Solar Radiation on Tilted Surface madeagusb - Thu, 06/19/2014 - 19:24 Dear Christiana Honsberg and Stuart Bowden, First of all I really appreciate your work for very complete information about photovoltaic system in this site. After I read about "Solar Radiation on Tilted Surface", I've tried to apply the equation to get best tilted angle for optimum PV system output. In that article, its said that S module = S horizontal (sin (a + b)/sin a). From that equation, we can see the optimum S module will be got if (sin (a + b)/sin a) is maximum (Because S Horizontal is given data). Then I've began to get maximum value of (sin (a + b)/sin a). These are what I've done : Given data : Latitude : -6 (S) Longitude : 106 (E) GMT : +7 Type of Module : Fixed Daily solar radiation - horizontal : NASA data. From data above a (elevation angle - at noon) is got from day 1 to 365, then I've calculated value of "(sin (a + b)/sin a)" where data of b (tilted angle of module) are varied from 0deg - 90deg (interval 1deg). After all data calculated and summarized in one year (day 1 to 365), the maximum output of PV is got when tilted angle of module b = 17. But in the other hand, that article also said that : "For a fixed tilt angle, the maximum power over the course of a year is obtained when the tilt angle is equal to the latitude of the location". SO from that statement, the best tilted angle for latitude = -6 (S) should be b = 6 (different with what I've got from calculation, b =17). Can u explain this matter? In another references, it also said that "the maximum power over the course of a year is obtained when the tilt angle is equal to the latitude of the location", how can we prove that this statement is true and can be accounted. Hope you can reply soon because I really need your answer. Thank u for your time.