To calculate solar panel wattage, multiply the panel's open-circuit voltage (VOC) by its short-circuit current (Isc). This gives the theoretical maximum wattage. For example, a panel with a VOC of 24 volts and Isc of 8 amps produces 24V x 8A = 192W. However, manufacturers rate panels at standard test conditions (STC: 1000W/sq.m, 25°C), which may differ from real-world output. Efficiency (typically 15–22%) also impacts actual power. For precise ratings, use the formula: Wattage = (VOC x Isc) x Efficiency Factor.

The calculation hinges on understanding panel specifications and environmental factors. Open-circuit voltage represents the maximum voltage when no current flows, while Isc is the current at maximum voltage. Efficiency accounts for energy loss due to heat, shading, and material defects. According to the International Electrotechnical Commission (IEC) 61215 standard, panels are tested under STC to ensure consistency. Real-world output often drops 10–25% due to temperature increases (each 1°C above 25°C reduces efficiency by ~0.3–0.5%), as noted by the National Renewable Energy Laboratory (NREL). For instance, a 300W panel at 35°C may produce ~270W. Additionally, shading can reduce output by 5–30% per cell, per the Solar Energy Industries Association (SEIA). These factors highlight the importance of accurate measurement and derating in system design.