What’s the cleanest energy?

When it comes to clean energy, there are a few main options to consider: solar, wind, hydroelectric, geothermal, and nuclear energy. Each has its own advantages and disadvantages in terms of environmental impact, safety, reliability, and cost. Determining the “cleanest” energy is not straightforward, as different metrics point to different energy sources as being optimal. However, by examining the key factors, we can get a sense of which energy sources are the most sustainable and have the least negative impact on the planet.

How is “clean” energy defined?

There are a few main criteria we can use to define how “clean” an energy source is:

  • Greenhouse gas emissions – How much carbon dioxide (CO2) and other greenhouse gases are emitted in generating electricity from each source? Less emission of these heat-trapping gases means lower impact on climate change.
  • Air pollution – Does generating electricity from the source produce air pollutants like sulfur dioxide, nitrogen oxides, and particulate matter? Less air pollution means lower impact on public health.
  • Water use – How much water is required to produce electricity from each source? Lower water use is preferable, especially in areas where water scarcity is an issue.
  • Waste – How much solid waste or other byproducts result from each energy source? Less waste and ability to recycle or reuse byproducts is preferable.
  • Land use – What is the land footprint required for facilities like solar farms, wind turbines, dams, etc? Lower land disturbance and impact on ecosystems is better.
  • Wildlife impact – Does each energy source affect natural habitats, migratory patterns, and mortality risk for birds, bats, marine life, etc? Minimal disruption of wildlife is key.

When examining the “cleanliness” of energy sources, we need to weigh all these factors. An ideal clean energy source would have low emissions, pollution, water use, waste production, land disturbance, and wildlife impact. Of course, finding a perfect option is difficult, so it’s about striking the best balance between these considerations.

How do solar and wind power stack up?

Two of the most popular and rapidly growing renewable energy sources are solar and wind power. Here is how they compare in terms of cleanliness:

Solar power

  • Greenhouse gas emissions – Produces no direct emissions while generating electricity. Manufacturing solar panels creates some emissions.
  • Air pollution – No air pollutants emitted during electricity generation. Some air pollutants from manufacturing panels.
  • Water use – Very low water usage once solar farms are installed and operating. Water is used to clean panels.
  • Waste – Can produce small amounts of hazardous waste during manufacturing. Panels can be difficult to recycle.
  • Land use – Utility-scale solar farms require significant land area, anywhere from 5 to 10 acres per megawatt produced. Can impact ecosystems.
  • Wildlife impact – Potential issues with birds flying into panels. Effects can be mitigated with proper panel positioning.

Wind power

  • Greenhouse gas emissions – No direct emissions during electricity generation. Manufacturing turbines creates some emissions.
  • Air pollution – No air pollutants emitted during electricity generation.
  • Water use – No water usage during electricity production. Minimal water used in manufacturing.
  • Waste – Negligible amounts of solid waste produced during manufacturing and construction of turbines.
  • Land use – Requires substantial land area for wind farms, but can co-exist with agriculture. Smaller footprint than solar overall.
  • Wildlife impact – Risk of bird and bat deaths from turbines. Careful site selection and updated turbine designs can minimize impacts.

In general, solar and wind energy score very well on most cleanliness criteria. Their primary downsides are land use, wildlife impacts, and manufacturing-related emissions – though rapid advances are being made on reducing all these factors.

How does hydropower compare?

Hydropower, or energy generated from flowing water in dams and reservoirs, has been used for decades and provides substantial electricity globally. Here is how it measures up:

  • Greenhouse gas emissions – Very low emissions during electricity generation. Reservoirs can emit methane though, a potent greenhouse gas.
  • Air pollution – No air pollutants emitted during electricity generation.
  • Water use – Requires routing river flows through turbines so has major impacts on water flows and aquatic ecosystems.
  • Waste – Generally no solid waste produced.
  • Land use – Dams and reservoirs flood substantial land areas and disrupt natural habitats.
  • Wildlife impact – Significant disruptions to fish migration and populations from blocked rivers, changes in water levels and quality.

Hydropower scores very well for air emissions but quite poorly for impacts on water, land use, and wildlife. Fish populations and natural river ecosystems can be severely damaged by large hydroelectric dams. However, “run-of-river” hydro projects can have much lower impact as they divert only a portion of flow.

How does geothermal energy compare?

Geothermal energy harnesses heat from under the earth’s surface for heating or to generate electricity. Here is how it fares for cleanliness:

  • Greenhouse gas emissions – Very low emissions, comparable to wind and solar.
  • Air pollution – Little to no air pollutants emitted.
  • Water use – Geothermal plants use water for pumping and cooling but substantially lower usage compared to fossil fuels.
  • Waste – Small amounts of solid wastes are produced from geothermal plants.
  • Land use – Geothermal plants have a relatively small footprint. Existing geothermal fields are used for decades.
  • Wildlife impact – Generally low impact, but high temperature geothermal systems can release trace amounts of contaminants and heavy metals that can impact ecosystems.

Geothermal performs very well on emissions, water use, and land use criteria. Rapid technical advances are also helping make geothermal energy economical in more locations. Overall, geothermal provides reliable clean energy with minimal environmental disturbance.

How does nuclear energy compare on clean metrics?

Nuclear power plants produce enormous amounts of steady, carbon-free electricity. But safety and radioactive waste concerns often overshadow their clean energy attributes. Here’s how nuclear stacks up:

  • Greenhouse gas emissions – Extremely low carbon emissions comparable to renewables.
  • Air pollution – No air pollutants are emitted during electricity generation.
  • Water use – Nuclear plants require substantial water for cooling and steam generation.
  • Waste – Produces small volumes of highly radioactive spent fuel that requires special long-term disposal.
  • Land use – Nuclear plants have a relatively small land footprint compared to solar and wind farms.
  • Wildlife impact – Generally low impact on natural habitats and wildlife.

While nuclear performs admirably for greenhouse gases and air pollution, major constraints are water use and radioactive waste management. Improved reactor designs continue to progress though, with potential to recycle fuel and use alternate coolants.

How do natural gas power plants compare?

Natural gas is often touted as a cleaner fossil fuel alternative to coal power. But how does it compare on sustainability metrics?

  • Greenhouse gas emissions – Emits about half the carbon dioxide compared to coal when burned for electricity.
  • Air pollution – Emits nitrogen oxides and particulate matter, but at lower levels than coal.
  • Water use – Uses water for steam generation and cooling at levels comparable to nuclear.
  • Waste – Produces negligible amounts of solid waste.
  • Land use – Compact facilities with small surface footprint.
  • Wildlife impact – Relatively low habitat disruption and wildlife impacts.

While cleaner than coal, natural gas power still produces substantial carbon emissions and air pollution compared to true clean energy sources. As a transitional energy supply, natural gas may provide cleaner electricity as renewable generation scales up.

And how about coal power generation?

Coal combustion produces large volumes of electricity globally, but carries heavy environmental impacts:

  • Greenhouse gas emissions – Very high emissions of CO2 when burned, roughly double that of natural gas.
  • Air pollution – Major source of sulfur dioxide, nitrogen oxides, particulate matter, and mercury.
  • Water use – Large amounts used for steam generation and cooling.
  • Waste – Burning coal produces huge amounts of coal ash waste.
  • Land use – Vast surface coal mines disrupt landscapes and habitats.
  • Wildlife impact – Air and water pollution from coal plants damage natural ecosystems.

Coal scores poorly across the board on sustainability metrics compared to other energy sources. Phasing out coal power in favor of clean renewables can bring major air, water, land, and wildlife benefits.


When all factors are weighed, a few clean energy sources rise to the top:

  • Wind and solar – Excellent for emissions and pollution, though require large land areas.
  • Geothermal – Low on all metrics though site-limited. Holds promise for growth.
  • Run-of-river hydro – Less habitat impact than big dams when done right.
  • Nuclear – Major clean power potential but constraints around water use and waste.

No energy source is perfect. But prioritizing growth in wind, solar, geothermal and sustainable hydro can put us on the path to a cleaner electric grid with minimal environmental footprint. Policies that encourage innovation and continued progress toward the most sustainable technologies can lead the transition to an eco-friendly energy future.

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