Is My Roof Right?

Determining whether solar makes sense…

The Sun is a Moving Target (More about Tilt)

Actually the sun is not a moving target. It’s the earth that is spinning and wobbling, but for this discussion, it’s fine to think of the sun as what’s moving. Since your objective is to have your panels facing the sun, and the sun never stays still, it’s useful to revisit high school science.

In the morning the sun is to the east, and in the afternoon the sun is to the west. And in the summer the sun’s path is high in the sky, and in the winter, low. What this means is that whatever tilt your panels are at, they’re only going to be perfect for, at most, two days a year.

The two “average sun-paths” are the equiluxes — the points where days and nights are equal in length. By tilting a panel to optimize this average sun path, the panel would still perform well when the sun was at its highest, and at its lowest. This optimal tilt is easy to determine: it’s your latitude! Sonoma and Marin are latitude 38, so that’s the optimal tilt to leverage both summer and winter sun.

But here in PG&E territory, there are three reasons that panels are typically tilted to favor the summer sun:

  1. There are more hours of sun to harvest during the summer.
  2. The weather is clearer (except perhaps on the coast!)
  3. PG&E needs energy during hot summer days, and thus designs its rebate to favor summer production.

(As an aside: If your home was “off-the-grid”, that is, not connected to PG&E, you might point your panels to maximize winter production, so that your battery bank could get you through those long winter nights.)

The optimal tilt for summer production in the North Bay is about 14°. A typical asphalt shingle roof is 18°. And as we’ve seen in the chart above, small changes in tilt don’t make that much of a difference. This is why you should never expect to see tilt-racks on any roof that’s already sloped.

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