Solar Photovoltaics (PV)
Solar energy comes in several forms: thermal/heat, chemical (e.g. the result of what plants do with sunlight to make food), and electrical. In nature, electrical storms are an indirect result of heating of the earth's atmosphere by the sun. In 1839 a 19 year-old French physicist, Edmond Becquerel, working in his father's laboratory, invented the first "solar cell" and discovered the photovoltaic effect, when sunlight shining on his invention produced an electrical current. Becquerel went on to discover other electrical effects and photographic processes, but his solar cell never 'saw much light' as it was impractical outside of a chemistry lab. It took Albert Einstein in 1905 to fully explain how and why it works, for which he was awarded the 1921 Nobel Prize. It wasn't until 1954 that Bell Laboratories publicly revealed the first practical solar cell, built using a silicon semiconductor and without fluids or any moving parts. So began the modern solar PV industry.
April 25, 1954: Bell Labs demos practical solar cell
Modern solar PV panels (also called modules) are generally made up of a series of silicon solar cells, soldered and laminated under tempered glass, then bound within an aluminum frame. PV technology has greatly benefited from the manufacturing advances driven by the electronics industry's need for greater purity for its silicon ingots. The electrical generation capacity (measured in watts) of a solar cell varies with its exposed area, light intensity, temperature, and the quality of the semiconductor material. Silicon solar cells are typically either mono-crystalline (a solid/single color, with energy conversion efficiency of upto 20-25%) or poly-crystalline (multiple shades from random shapes within the cell; less efficient per unit area, but also less costly). Solar cells can also be made from flexible thin-films or even 'printed' with an ink-jet, but these are even less efficient. Some super efficient (approaching 45%) solar cells, such as for spacecraft and the International Space Station, are instead made from gallium arsenic (GaAs), but are extremely expensive and not at all common. This video from the US DoE provides some basic info on how PV panels work.
Today's typical PV modules weigh about 45-50lbs each, are approx. 1.25 inches thick, by 60-66 inches by 35-40 inches, and vary in capacity from 200W to 350W (or more), as tested under "standard test conditions" (STC) by an external testing lab. UL listed manufacturers are required to include the module's rated capacity and other important electrical specifications on the label attached to every panel. Because the modules have no moving parts to break, manufacturers usually warranty the production capacity of their modules for 25 years, with only 0.5% decrease/year. Some modules installed in the 1970s are still operating today and often have better production than that.
PV modules produce direct current (DC) electricity, but most of the electricity we use, and that a utility supplies to our homes and businesses is alternating current (AC). Therefore most PV systems include one or more "inverters" to convert from DC to AC electricity; but in that process (and in the wiring & other components too), some power is lost (converted to waste heat). This, and the fact that sunshine intensity depends upon the location and orientation of the modules, and with weather and by season, means the "nameplate" (labelled) capacity of the modules can not just be added together to determine a PV system's power output (measured in kilowatts) or determine its electricity production (measured in kilowatt-hours, or kWhr). Fortunately, the National Renewable Energy Labs has a comprehensive web-based modelling tool, named PVWatts, that forecasts PV system production, using local historical weather data and other system design information we provide.