Our petro-industrial civilization produces and consumes a seemingly diverse suite of energies: oil, coal, ethanol, hydroelectricity, gasoline, geothermal heat, hydrogen, solar power, propane, uranium, wind, wood, dung.  At the most foundational level, however, there are just two sources of energy.  Two sources provide more than 99 percent of the power for our civilization: solar and nuclear.  Every other significant energy source is a form of one of these two.  Most are forms of solar.

When we burn wood we release previously captured solar energy.  The firelight we see and the heat we feel are energies from sunlight that arrived decades ago.  That sunlight was transformed into chemical energy in the leaves of trees and used to form wood.  And when we burn that wood, we turn that chemical-bond energy back into light and heat.  Energy from wood is a form of contemporary solar energy because it embodies solar energy mostly captured years or decades ago, as distinct from fossil energy sources such as coal and oil that embody solar energy captured many millions of years ago.

Straw and other biomass are a similar story: contemporary solar energy stored as chemical-bond energy then released through oxidation in fire.  Ethanol, biodiesel, and other biofuels are also forms of contemporary solar energy (though subsidized by the fossil fuels used to create fertilizers, fuels, etc.).

Coal, natural gas, and oil products such as gasoline and diesel fuel are also, fundamentally, forms of solar energy, but not contemporary solar energy: fossil.  The energy in fossil fuels is the sun’s energy that fell on leaves and algae in ancient forests and seas.  When we burn gasoline in our cars, we are propelled to the corner store by ancient sunlight.

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