The net primary productivity of an ecosystem can be viewed as the

Primary productivity is the process resulting from the photosynthesis activity of plants and it also determines accumulation of biomass in a forests. Net primary productivity (NPP) is a gross primary productivity minus the rate of energy loss by maintenance and metabolism. In other words, NPP is the rate at which energy is stored as biomass by plants and made it available to all consumers present in the ecosystem. 

Hence, the net primary productivity of an ecosystem is viewed as the amount of energy available to the consumers.