Answer to Question 1
Firstly, solar power has to be cost effective. We will not switch to more environmentally friendly forms of energy unless renewable energy is cheaper. Secondly, because solar power is only available during the day, a backup form of energy is required for nighttime use. Finally, some areas of the world simply do not get enough consistent sunlight for solar power to be a viable option.
Answer to Question 2
Since humans are at the top of the biomass pyramid it can be expected that a very high percentage of Earth's primary productivity is used by humans and other top-level organisms. However, because of their place in the ecosystem and tendency to treat the ecosystem in an unsustainable fashion, humans use and preempt the Earth's primary productivity at unreasonably high rates: approximately 30 of the Earth's total production.
Solar Energy Input Varies with Latitude
Light-Gathering Power
Origin of a Solar Flare
Variations in Solar Angle with Latitude
Active solar technologies use for heat, cool, and produce electricity
Social Problems: A Critical Power-Conflict Perspective