Roof Constraints and Solar Payback: Shade, Orientation, and Age

A south-facing unshaded roof is not the only roof that works for solar. But shade, orientation, and structural condition each hit production in ways that make conservative quotes matter more.

The three roof factors that separate a great solar site from a marginal one ??and why a site-specific shade analysis beats an average production estimate every time.

Key takeaways

A system sized for full sun on a shaded roof is not a good deal. Ask specifically how much of the roof area is shaded between 9am and 3pm in summer months, and whether the design uses panel-level electronics to handle it.

Ask the installer: what shade analysis tool did you use?

Ask: did the analysis include winter sun angles, or only summer peak production?

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Roof orientation: south is best, east and west are viable

A south-facing roof at 30Â° tilt produces the most annual energy at most US latitudes. PVWatts models this as the reference case. East and west-facing roofs produce 10??0% less annually, but they spread production more evenly across morning and afternoon hours rather than concentrating it at solar noon.

In utilities with time-of-use pricing, a west-facing array can actually be more valuable per kWh than a south-facing one: it generates in late afternoon when peak TOU rates apply. If you are on a flat rate plan, south is still optimal.

Shading: the biggest production variable

Shade from trees, chimneys, dormers, or neighboring structures affects solar production in two ways. First, shaded panels produce less power directly. Second, with string inverter systems, a shaded panel drags down the output of every panel connected to the same string ??sometimes reducing whole-string output by 50??0% during the shade event.

Microinverters and power optimizers (panel-level electronics) largely eliminate the string effect: each panel operates independently, so a shaded panel affects only its own output.

How shade analysis should be done

Professional shade analysis uses either a device like the Solmetric SunEye (physically measures shade on the roof) or shading software like Aurora or Helioscope, which models shade from satellite and LiDAR data.

Satellite-based shade models are faster and less expensive, but can miss seasonal shade from deciduous trees, new construction, or obstructions that do not appear clearly in imagery. An in-person roof assessment is more accurate for shade-sensitive sites.

Roof age and structural condition

Solar panels are warrantied for 25 years. If your roof is 15+ years old and made of asphalt shingles with an expected lifespan of 20??5 years, the roof may need replacement while panels are installed ??triggering a $5,000??15,000 panel removal and reinstallation cost.

Reroofing after solar installation is not impossible, but it adds cost and complexity. Most solar installers recommend replacing an aging roof before installation, particularly if the roof is within 7 years of expected end-of-life.

When roof constraints make solar not viable

North-facing roofs in the northern US (above 35Â° latitude) are rarely economically viable ??production is too low. Heavy year-round shading from mature trees or nearby buildings can reduce production enough that payback periods exceed 20 years on a realistic basis.

Flat commercial-style roofs on residential buildings are workable with tilted racking, but add cost and sometimes require structural assessment. Very complex roofs with many small planes are harder to design efficiently.