Firewood Energy

Hardwood trees store about 1 trillion BTU after 70 years of growth. That's pretty amazing. That's the equivalent of 3000+ gallons of gasoline. A cord of firewood is something like 128 cubic feet, and contains roughly 20 million BTU of heat.

The average american uses about 1 gallon of gasoline per day for travel, so 365 gallons per year (more or less).

If a mature forest is storing somewhere around 3000 gallons/70 years per acre, that's only around 43 gallons of gasoline energy equivalent per year, so it takes something like 8 acres of forest growth to provide the energy equivalent of the driving fuel for just one person.

Obviously, firewood isn't a very good option as a fuel for motor vehicles, but it provides a good data point that shows how solar to biomass conversion is a pretty slow process.