<p>Flat plate solar collectors (FPSC) remain a cost-effective option for harnessing solar energy, yet their performance is strongly influenced by climatic conditions and system design. In high-altitude semi-arid regions such as Abha, Saudi Arabia, conventional FPSCs experience reduced output due to moderate irradiance and significant heat losses. To address these limitations, a modified collector was developed by integrating a layer of dark natural pebbles on the absorber plate and side-mounted polished aluminum reflectors. The experimental system was installed and monitored under real outdoor conditions, and its performance was compared against a baseline collector. Thermal efficiency was determined using first-law analysis, while supporting simulations were carried out using ANSYS Fluent to validate heat transfer trends and temperature distributions. The baseline collector exhibited the lowest efficiency, averaging ~ 0.45. The pebble-modified collector showed clear improvement, reaching an average of ~ 0.54, owing to enhanced heat absorption and thermal storage that stabilized outlet temperatures. The combined pebble–reflector system achieved the highest performance, with efficiencies between 0.55 and 0.75 and a mean of ~ 0.66. Reflectors increased incident solar flux and raised absorber temperatures, while pebbles acted as a sensible heat reservoir, extending useful operation into late hours. Uncertainty analysis confirmed the robustness of the measurements, with efficiency errors within ± 2–3%. The results demonstrate that low-cost, locally available materials can significantly enhance FPSC efficiency in challenging climates. The integration of optical concentration and thermal storage strategies provides a practical pathway toward sustainable and high-performance solar water heating.</p>