What is Renewable Energy?

Renewable energy comes from renewable sources.  That much seems self-evident.  However, I often get the questions:

  • What is renewable energy
  • What is alternative energy
  • What is green energy
  • What is sustainable energy

Let’s examine each type of energy and attempt to come up with a definition for each.  There will always be disagreement with any definition of these terms because there are numerous definitions being circulated in the world today.  My definitions might vary slightly from others you find, so I’ll try and explain why I categorize the way I do.

Renewable Energy

The key feature of renewable energy is that it comes from a source that is continuously replenished.  Think solar, wind, geothermal, ocean tides, hydroelectric and many forms of biomass.  Each of these sources of energy are automatically replaced when they are used to produce power for our consumption.  In fact, another way to look at it is that renewable sources are wasted if we don’t use them.  If we don’t use the water flowing down a river to produce electricity from hydroelectric turbines, well, a lost opportunity.  If we don’t use sunlight to produce electricity with photovoltaic cells, well, a lost opportunity.  The source of the energy acts upon the earth in one way or another whether we take advantage of it for our needs or not.

Renewable energy may or may not have an environmental impact.  Damming up a river for a hydroelectric power plant greatly changes the environment both upstream and downstream of the dam, affecting fish and wildlife.  It has beneficial effects for some creatures and detrimental effects for others.  But, bottom line, the dam has environmental impacts.  Likewise, wind generators create noise and can kill birds, so wind energy also has an environmental impact, although it is considered minimal.

So renewable energy is energy that comes from a source that is continuously replenished, and may have an environmental impact.

Alternative Energy

It is generally accepted that alternative energy includes any energy source that is used as an alternative to fossil fuel (gas, oil, and coal).  That means that nuclear energy (from enriched uranium) would fall under alternative energy.  I do not consider nuclear fission energy renewable because there is a limited source of uranium for fuel, but it is an alternative to fossil fuel sources.  In addition, the radioactive waste produced by nuclear fission reactors is continually produced and must be stored somewhere.

Someday, when we get nuclear fusion energy working commercially, it will also fall into the category of alternative energy.  Also, because of an inexhaustible fuel supply and minimal radioactive waste, it can be argued that it will be renewable.

So alternative energy falls into a broader classification of energy.  All renewable energy is alternative energy.  But alternative energy also includes other non-fossil fuel sources of energy like nuclear fission.

Green Energy

Green energy is produced from sources that are environmentally friendly.  They may or may not be renewable, but the differentiator is that they have little impact on the environment.  A large dam built to support a hydroelectric plant has a significant impact on fish and wildlife.  So hydropower requiring large dams is not considered green energy.  Small hydroelectric installations, on the other hand, might be considered green if they have minimal environmental impact.

For all practical purposes, renewable energy, excluding hydropower that requires large dams, is green energy.

Sustainable Energy

Sustainable energy comes from sources that do not run out of fuel, and have minimal impact on the environment.  Sustainable energy is viable over the long term, both for the current generation and future generations.

Energy sources that are both renewable and green are sustainable.  For the list of energy types we are considering in this article, green energy and sustainable energy are the same.


Renewable Energy includes:

  • Solar
  • Wind
  • Hydropower (includes wave and ocean tidal power)
  • Geothermal
  • Biomass (except from solid waste*)
  • Biofuels
  • Nuclear fusion (future possibility)
  • Zero Point (future possibility)

Sustainable and Green Energy includes all the above Renewable Energy, except hydropower that requires large dams with a significant environmental impact.

Alternative Energy includes the Renewable Energy list above, plus:

  • Nuclear fission
  • Biomass (from solid waste*)

*Note:  I explain why Biomass energy from solid waste is categorized differently from other biomass energy in my blog article Biomass Energy.