Uncertainty Estimates for Global Solar Irradiance Measurements Used to Evaluate PV Device Performance

Broadband (0.3 - 3.0 μm) global solar irradiance measurements are used in the evaluation of solar energy conversion devices. The uncertainty attached to such measurements is important in evaluating whether conclusions associated with the measurements are statistically valid. A standardized uncertainty analysis method, developed over the past 15 years in the arena of consensus standards and professional society organizations, is described and applied. The results of the uncertainty analysis for the instrument calibration and field data measurement process indicate that the total measurement uncertainty in pyranometry (i.e. the measurement of global solar irradiance) can approach 5%. Thus comparisons of results between laboratories using different pyranometers can have a total uncertainty of up to 10%. Statistically valid conclusions on a conversion device's performance may be drawn only if such results account for known bias errors or exceed the uncertainty limits derived using this methodology.