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Roof orientation and tilt: how much does output change?

· 5 min read ·

Find out how much roof direction and tilt change solar output, with numbers for each case and a calculator to test your own roof.

Roof direction matters, but less than you might think

When in doubt about a roof, the most common question is: “does it face the wrong way?” The answer is usually less dramatic than the fear. A roof facing east or west still produces about 90% of the best case.

≈ 100%facing the equator, at the ideal tilt
≈ 90%facing east or west
≈ 68%facing away from the equator

The numbers come from a simple model for a latitude of 30° and a roof tilted at 30°. They give the order of magnitude, not a promise of output.

The ideal orientation

The sun spends more time on the equator side. So it is best to point the panels that way. Outside the tropics the rule is simple:

Southern Hemisphere

Facing north

  • The sun is to the north at noon
  • True north, not magnetic north
  • East and west lose about 10%

Northern Hemisphere

Facing south

  • The sun is to the south at noon
  • True south, not magnetic south
  • East and west lose about 10%
The rule holds outside the tropical band; near the equator orientation matters little. The percentages come from the illustrated model.

Output by deviation from the ideal orientation (30° tilt)

Ideal orientation (0°)≈ 100%
45° deviation from the ideal orientation≈ 97%
90° deviation (east or west)≈ 90%
135° deviation≈ 80%
180° deviation (opposite side)≈ 68%

The ideal tilt

The tilt that produces the most over the year is close to the site latitude, a little lower. A simple approximation is 0.76 times the absolute value of the latitude, plus about 3° (the calculator rounds to whole degrees):

0.76 × 30°latitude of 30°+3°adjustment≈26°ideal tilt

Output by tilt (latitude 30°, facing the equator)

Flat, no tilt (0°)≈ 92%
10°≈ 97%
26° (ideal)100%
45°≈ 96%
60°≈ 86%
90° (wall)≈ 51%

The curve is very flat near the ideal: missing by 10° costs about 1.2% in this model. That is why the roof’s own angle is usually good enough.

Test your roof

Choose the latitude, the tilt and the orientation deviation of your roof. The calculator shows the output relative to the best case and the ideal tilt for the site.

Interactive

Explore orientation and tilt

Choose the latitude, the tilt and the deviation from the ideal orientation. The result uses a simple model and shows how much of the best case’s annual generation is left.

100%of the best case’s generation
0%lost to orientation and tilt
26°ideal tilt for the latitude

Simplified, illustrative model for mid latitudes. For a real project, use a simulation for your site, such as PVGIS.

And when the roof is not ideal

  • East and west: generation is spread between morning and afternoon, which suits homes that use energy early and late in the day
  • Separate MPPT inputs: connect each roof slope to its own inverter input
  • Flat roof: a tilt of 10° to 15° gives almost the ideal output and helps reduce shade between rows (spacing also matters)
  • More panels: if the ideal roof has no room, adding a panel on the weaker side usually costs little
  • Shade matters more than orientation: see the article on shading

Draw it in Editor Solar

This is Editor Solar itself, already loaded with the diagram from this article. Move the equipment, edit labels, connect new parts and export to PNG, SVG or PDF. It is free and runs right in your browser.

A two-slope roof, one facing east and the other west, each on its own MPPT input of the inverter.

Try it: Adjust the number of panels on each side to your roof and write each slope’s direction on the diagram. Note the tilt of each one.

Ready-made templates: see all diagram templates

Open this diagram in the full editor ↗

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