).

Ember’s Global Electricity Review 2025 called 2024 the first year since the 1940s that low-carbon sources topped 40% of world power. Hydro was the largest clean source at 14.3%, then nuclear at 9.0%, wind at 8.1% and solar at 6.9%. Solar was the largest source of new generation for the third year running, adding 474 TWh in 2024.

Cost is a big reason for that growth. IRENA’s Renewable Power Generation Costs in 2024 found that 91% of new utility-scale renewable capacity commissioned in 2024 produced electricity more cheaply than the cheapest new fossil fuel option, with onshore wind averaging $0.034/kWh and solar PV $0.043/kWh worldwide. Since 2010, IRENA’s figures show the global average cost of new solar PV electricity falling 90% and onshore wind 70% (IRENA summary).

Storage is the other half. Because wind and solar output varies, grids add batteries to shift clean power into the evening. US power providers added a record 10.3 GW of utility-scale battery storage in 2024, according to the EIA.

Who is the largest producer of clean energy?

China is the largest producer of clean energy: it generated about 3,920 TWh of renewable electricity in 2025, more than three times the United States in second place at about 1,159 TWh (Our World in Data, from Ember and Energy Institute data). By share, countries built on hydro or nuclear lead instead.

Country	Low-carbon share of electricity, 2025	Main clean sources
Sweden	About 99%	Hydro, nuclear, wind
France	About 95%	Nuclear, hydro
United States	About 43%	Nuclear, wind, solar, hydro
China	About 42%	Hydro, wind, solar, nuclear
World	About 43%	Hydro, nuclear, wind, solar

Source: Our World in Data, share of electricity from low-carbon sources (renewables plus nuclear; includes small-scale solar, so the US share is slightly above the EIA utility-scale figure).

Within the US, the leader depends on the source. Texas generates the most wind power, and Texas, Iowa, Oklahoma, Kansas and Illinois together produced about 52% of US wind electricity in 2025 (EIA). Illinois generates more nuclear electricity than any other state, about 13% of the US total in 2024 (EIA). For countries running almost entirely on renewables, see the country table in our renewable energy guide.

What are the benefits and drawbacks of clean energy?

Clean energy’s benefits are lower emissions, cleaner air, no fuel cost for wind and solar, and falling prices; its drawbacks are variable output for wind and solar, storage and grid costs, land use and mineral demand.

Benefits	Drawbacks
Lifecycle emissions 11–48 g CO2e/kWh vs 490–820 for fossil fuels (IPCC, 2014)	Wind and solar vary with weather; US capacity factors 33.8% and 24.0% (EIA, 2025)
No smokestack pollution from wind, solar, hydro or nuclear	Batteries, transmission and backup add system cost
No fuel to buy for wind, solar and hydro	High up-front capital cost per project
Cheapest new power in most markets (IRENA, 2024)	Nuclear brings long-lived waste and high build costs
Domestic resource, less exposure to fuel imports	An onshore wind plant needs about 9 times the minerals of a gas plant (IEA, 2021)

The drawbacks are mainly about integration rather than the energy itself. Our full breakdown is in advantages of renewable energy. To measure your own emissions, use the carbon footprint calculator. The build-out also creates renewable energy jobs, and the largest clean energy companies are profiled in a separate guide.

Clean energy also matters for health, not just climate. A review in The Lancet found that electricity from lignite, coal and oil carries by far the largest burden of deaths and serious illness per TWh in Europe, with gas much lower and nuclear lowest of the sources assessed (Markandya & Wilkinson, 2007).

What is carbon capture, and does it make fossil fuels clean?

Carbon capture and storage (CCS) traps CO2 from a power plant or factory and stores it underground; it can reduce emissions from fossil plants but does not remove the upstream emissions from mining and drilling.

A CCS-equipped plant separates CO2 from its exhaust, compresses it and injects it into deep rock formations. Some clean electricity standards count fossil plants with capture as clean, at least partly. Critics point out that capture rates below 100%, energy used to run the capture equipment, and methane leaks from gas supply all leave a residual footprint. Because few power plants use CCS at commercial scale today, most clean energy growth comes from wind, solar and storage.

What is the difference between clean energy and sustainable energy?

Sustainable energy is energy that can meet today’s needs without compromising future generations, which adds long-term resource and social tests on top of low emissions.

Wind and solar are usually called both clean and sustainable. Nuclear is clean but debated as sustainable because of its finite fuel and waste. Biomass can be sustainable if forests and crops regrow as fast as they are harvested, yet it may not be clean if burning it pollutes the local air. The terms overlap heavily, so check the definition any program or product uses. A useful rule: look for the lifecycle emissions number, since that is the measurable part. Our sustainable energy guide scores each source against the full set of sustainability tests.

Is hydropower clean energy?

Yes. Hydropower’s median lifecycle emissions are about 24 g CO2e/kWh (IPCC, 2014), and it was the world’s largest clean electricity source in 2024 at 14.3% of generation (Ember).

Hydropower plants use falling or flowing water to spin turbines. Reservoir dams can store water and release it when demand peaks, which makes hydro one of the few clean sources that can be dispatched like a fossil plant. The caveats are ecological: dams block fish migration, change river temperatures and sediment flows, and reservoirs in warm climates can emit methane from decaying vegetation. Drought also cuts output. US conventional hydro ran at a 34.4% capacity factor in 2025 (EIA). Read more in our hydroelectric power guide.

How can you switch to clean energy at home?

You can generate clean power yourself, buy it from your utility, or cut how much energy you need — ideally all three. A practical order:

1. Measure your usage. Find your monthly kWh on your bill, then use the electricity cost calculator to see what each appliance costs.
2. Cut waste first. LEDs, smart thermostats, insulation and efficient appliances reduce the size of any solar system you will need.
3. Size rooftop solar. The solar panel calculator estimates panel count from your usage and location. See our solar energy pros and cons before you buy.
4. Consider storage or off-grid. A home battery stores midday solar for the evening; the off-grid solar calculator sizes a full stand-alone system.
5. Buy green power if you rent. Many utilities offer green pricing or community solar subscriptions backed by renewable energy certificates (RECs), each representing 1 MWh of renewable generation. Even small loads can follow the grid: US iPhones use Clean Energy Charging to finish overnight charges in lower-carbon hours.
6. Electrify heat and transport. Heat pumps and electric vehicles move your energy use onto a grid that gets cleaner every year.

Scientific references

1. Schlömer, S., Bruckner, T., Fulton, L., Hertwich, E., McKinnon, A., Perczyk, D., Roy, J., Schaeffer, R., Sims, R., Smith, P., & Wiser, R. (2014). “Annex III: Technology-specific cost and performance parameters.” In Climate Change 2014: Mitigation of Climate Change. Contribution of Working Group III to the Fifth Assessment Report of the IPCC. Cambridge University Press, 1329–1356. IPCC PDF
2. Whitaker, M., Heath, G. A., O’Donoughue, P., & Vorum, M. (2012). “Life Cycle Greenhouse Gas Emissions of Coal-Fired Electricity Generation: Systematic Review and Harmonization.” Journal of Industrial Ecology, 16(S1). doi:10.1111/j.1530-9290.2012.00465.x
3. Dolan, S. L., & Heath, G. A. (2012). “Life Cycle Greenhouse Gas Emissions of Utility-Scale Wind Power: Systematic Review and Harmonization.” Journal of Industrial Ecology, 16(S1). doi:10.1111/j.1530-9290.2012.00464.x
4. Warner, E. S., & Heath, G. A. (2012). “Life Cycle Greenhouse Gas Emissions of Nuclear Electricity Generation: Systematic Review and Harmonization.” Journal of Industrial Ecology, 16(S1). doi:10.1111/j.1530-9290.2012.00472.x
5. Hsu, D. D., O’Donoughue, P., Fthenakis, V., Heath, G. A., et al. (2012). “Life Cycle Greenhouse Gas Emissions of Crystalline Silicon Photovoltaic Electricity Generation: Systematic Review and Harmonization.” Journal of Industrial Ecology, 16(S1). doi:10.1111/j.1530-9290.2011.00439.x
6. Pehl, M., Arvesen, A., Humpenöder, F., Popp, A., Hertwich, E. G., & Luderer, G. (2017). “Understanding future emissions from low-carbon power systems by integration of life-cycle assessment and integrated energy modelling.” Nature Energy, 2(12), 939–945. doi:10.1038/s41560-017-0032-9
7. Markandya, A., & Wilkinson, P. (2007). “Electricity generation and health.” The Lancet, 370(9591), 979–990. doi:10.1016/S0140-6736(07)61253-7

Bottom line

Clean energy means low lifecycle emissions and low pollution: wind, solar, hydro, geothermal and nuclear all qualify, with medians between about 11 and 48 g CO2e/kWh against 490 g for gas and 820 g for coal. Renewable is about whether a source refills, and green is about overall environmental impact, so nuclear is clean but not renewable and biomass is renewable but not always clean. Clean sources now supply about 41% of world electricity, and falling costs plus storage are pushing that share higher. To see how each source compares in depth, explore our renewable energy sources guide.