The amount of energy in an ecosystem determines how much energy an ecosystem can support. The amount of energy that the producers in an ecosystem capture is called the gross primary productivity (GPP).
When plants take in sunlight, some of the energy is always lost through respiration, so, the net primary productivity (NPP) is calculated by subtracting the amount of energy respired from the gross primary productivity. Since the net primary productivity factors in respiration, it is a more accurate representation of the amount of energy available in an ecosystem.
Ecosystems are generally very unproductive. 99% of the sunlight that hits the earth is reflected, or it passes through the producers without being absorbed. Only the remaining 1% is captured by photosynthesis and becomes part of the GPP. However, 60% of the GPP is lost to respiration, so only 40% of the GPP, or 0.4% of total solar energy, supports the growth and reproduction of producers.