Shading is the single largest controllable yield loss in solar system design. A poorly spaced array can lose 5–15% of annual generation to inter-row shading — equivalent to discarding months of output over a 25-year project life. Getting row spacing right is one of the highest-value decisions in the design phase.
Why shading losses are disproportionate
In a string inverter system, modules in a string operate in series. When the bottom row of a module (or the bottom cell in a portrait module) is shaded by the row in front, it does not simply lose its proportional output. The shaded section becomes a high-resistance element in the series circuit, activating bypass diodes and potentially dropping the output of the entire shaded module — not just the shaded cell area. This disproportionate loss is the "string effect" of shading.
For example: a string of 20 modules where 1 module loses 50% output due to partial inter-row shading at a low sun angle may cause 15–25% string output loss — not 2.5% (1/20 × 50%). This is why row spacing optimisation matters so much more than it might initially appear.
The solar angle geometry
Inter-row shading occurs when the sun is at a low elevation angle — typically in the morning and evening, and all day in winter at mid-to-high latitudes. The critical design parameter is the minimum solar elevation angle above which the row in front casts no shadow on the modules behind.
The relationship between row pitch, module tilt, module height, and shadow is:
- Shadow length cast by a row = module height × sin(tilt) / tan(solar elevation)
- For zero shading: row pitch ≥ shadow length + module base footprint
Designing for zero shading at all times would require enormous spacing — economically impractical. The accepted industry approach is to design for no inter-row shading for at least 6–8 hours per day, with small losses at low sun angles in early morning and late afternoon (when irradiance is also low and the energy penalty is minor).
Row spacing by latitude
| Site latitude | Typical tilt angle | GCR (ground coverage ratio) | Estimated inter-row shading loss |
|---|---|---|---|
| 10–20° (tropics) | 10–15° | 0.40–0.50 | 1–3% |
| 20–35° (subtropics) | 20–25° | 0.35–0.45 | 2–4% |
| 35–45° (Mediterranean) | 25–30° | 0.30–0.40 | 3–6% |
| 45–55° (continental Europe) | 30–35° | 0.25–0.35 | 4–8% |
| 55°+ (Northern Europe) | 35–40° | 0.20–0.30 | 5–10% |
Single-axis trackers: different problem, different solution
Single-axis trackers (SAT) change the shading geometry because row pitch relative to a horizontal reference changes throughout the day. Most SAT installations use a "backtracking" algorithm: at low solar elevation angles, the tracker tilts back toward horizontal to avoid casting a shadow on the adjacent row — trading some output angle optimisation for elimination of inter-row shading. Backtracking typically recovers 2–5% of annual generation that would otherwise be lost to shading in SAT systems.
Bifacial modules: spacing affects rear-side yield
Bifacial modules generate power from both the front and rear surfaces. The rear side captures reflected irradiance from the ground — the "albedo" contribution. Higher ground coverage ratio (tighter rows) reduces the ground area visible from the rear of each module, reducing rear-side irradiance. For bifacial modules, the optimal GCR is lower than for monofacial — typically 0.25–0.35 — to allow adequate albedo contribution of 5–15% additional rear-side generation. Simulation tools (PVsyst, Bifacial_radiance) model this explicitly.
String design to minimise shading impact
Beyond row spacing, string configuration affects how shading losses propagate:
- Horizontal stringing (east-west): Each string contains modules at the same height position in every row — when the bottom row is shaded, the loss is distributed across all strings proportionally. Best for uniform shading impact.
- Vertical stringing (north-south, per row): Each string is contained within a single row — when a row is shaded, only that string is affected. Can be better or worse depending on inverter MPPT architecture.
- Half-cut modules: Split each module into two independent electrical halves, with bypass diodes protecting each half. When inter-row shading affects only the bottom half of portrait modules, only the bottom half's bypass diode activates — halving the output loss per shaded module vs full-cut cells.