Backtracking

Backtracking Mechanics

At low sun angles (early morning, late afternoon):

1. Naive tracking aims modules toward the sun → casts shadow on next row.
2. Backtracking rotates modules slightly off-sun → no inter-row shadow.
3. Module receives less direct beam but avoids self-shading loss.
4. Net positive recovery: 3–5% annual.

This logic is what separates a modern single-axis tracker from the older single-axis designs that simply chased the sun and accepted the shading loss; the difference in behavior is easiest to see side by side in an explanation of how auto solar tracking systems work.

Sun-Angle Threshold

Backtracking activates when:

sun_elevation < arctan(module_chord / row_pitch_horizontal)

Below this angle, naive tracking would cast row-to-row shadows. The threshold is a direct function of row pitch relative to module width, which is why tighter GCR (ground coverage ratio) layouts trigger backtracking earlier in the day and recover more energy from it — closer rows shade each other sooner as the sun drops.

Implementation

• Controller computes solar position from time + GPS.
• Knows row geometry (chord length, pitch, ground slope).
• Computes optimal tilt: maximum tracking until shadow onset, then backtrack.
• Smooth transition between modes.

Before any of this is coded into a controller, the row pitch and backtracking gain are already baked into the yield model an EPC bids against — the PVsyst tracker yield study methodology walks through how that 3–5% recovery gets modeled hour-by-hour rather than applied as a flat derate.

Validation

Field commissioning should verify:

• Tracker angle vs. sun position log.
• Energy production matches simulation.
• Visual check at sunrise/sunset: no row-to-row shadows.

Discrepancies here almost always trace back to a bad row-pitch or slope input, and the same shading analysis discipline used to catch obstruction losses elsewhere on site applies to confirming backtracking behaves as designed.

Key Takeaways

• Backtracking rotates trackers off-sun at low angles to avoid inter-row self-shading.
• Recovers 3–5% annual energy on typical utility-scale tracker plants.
• All modern utility trackers include backtracking; budget trackers may not.
• Calibrated against row geometry and terrain.
• Validation during commissioning is critical.

Related Reading

For a deeper technical walkthrough of the same rotation logic described above, see the dedicated piece on how the backtracking algorithm works for solar trackers, which expands the geometry in more detail. Backtracking is only one input to the broader shading picture a design has to close before construction — the wider set of obstruction, near-shading, and horizon-shading checks a tracker plant needs is covered in our review of solar shading analysis software. And because the whole point of backtracking is to protect the yield number a project gets financed against, it’s worth understanding how that number is validated in the first place: our guide to P50/P90/P99 in solar yield reports explains how backtracking gains flow through into the lender-facing P-values PVsyst produces.