Solar Radiation Basics

Solar radiation is a general term for the electromagnetic radiation emitted by the sun. We can capture and convert solar radiation into useful forms of energy, such as heat and electricity, using a variety of technologies. The technical feasibility and economical operation of these technologies at a specific location depends on the available solar radiation or solar resource.
Basic Principles

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 these factors:

* 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 0º (just above the horizon) to 90º (directly overhead). When the sun’s rays are vertical, the Earth’s surface gets all the energy possible. The more slanted the sun’s rays are, the longer they travel through the atmosphere, becoming more scattered and diffuse. Because the Earth is round, the frigid polar regions never get a high 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 and is closer to the sun during part of the year. When the sun is nearer the Earth, the Earth’s surface receives a little more solar energy. The Earth is nearer the sun when it’s summer in the southern hemisphere and winter in the northern hemisphere. However the presence of vast oceans moderates the hotter summers and colder winters one would expect to see in the southern hemisphere as a result of this difference.

The 23.5º tilt in the Earth’s axis of rotation is a more 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 six 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 like the United States, which lie in the middle latitudes, receive more solar energy in the summer not only because days are longer, but also because the sun is nearly overhead. The sun’s rays are far more slanted during the shorter days of the winter months. Cities like Denver, Colorado, (near 40º latitude) receive nearly three times more solar energy in June than they do in December.

The rotation of the Earth is 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. On a clear day, the greatest amount of solar energy reaches a solar collector around solar noon.

Source: EERE, U.S. Department of Energy

How to Evaluate Your Site’s Solar Resource for Solar Electricity

The solar resource across the U.S. is ample for solar electric systems—also known as photovoltaic (PV) systems—because they can use both direct and scattered sunlight. However, the amount of electricity generated at a particular site depends on how much of the sun’s energy reaches it. Thus, PV systems function most efficiently in the southwestern United States, which receives the greatest amount of solar energy.

Before you buy a PV system, you’ll want to be sure your site has enough solar energy to meet your electricity needs efficiently and economically. Your local system supplier can perform a solar site analysis for you or show you how to do so on your own.

When evaluating your site, you’ll also need to consider both the geographic orientation and the tilt of your solar panels—PV modules—as both can affect your system’s performance.

Source: EERE, U.S. Department of Energy

Considering a Small Solar Electric System

To help evaluate whether a small solar electric system will work for you, you should consider the following:

  • Your available solar resource—do you have clear and unobstructed access to sunlight for most or all of the day, throughout the year?
  • The system size—do you have a roof or area large enough to accommodate it?
  • The economics—is it worth the investment?
  • Local permits and covenants—are there any issues with installing a system?

Source: EERE, U.S. Department of Energy

How Small Solar Electric Systems Work

Solar electric systems, also known as photovoltaic (PV) systems, convert sunlight into electricity.

Solar cells—the basic building blocks of a PV system—consist of semiconductor materials. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms. This phenomenon is called the “photoelectric effect.” These free electrons then travel into a circuit built into the solar cell to form electrical current. To see a simulation of the photoelectric effect, please view our animation. Only sunlight of certain wavelengths will work efficiently to create electricity. PV systems can still produce electricity on cloudy days, but not as much as on a sunny day.

The basic PV or solar cell typically produces only a small amount of power. To produce more power, solar cells (about 40) can be interconnected to form panels or modules. PV modules range in output from 10 to 300 watts. If more power is needed, several modules can be installed on a building or at ground-level in a rack to form a PV array.

PV arrays can be mounted at a fixed angle facing south, or they can be mounted on a tracking device that follows the sun, allowing them to capture the most sunlight over the course of a day.

Because of their modularity, PV systems can be designed to meet any electrical requirement, no matter how large or how small. You also can connect them to an electric distribution system (grid-connected), or they can stand alone (off-grid).

Source: U.S. Department of Energy – Energy Efficiency and Renewable Energy

Small Solar Electric Systems

A small solar electric or photovoltaic (PV) system can be a reliable and pollution-free producer of electricity for your home or office. And they’re becoming more affordable all the time. Small PV systems also provide a cost-effective power supply in locations where it is expensive or impossible to send electricity through conventional power lines.

Because PV technologies use both direct and scattered sunlight to create electricity, the solar resource across the United States is ample for small solar electric systems. However, the amount of power generated by a solar system at a particular site depends on how much of the sun’s energy reaches it. Thus, PV systems, like all solar technologies, function most efficiently in the southwestern United States, which receives the greatest amount of solar energy.

Source: EERE, U.S. Department of Energy

Energy Performance Testing, Certification and Labeling

The National Fenestration Rating Council (NFRC) operates a voluntary program that tests, certifies, and labels windows, doors, and skylights based on their energy performance ratings. The NFRC label provides a reliable way to determine a window’s energy properties and to compare products.

The NFRC label can be found on all ENERGY STAR® qualified window, door, and skylight products, but ENERGY STAR bases its qualification only on U-factor and SHGC ratings.

Source: Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy

A window’s, door’s, or skylight’s ability to transmit sunlight into a home can be measured

A window’s, door’s, or skylight’s ability to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:

* Visible transmittance (VT)

A fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through a window’s, door’s, or skylight’s glazing. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home’s daylighting requirements and/or whether you need to reduce interior glare in a space.

* Light-to-solar gain (LSG)

The ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn’t always provided.


Source: Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy

Energy Performance Ratings for Windows, Doors, and Skylights

You can use the energy performance ratings of windows, doors, and skylights to tell you their potential for gaining and losing heat, as well as transmitting sunlight into your home.
Heat Gain and Loss

Windows, doors, skylights can gain and lose heat in the following ways:

* Direct conduction through the glass or glazing, frame, and/or door

* The radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls

* Air leakage through and around them.

These properties can be measured and rated according to the following energy performance characteristics:

* U-factor

The rate at which a window, door, or skylight conducts non-solar heat flow. It’s usually expressed in units of Btu/hr-ft2-ºF. For windows, skylights, and glass doors, a U-factor may refer to just the glass or glazing alone. But National Fenestration Rating Council U-factor ratings represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the window, door, or skylight.

*Solar heat gain coefficient (SHGC)

A fraction of solar radiation admitted through a window, door, or skylight—either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat gain during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gained from the sun. Therefore, what SHGC you need for a window, door, or skylight should be determined by such factors as your climate, orientation, and external shading. For more information about SHGC and windows, see passive solar window design.

*Air leakage

The rate of air infiltration around a window, door, or skylight in the presence of a specific pressure difference across it. It’s expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter than one with a high air leakage rating.


Source: Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy

Heat-Absorbing, Tinted Window Glazing or Glass

Heat-absorbing window glazing contains special tints that change the color of the glass. Tinted glass absorbs a large fraction of the incoming solar radiation through a window. This reduces the solar heat gain coefficient, visible transmittance, and glare.

Some heat, however, continues to pass through tinted windows by conduction and re-radiation. Therefore, the tint doesn’t lower a window’s U-factor. However, inner layers of clear glass or spectrally selective coatings can be applied on insulated glazing to help reduce these types of heat transfer.

Gray- and bronze-tinted windows—the most common—reduce the penetration of both light and heat into buildings in equal amounts (i.e., not spectrally selective). Blue- and green-tinted windows offer greater penetration of visible light and slightly reduced heat transfer compared with other colors of tinted glass. In hot climates, black-tinted glass should be avoided because it absorbs more light than heat.

Tinted, heat-absorbing glass reflects only a small percentage of light, so it does not have the mirror-like appearance of reflective glass.

Note: when windows transmit less than 70% of visible light, indoor plants can die or grow more slowly.

Source: Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy