Is My Roof Right?

Determining whether solar makes sense…

Orientation and Tilt

Let’s look at a more typical situation: homes that are built on flat land. I’ve moved across my map to a nearby neighborhood that’s flat.

Wow — Nearly every roof seems perfect for solar — no hillside, no trees, and very few penetrations on the roof! The biggest decision for most of these homes is whether to put to solar on the front roof or back roof. You’ve probably heard that south is the best orientation – and that’s true. Since Google orients its satellite images with south at the bottom, we can see that the front-roof / back roof decision is actually a facing-east / facing west decision.

How bad is this? Does the wrong orientation make solar a poor investment? Let’s see.


Flat
18°
4:12
30°
7:12
45°
12:12
60°
21:12
90°
Vertical
0° (South) 0.89 0.97 1.00 0.97 0.89 0.58
23° (SSE, SSW) 0.89 0.97 0.99 0.96 0.88 0.59
45° (SE, SW) 0.89 0.95 0.96 0.93 0.85 0.60
68° (SE, WSW) 0.89 0.92 0.91 0.87 0.79 0.57
90° (E, W) 0.89 0.88 0.84 0.78 0.70 0.52

Source: A Guide to Photovoltaic (PV) System Design and Installation California Energy Commision.

Lots of information here, so let’s chip away at it:

The columns are percentages for common roof slopes. The rows are different orientations.

The first column is for flat roofs. Flat roofs are common on commercial properties, but a fair number of California residences have them too. Note that the percent efficiency for a flat roof isn’t impacted by orientation, since, by definition, a flat roof has no orientation.

The next column is for 18° — sounds like a random number, but roofs are typically measured in number of inches vertical for every 12 inches horizontal, and asphalt shingle roofs must, by code, be sloped at a minimum 4:12, so 18° is very common.

In the satellite image on the last page, these look like asphalt shingle roofs, so they’re likely 18°. But before sweating whether a particular roof is 18° or 30°, look at how closely the percentages are grouped. If the roof is pointed perfectly south, 3% of potential is lost if the slope is 18° or 45°, rather than the optimal 30°. But as the orientation changes from perfectly south, the impact of a non-ideal slope actually decreases. For example, for a roof that is southeast or southwest, the loss by having an 18°roof rather than a 30° roof is only 1%.

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