Transposition

Using solar geometry, a photovoltaic simulation tool can calculate the amount of irradiance received by a module when it is tilted rather than laid flat. The tilt and orientation of the module toward the equator generally increase the irradiance received on its surface, as the module aligns more closely with the Sun’s average path across the sky. The optimal tilt depends on latitude: sites at high latitudes benefit from steeper tilts, while regions near the equator require only a moderate tilt.

To quantify the gain—or loss—in irradiance associated with changes in tilt and orientation, the transposition factor is used, which expresses the relationship between the irradiance received on a horizontal plane and that received on the module’s inclined plane. Optimizing this factor is a key step in the design of photovoltaic systems.