A salesperson is building a preliminary production estimate during a home visit in the Northern Hemisphere. The array will fit on only one plane: the 6-year-old asphalt shingle roof has a 6:12 south face shaded only over a small area by a chimney from 4 to 6 p.m., while the detached garage offers an unshaded 5:12 east face. Which roof plane should anchor the preliminary production estimate for the array?

The south roof plane should anchor the estimate because, in the Northern Hemisphere, a south-facing plane generally receives stronger annual solar exposure than an east-facing plane, and the array is constrained to a single plane. The chimney's limited late-afternoon shade should be represented in the production model rather than ignored; its small affected area and restricted hours do not, by themselves, make the unshaded east plane the better anchor. A standard monthly shade-loss factor is a common screening method, but it fails here because a generic factor does not specifically represent the chimney's stated time-of-day shading. Selecting the east garage plane to avoid shade is also a defensible design choice in some cases, but it misses the south plane's more favorable orientation for this preliminary estimate. Applying a fixed annual shade-loss percentage is another convenient early-stage shortcut, yet it does not derive the loss from the chimney's limited area and 4-to-6 p.m. exposure, so it can misstate production. Exam caveat: orientation alone does not guarantee greater output; local irradiance, roof geometry, and quantified shade can change the comparison. Practical check: model both planes with site-specific weather and a shade survey, then compare annual production using the proposed array layout.