EIA: South orientation yields 11% more energy

The value of the higher gain from a south orientation is greatest for customers on time-of-day billing, within which some utilities penalize late afternoon and early evening usage with rates up to seven times higher than base rates. For those customers selling energy into the grid, these peak hours should yield the greatest revenue per kilowatt, although such policy is utility and public utility commission dependent and varies widely across the country.

The study also showed that a predominatly west-oriented system yields little more energy than a flat orientation, and that an east-orientation yields substantially less. The study was based on the National Renewable Energy Laboratory’s PVWatts model, and assumed a 20-degree tilt from horizontal on a hypothetical installation located in Los Angeles. Owen Comstock was identified as the principal contributor to the study.

Statistics quoted within the study from the California Solar Initiative’s (CSI) California Solar Statistics database indicated that over 40% of installations in the state are south oriented, and 30% are oriented in multiple directions.

The CSI database statistics in the study also showed that 40% of installations in California are in the 11 degree to 20 degree tilt range, while about 25% are at a 30 degree tilt. While residential and small commercial systems typically have high tilt angles, two-thirds of the fixed-tilt systems that have been installed in California this year to date are tilted between 11 and 30 degrees, the study noted. Residential tilts are typically dictated by the slope of the roof rather than racking, while typically flat commercial roof installations incorporate tilts optimized through the use of racking.

Solmetric Corp., based in Sebastopol, California, a producer of solar expert tools, shows that for the EIA study’s hypothetical Los Angeles location, a 30 degree tilt is optimal, with an orientation 10 degrees west of due south, or 190 degrees (see chart). The company cites NREL’s TMY3 National Solar Radiation Data Base (NSRDB) as its solar isolation reference. The company offers a free online tool to calculate optimal tilt and orientation for U.S. locations on its website. As a general rule of thumb, Beyond Oil Solar suggests that “the best tilt angle is equal to your latitude.”

The EIA study observed that tracker adoption is relatively strong for commercial scale or larger installations. “For systems greater than 10 kW installed in California so far this year that were eligible for support from the CSI, 30% of the capacity was installed using single-axis tracking systems and 4% either used dual-axis tracking or a mix of tracking and fixed mounts, but the majority of the capacity (66%) was on fixed mounts."

In contrast, "Virtually all (more than 99.9%) of the capacity under 10 kW installed in California this year have fixed mounting without any tracking," the EIA study concluded.