In what form do biomass fuels store energy from sunlight?

Chemical Energy. Biomass fuels come from things that once lived: wood products, dried vegetation, crop residues, aquatic plants and even garbage. Plants used up a lot of the sun's energy to make their own food through photosysnthesis, and stored the foods in the plants in the form of chemical energy. As the plants died, the energy is trapped in the residue. This trapped energy is usually released by burning and can be converted into biomass energy.

C14 dating since carbon is present in both living and no living objects .