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How much solar energy is there?

Around 1,000 times the daily energy humans consume reaches the earth as sunlight.

Solar radiation is the energy from the sun that reaches a particular location at any time.

How is solar radiation measured?

The amount of radiation, which depends on the atmospheric conditions and the location of the sun, is called the solar resource. Radiation is measured in kilowatt-hours per square meter (kWh/m2) for PV systems, and in British thermal units per square foot (Btu/ft2) for solar thermal systems.

The atmospheric conditions determine the type of radiation. Total solar radiation – also called global radiation – is made up of diffuse and direct radiation. As sunlight travels through the atmosphere, some is scattered and reflected by:

  1. dust, pollutants, smoke from forest fires, and volcanic ash

  2. clouds: local geography and season impact cloud formation

What is the difference between direct and diffuse radiation?

This obstructed light is diffuse radiation. Radiation that reaches the Earth’s surface unobstructed is called direct beam solar radiation.


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Due to the shape of the Earth and the rotations around the sun, sunlight reaches the earth at different angles. The longer rays travel through the atmosphere, the less concentrated the radiation. The location of the sun in respect to the Earth determines how far rays must travel. Most energy reaches a point when the sun is directly above it. Radiation is also strongest during summer, when there are more hours of sunlight.

The solar resource is key to the functioning of PV and solar thermal systems. The more radiation that reaches the panels, the more electricity is produced. In the northern hemisphere, panels should face south, though other factors also determine the best solar panel orientation. A solar resource assessment of your location will determine whether using solar energy to generate electricity or thermal energy is viable.

For more information about the solar radiation in your state, check out this solar radiation map from the National Renewable Energy Laboratory.


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