This article explains the fundamental terms related to sunlight and residential solar energy systems. You will learn about units of measure, forms of solar radiation, spectral distribution, energy distribution, and solar position.

Germany and Japan are often cloudy or foggy but is where most solar panels now sold in the United States are produced. Because of the way solar modules produce power from direct, diffused and reflected light it works great in these conditions. A very high performing solar array can be found on a foggy beach in California.

Measuring Light and How Solar Panels Use It

The following terms related to different aspects of light and their characteristics as they come in contact with different physical formations on Earth. The definitions below will help explain how some residential solar energy systems seemingly in areas with poor sun quality can actually achieve respectable power output.

Irradiance - The rate of solar radiation falling on a given area at a moment in time. Irradiance is measured in units of kW/m2 (read kilowatts per square meter).

Irradiation - The amount of solar energy over time. Irradiation is measured in units of kWh/m2/day and read kilowatt-hours per square meter per day. Different locations through the United States (and the World) have different levels of irradiation. As in previous articles, you can find many maps on the Internet that will show the average annual solar irradiation throughout the United States.

Light Characteristics and Solar Fundamentals

Deflection - The amount of light lost when the solar panels aren't facing the sun squarely as the sun moves across the sky. This loss of light is called deflection. Even though solar cells are etched on the surface into little pyramids to collect light from all angles, coated with a light diffusing coating, and protected with light diffusing glass, more light is deflected when the sun is not directly in front of the panels.

Spectral distribution - The bulk of the solar light spectrum which reaches the Earth's surface is ideally suited for conversion by solar (PV) cells into electrical energy.

The Sun's Position as It Relates to Solar Energy Systems

Two coordinates describe the position of the sun at any given time. These coordinates allow solar technicians to calculate and orientate the solar panels to optimal attract and convert the sun's light into solar generated electricity.

Azimuth - Describes the direction from east to west in degrees (°). North is 0 degrees. East is 90 degrees. South is 180 degrees. West is 270 degrees. This orientation is similar to what can be seen when looking at a compass with all of the degrees marked on its face.

Altitude - Measures how high the Sun is from horizon to zenith. Altitude is measured in degrees as well. Altitude is essentially the height the sun is in the sky. Just like a plane has an altitude that it travels at across the sky as it is measured above the horizon. So does the sun.

Irradiation Example for a Solar Energy System

In the coast area of California, residential solar energy systems are sized using an average of 5.5 sun hours per day. More often than not, most people think or comment that their house gets more like 8 to 15 hours of sunlight per day. This is true but sunlight is different than solar sun hours per day.

The sun hour numbers take into account things like fog, rain, night and, most importantly, deflection. The sun hour ratings listed for different places throughout the United States is the amount of solar irradiation available to be converted into electricity by a fixed mounted solar array facing true south at the optimal tilt angle. A solar module produces its full rated power only when in direct sunlight, so when the sun is to the East or the West of the module it is not at full production.

reThinking the Economics of Residential Solar Energy

Doing your part to choose green, solar power can be a bit confusing and complex at times. There are many terms, aspects of the installation, and details about your sun light hours per day that all influence decisions to upgrade your home to solar energy. The great thing emerging in today's marketplace for solar energy systems are the new solar panel rental services.

The economics of residential solar power have changed. Every American homeowner can decide to upgrade to solar energy and avoid large purchase of a solar energy system. New rental programs are now available that give homeowners the chance to adopt residential solar energy without having to make a huge investment.