Revised 24 May 2019

The total quantity of solar energy entering the Earth’s atmosphere is in the order of 15.3 x 108 calories/m2/year. Much of this energy is absorbed by the atmosphere, or reflected back to space by clouds. The actual quantity of solar energy available to plants ranges from 2.5 - 6.0 x 108 calories/m2/year depending upon the geographic location. However, much of this available energy is not used in the process of photosynthesis. As much as 95% to 99% of this available energy is lost from the plants in the form of sensible heat and heat of evaporation. The remaining 1% to 5% of energy is used in photosynthesis and is transformed into the chemical energy of plant tissues (phytomass). Even then, not all this energy is continuously available to herbivores as this energy represents the Gross Primary Production (GPP) of the plant.