Most roofs are not perfectly tilted and aimed due south, and this calculator estimates the energy cost of that compromise. It compares your array’s tilt and azimuth against the latitude-tilt, south-facing optimum using a simplified geometric transposition model.
How it works
Two penalties combine. The tilt penalty falls off as your tilt departs from the latitude-equal optimum, and the azimuth penalty follows a cosine-shaped curve as you turn away from due south:
tiltFactor = cos(tilt − latitude) clamped to a diffuse floor
azimuthFactor = 1 − k × (1 − cos(azimuth))
yield % = tiltFactor × azimuthFactor × 100 (relative to optimum)The diffuse floor keeps even a poorly tilted array from dropping to zero, since panels always collect scattered sky light from the whole sky dome even when aimed poorly.
What “optimum tilt” actually means in practice
The latitude-equal tilt rule maximises the annual average irradiance on a fixed surface. But it is a single-orientation average over a whole year. In reality:
- A lower tilt (5–10 degrees below latitude) slightly favours summer production when irradiance is highest, and self-cleans better in rain.
- A higher tilt (5–10 degrees above latitude) slightly favours winter production and is preferred for off-grid systems that need better winter yield.
- A flat array (0 degrees) is azimuth-independent and produces around 85–90% of the optimum in most climates, at the cost of heavier soiling.
For grid-connected systems in temperate climates, the tilt is often constrained by the roof pitch anyway, and the loss from a non-optimal pitch is usually smaller than the variability in local weather.
Azimuth sensitivity by orientation
The financial impact of facing away from south varies more than most people expect:
| Azimuth deviation from south | Approximate yield vs. optimum |
|---|---|
| 0° (due south) | 100% |
| 15° east or west | ~98% |
| 30° east or west | ~94% |
| 45° east or west | ~88% |
| 90° (due east or west) | ~78–83% |
| 135° east or west | ~60–65% |
| 180° (due north) | ~50–60% |
The cosine relationship is flat near 0° and falls more steeply past 45°, which is why modest deviations are forgiving but a wall-mounted east or west array is a meaningful sacrifice. These percentages assume a climate with a reasonable proportion of diffuse light; very high-irradiance desert climates are even more sensitive to orientation.
When a non-optimal orientation is still worth it
Even at 80% of the optimum, a poorly oriented roof often still delivers a compelling return on a solar installation. The economics change more than the physics: lower yield simply extends the payback period by a proportional amount. A west-facing array also shifts generation toward the afternoon, which can improve self-consumption for households with evening-heavy loads — an orientation inefficiency that is partially offset by better load matching.
This is a screening estimate to compare orientations quickly. For a final design use an hourly simulation tool such as PVWatts or PVsyst with local meteorological data, which accounts for real diffuse fractions and shading profiles.