Understanding the long-term degradation of photovoltaic (PV) technologies under harsh climatic conditions is essential for improving reliability assessment and lifetime prediction. This study investigates the ageing behavior of three silicon-based PV technologies monocrystalline (m-Si), polycrystalline (p-Si), and amorphous silicon (aSi) using an eleven-year outdoor dataset from rooftop installations operating under identical semi-arid conditions in Marrakech, Morocco. The methodology combines continuous system monitoring, standardized current-voltage (I-V) measurements under standard test conditions (STC), visual inspection, and electroluminescence imaging to quantify degradation rates and identify technology-dependent ageing mechanisms. Crystalline silicon modules demonstrate strong long-term stability, with annual STC power degradation rates of 0.612%/year for mSi and 0.535%/year for p-Si, resulting in cumulative power losses below 7% after eleven years. Degradation is primarily associated with encapsulant ageing, persistent soiling, and localized microcracks, without extensive electrically inactive regions. In contrast, amorphous silicon modules exhibit significantly higher and more heterogeneous degradation. System-level STC measurements indicate an annual power decline of 2.40%/year and a cumulative loss of approximately 26% over the monitoring period, while module-level analysis yields an average degradation rate of 2.17 +/- 0.06%/year. Electroluminescence imaging reveals worm-like defects, internal shunting, and substantial electrically inactive areas in the most severely affected modules. Performance ratio analysis further indicates accelerated degradation of a-Si modules after approximately 38 months of operation, whereas crystalline technologies display gradual and predictable ageing trends. These findings provide robust long-term field benchmarks and confirm the strong dependence of photovoltaic durability on technology selection under semi-arid climatic conditions.