New rules for solar system design

In the past, design choices were based on a project’s latitude. Solar engineers believed a module’s tilt had to be equal to the latitude of the location. Also, the row spacing was calculated by the shadows from the rows on the winter solstice. With advanced technology and more economical hardware and improved software tools, the design is evolving. 
 

Historical hardline of solar panel design rules 

Modules were tilted at or near the latitude of the project’s location. For instance, this means a 25° tilt in Miami and a 35° tilt in Raleigh. This orientation maximizes the sun angles annually. This points the modules as close as possible to the sun’s position in the sky.
Prior, the row spacing was calculated by a rule of not permitting inter-row shade on the winter solstice from 10 a.m. to 2 p.m. Modules tilted more heavily at higher latitudes, but since the sun is lower in the horizon in northern locations, the shadows were longer. The row spacing were very large. Such rules maintained a high energy productivity per watt, yet resulted in smaller systems on space restricted rooftops. When modules were expensive, the rules made more sense. But as prices drastically decreased, the high-tilt approach became less practical. 

About lower tilt with time-of-day spacing

With cheaper modules, a lower tilt is optimal. This squeezes more watts into a unit of area, sacrificing yield per peak-watt to optimize total energy yield. Most were installed at low tilts, but the row spacing rule of avoiding shade on the winter solstice from 10 a.m. to 2 p.m. remained. This improved alignment between the systems. While the sun angles on the winter solstice differ based on latitude, the net impact on row spacing is modest. This results  in system sizes that’s around 10% different between Miami and Seattle.
 

Maximizing row spacing

As software tools enabled system designers evaluate various combinations of row spacing and tilt, the final stage of design is developed on iteration and optimization. It’s only the most northern locations, Spokane and Boston, where a higher tilt and wider spacing are ideal. As an outcome, a rooftop in Miami versus Seattle vary in size by 14%. Ideal designs for different locations diverge based on various aspects of each site. Locations have contrasting weather patterns, temperatures and sun angles. The designs don’t alter too much. Optimized systems are trending toward looking more similar.

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