Historical design rules varied greatly by latitude
Historically, modules were tilted at or near the latitude of the project’s location. This would mean a 25° tilt in Miami, a 35° tilt in Raleigh, North Carolina and a 45° tilt in Seattle. This orientation optimizes for the sun angles throughout the year, pointing the modules as close as possible to the sun’s average position in the sky.
Then, the row spacing was typically defined based on a rule of not allowing inter-row shade on the winter solstice from 10 a.m. to 2 p.m. This created a double-whammy impact on row spacing. Not only are modules tilted more steeply at higher latitudes, but because the sun is lower in the horizon on the winter solstice in those northern locations, the shadows they cast are longer. As a result, the row spacing in northern latitudes was typically very large—in the example here, nearly 9’ in Seattle, compared to under 2’ in Miami.
These design rules are good for maintaining a high energy productivity per watt (specifically, optimizing kWh/kWp), but result in smaller systems when working on space-constrained rooftops. For example, the same rooftop that could support a 545-kWdc array in Miami would field just a 240-kWdc array in Seattle—a difference of over 55%! When modules were expensive, this was the right tradeoff to make. But as prices fell over time, the high-tilt approach made less and less sense.
Lower tilt with time-of-day spacing
With cheaper modules, a lower tilt becomes optimal: Squeeze more watts into a unit of area, sacrificing yield per peak-watt (kWh/kWp) to optimize total energy yield. Most systems were installed at relatively low tilts (typically 10 to 15°), yet the row spacing rule of avoiding shade on the winter solstice from 10 a.m. to 2 p.m. still persisted. This resulted in far greater harmonization between different systems.
While the sun angles on the winter solstice differ based on latitude, the net impact on row spacing is relatively modest (changing row spacing from ~1.2’ to ~1.6’), resulting in system sizes that are only about 10% different between Miami and Seattle.
Finally, as modern software tools enabled system designers to quickly evaluate different combinations of row spacing and tilt, the final stage of design has been built around iteration and optimization.
The more things change, the more they stay the same
Certainly, any system engineer who truly wants to squeeze out every dollar of their project should ensure that the tilt and spacing are optimized—after all, a 20-minute analysis could improve a project’s margin by 3 to 5%. But all in all, optimized systems across the country are trending toward looking more similar than different.
Tilt: We are only looking at a minimum of 5° tilt. While lower tilts are theoretically possible, they increase soiling losses because rain does not wash off the dirt and dust from the modules.
Costs: This optimization analysis is based on the default cost structure from NREL’s System Advisor Model. We’re using identical cost assumptions for each location to keep it an apples-to-apples analysis, since we’re trying to isolate the impact of irradiance and sun angles—but we certainly understand that labor, permitting and interconnection costs can vary, sometimes significantly, from state to state.
you haven’t factored in snow, first for the simple act of clearing the snow but also in locations where the mean temperature is below freezing for months at a time the depth of snow piling in the space between rows.
can helioscope generate the tilt vs row spacing matrix?