No form of energy really emits zero emissions, and that's a point that's both missed by casual advocates and overstated by strident critics.

It takes a whole lot of energy — quite often of the coal burning variety — to test, build, transport, install and dismantle all those clean sources of renewable energy. But how do the carbon emissions produced over the lifetime of a clean source of energy compare with various forms of fossil fuels over that same period?

This week, the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) released an analysis that lets users compare, contrast and better understand what a lifespan of emissions means for project permitting, environmental impacts and future policy. And at worst, it’s a great way to kill a rainy afternoon. See for yourself with the online database.

A quick look at some of the findings:

  • Greenhouse gas emissions over the life of a solar project are about 5 percent of those of a coal plant;
  • Solar and wind have about the same amount of lifetime emissions;
  • Nuclear power rates about as well as renewable energy;
  • Though natural gas generation wasn't included in the final analysis, NREL says it is generally assumed to emit about half as much greenhouse gas per kilowatt-hour as coal.

To get to these numbers, the researchers at NREL applied their modeling to more than 2,000 studies across energy technologies like bio, solar, geothermal, hydropower, ocean energy, wind, nuclear, natural gas and coal. Then they set up a database that allows viewers to compare the differences between, say, concentrated solar power trough installations and emerging ocean wave technology.

Researchers measured their findings by grams of carbon dioxide emitted per kilowatt-hour of electricity generated. For the PV projects included, the study found a median of 45 grams emitted versus a 1,001-gram median for coal. Technology specific medians for solar included 26 grams for CSP trough, 38 for CSP power towers, 20 for amorphous silicon, 14 for cadmium-telluride and 26 for copper indium gallium diselenide.

Wind energy had a harmonized range of 3 to 45 grams with a median of 11 grams. Nuclear had a very large harmonized range of 4 to 110 grams, with a median of 12.

“As a society, we need to better understand what the effects of our energy choices are. Greenhouse gases and climate change are a part of the discussion. As we try to envision what our future energy system will look like, we need an accurate picture of what that transition will mean,” said Garvin Heath, a scientist with NREL and a leader of the project. “This methodology allows you to arrive at a better precision, so you can say with more certainty that this is the benefit you get from using this technology rather than that technology,”

The findings were published in six articles and an editorial in the May special supplemental issue on Meta-Analysis of Life Cycle Assessment of the Journal of Industrial Ecology.