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Photovoltaic panels are devices that convert the light from the sun into electrical energy. Irradiance is the measure of the sun's power at the surface of the earth and averages 1000 watts per square meter of ground. Insolation is the amount of energy available at a specific location and is measured in "full sun hours".
The amount of electrical energy produced by the cell compared to the amount that is available is called the solar cell efficiency.
The typical cell captures 14-16% of the available energy or 140-160 watts of power for each square meter of panel area.
We are probably most familiar with the solar cells that are used to power calculators but the technology was originally developed for the space program as a way of supplying power to a space station.
When light strikes a solar cell, some of the photons knock electrons free from their orbit around the atoms of the material. These free electrons (electricity) are then directed to devices to perform work for us. The typical cell produces about .5 vdc while the current output is directly proportional to the surface area of the cell.
Individual solar cells are normally electrically connected to each other to form panels called photovoltaic modules. There are basically three types of modules available to the homeowner. The most common are the typical roof mounted modules or ground mounted modules. For situations where you do not want to see the panels, you can get building integrated photovoltaic panels (BIPV). These modules are designed to take the place of traditional roofing materials and blend into the roof.
These assemblies are then combined to form photovoltaic arrays which combine all of the system's electrical energy to provide useful power for your home.
This DC power can be used to charge batteries or it can be converted to alternating current (AC power) through a device called an inverter. This AC power is then used to supply the electricity to the appliances in the home.
Systems that use batteries are commonly called off-grid systems because they can store energy in the batteries for the times when the sun is not shining. Other systems use batteries as a backup in case the utility company cannot provide power. The most common type of system does not use batteries and is called a grid tie system. **NOTE** A grid tied system, without battery backup, must shut down when power from the utility company goes out. If you live in an area where power outages are common, you would want a system with battery backup.
To find the proper size of photovoltaic panels for your home, you must know your normal energy usage. (The normal U.S. home uses approximately 13000 kwh/year or 5 kwp.) This is usually found by looking at your utility bills for a month or a year. (It is best to use a full year.) This will allow you to calculate your daily energy usage. Then you need to know the amount of power from the sun that is available at your location. This is typically found using special maps that give you the equivalent full sun hours.
Most of the system manufacturer's provide design services and they offer systems as complete packages with different electrical outputs. A simple site survey is performed to ensure that you have a suitable location (which most people do) and a package is selected based on your home's needs and location.
There are many brands and system options available but to cut through all of the sales hype you can use our residential solar system ratings. You can see how the systems stack up through a side by side comparison of the photovoltaic panels and other system components.