Typical Solar System Solar systems on schools in Rhode Island and elsewhere include some common elements, even though each system is designed specifically for the particular building on which it will be placed on. Systems vary in size, from small demonstration models of less than a kilowatt to very large systems of 50 kilowatts, depending upon the needs of a school and its budget. Most of the schools in Rhode Island have 2-kilowatt systems that are connected to the utility grid. Here, we describe a typical solar electricity system for a school, using one of the 2-kilowatt systems in Rhode Island as an example. For definitions of highlighted terms below, see the Glossary of Solar Terms. What's On the Roof? On the Rhode Island schools, twenty 100-watt photovoltaic modules manufactured by the AstroPower company have been used. Each module weighs 26 pounds and measures 26" wide and 58" high. Each module consists of many, connected photovoltaic cells covered by a sheet of glass and placed in a black heavy-duty aluminum frame. The entire solar array will cover 240 square feet. Mounting the Modules Depending upon the type of building each Rhode Island school is and its orientation to the sun, the solar modules are mounted on it in one of four ways. A two-kilowatt system produced by AstroPower and installed by Solar Works Inc. on a slanted roof on the Cabot School in Vermont. For sloped roofs at a 30° to 45° angle, aluminum mounting rails are used. The modules are affixed to the rails, which are then bolted to the roof. All penetrations of the roof by the bolts are covered with a roofing sealant to ensure that the roof remains watertight. A two-kilowatt system produced by AstroPower and installed by Solar Works Inc. using wooden beams on the roof of Northampton High School in Massachusetts. For some flat roofs, a series of pressure-treated wooden beams that lay flush against the roof are used. The beams are bolted into supporting rafters. A series of aluminum legs are used to raise the modules upward. These legs are themselves bolted to the beams. A one-kilowatt ballast tray system produced by AstroPower and installed by Solar Works Inc. on Goffstown High School in New Hampshire. A ballast tray system can also be used for flat roofs. Aluminum trays are placed flat on the roof, and weighed down with concrete blocks, rocks, or other suitable ballast. The modules are raised to a 30 to 40 degree angle by using aluminum legs. A one-kilowatt system produced by AstroPower and installed by Solar Works Inc. on a vertical wall on Monadnock Regional High School in New Hampshire. Solar modules can be attached to a vertical wall, as an awning. In this case, the modules may provide the extra benefit of providing some shading to windows. No matter which mounting method is used, the PV modules are mounted as close as possible to 15 degree west of true south. Getting Electricity from Photovoltaic Cells The photovoltaic cells convert solar radiation into electricity. When the sun is hitting them, they produce a stream of direct current (DC) electricity. This electricity then needs to be converted to alternating current (AC) because the appliances, computers, and lights in the school require alternating current. Wires from the photovoltaic modules are fed into an inverter, which is device used to make the conversion from DC to AC electricity. In some of the Rhode Island schools, two 1000-watt inverters are used. Each one weighs 43 pounds and stands 28.5" high, 6.5" wide and 8" deep. The schools' electrical distribution panel receives electricity from the photovoltaic system, that has gone through the inverter, and combines it with other electricity supplied through the utility company's power lines from the electricity supply company. A data monitor in each school allows students to monitor the daily and cumulative production of electricity from the system. The data monitor provides performance information, user control, and diagnostics the inverters. The information gathered by the monitor is displayed in real time on a built-in 4-line by 20-character LCD display. A special computer program allows the data from the inverter to also be downloaded on to a IBM-compatible 486 computer. Networking this computer can then allow the data to be loaded onto the school's server and be made available to any classroom in real-time. The data is also placed on our website.