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A given energy source doesn't simply pollute—or not—as it's converted into electricity. There's also mining, plant construction, transportation, and other factors to consider. Looking at the big picture is the point of life-cycle numbers like these, expressed in grams of carbon dioxide equivalent per kilowatt-hour of electricity created.
Lifetime carbon emissions by source (g/kwh)
Natural gas: 466
Solar: 17 to 39
Nuclear: 16 to 55
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