to spend a hundred thousand dollars to gain a two-hour advantage, and equally easy to lose two hours due to reliability issues. Solar array The solar array consists of hundreds (or thousands) of photovoltaic solar cells converting sunlight into electricity. Cars can use a variety of solar cell technologies; most often polycrystalline silicon, mono-crystalline silicon, or gallium arsenide. The cells are wired together into strings while strings are often wired together to form a panel. Panels normally have voltages close to the nominal battery voltage. The main aim is to get as much cell area in as small a space as possible. Designers encapsulate the cells to protect them from the weather and breakage. Designing a solar array is more than just stringing a bunch of cells together. A solar array acts like many very small batteries all hooked together in series. The total voltage produced is the sum of all cell voltages. The problem is that if a single cell is in shadow it acts like a diode, blocking the current for the entire string of cells. To design against this, array designers use by-pass diodes in parallel with smaller segments of the string of cells, allowing current around the non-functioning . Another consideration is that the battery itself can force current backward through the array unless there are blocking diodes put at the end of each panel. The power produced by the solar array depends on the weather conditions, the position of the sun and the capacity of the array. At noon on a bright day, a good array can produce over 2 kilowatts (2.6 hp). A 6 m2 array of 20% cells will produce roughly 6 kW·h (22 kJ) of energy during a typical day on the WSC. Some cars have employed free-standing or integrated sails to harness wind energy. Races including the WSC and ASC, consider wind energy to be solar energy, so their race regulations allow this practice. Aerodynamics Aerodynamic drag is the main source of losses on a solar race car. The aerodynamic drag of a vehicle is the product of the frontal area and its Cd. For most solar cars the frontal area is 0.75 to 1.3 m2. While Cd as low as 0.10 have been reported, 0.13 is more typical. This needs a great deal of attention to detail. Mass The vehicle's mass is also a significant factor. A light vehicle generates