Solar Radiation Basics

What Is Solar Radiation?

Solar radiation, often called the solar resource or just sunlight, is a general term for the electromagnetic radiation emitted by the sun. Solar radiation can be captured and turned into useful forms of energy, such as heat and electricity, using a variety of technologies.

Learn More

How Does Solar Radiation Work?

Every location on Earth receives sunlight at least part of the year. The amount of solar radiation that reaches any one spot on the Earth's surface varies according to:

  • Geographic location
  • Time of day
  • Season
  • Local landscape
  • Local weather

Because the Earth is round, the sun strikes the surface at different angles, ranging from 0° (just above the horizon) to 90° (directly overhead). When the sun's rays are vertical, the Earth's surface gets the most energy possible. More slanted rays travel through the atmosphere longer, making them more scattered and diffuse. Frigid polar regions never get direct overhead sun, and, because of the tilted axis of rotation, these areas receive no sun at all during part of the year.

The Earth revolves around the sun in an elliptical orbit, which means it is closer to the sun during part of the year. When the sun is nearer to the Earth, the Earth's surface receives a little more solar energy.

The 23.5° tilt in the Earth's axis of rotation is a significant factor in determining the amount of sunlight striking the Earth at a particular location. Tilting results in longer days in the northern hemisphere from the spring (vernal) equinox to the fall (autumnal) equinox and longer days in the southern hemisphere during the other 6 months. Days and nights are both exactly 12 hours long on the equinoxes, which occur each year on or around March 23 and September 22.

Countries that lie in the middle latitudes such as the United States receive more solar energy in the summer not only because days are longer, but also because the sun is almost directly overhead. The sun's rays are far more slanted during the shorter days of the winter months. For example, Denver, Colorado, (near 40° latitude) receives nearly three times more solar energy in June than they do in December.

The rotation of the Earth is also responsible for hourly variations in sunlight. In the early morning and late afternoon, the sun is low in the sky. Its rays travel further through the atmosphere than at noon, when the sun is at its highest point. For this reason, the greatest amount of solar energy reaches the Earth around solar noon.

Types of Solar Radiation

As sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by air molecules, water vapor, clouds, dust, and pollutants. This is called diffuse solar radiation.

Sunlight that reaches the Earth's surface without being diffused is called direct beam solar radiation. Atmospheric conditions can reduce direct beam solar radiation by 10% on clear, dry days and by 100% during thick, cloudy days.

The sum of the diffuse and direct solar radiation is called global solar radiation.

Measuring Solar Radiation

Scientists measure the amount of sunlight falling on specific locations at different times of the year. They then estimate the amount of sunlight falling on regions at the same latitude with similar climates. Measurements of solar energy are typically expressed as total radiation on a surface area.

Radiation data for photovoltaic (PV) systems are often represented as kilowatt-hours per square meter (kWh/m2). Direct estimates of solar energy may also be expressed as watts per square meter (W/m2).

Radiation data for solar water heating and space heating systems are usually represented in British thermal units per square foot (Btu/ft2).