Field Estimation of the Power Temperature Coefficient of a Liquid-Cooled Photovoltaic–Thermal Collector: Sensitivity to Irradiance Normalisation and Model Specification

The power temperature coefficient (γ) of a photovoltaic–thermal (PVT) collector is usually taken from the datasheet. Determining it outdoors is difficult because irradiance and temperature vary together. This study evaluates how reliably γ can be estimated from a few field observations. Thirty observations were collected during three campaigns in Bacău, Romania, in 2024 on a 240 Wp liquid-cooled PVT collector operated at a fixed pump setting. Power normalised to 1000 W/m2 was regressed on front-surface temperature with campaign-specific intercepts. This conventional analysis yielded γ = −0.64%/°C (95% CI: −0.82 to −0.46), compared with the declared −0.45%/°C. The result was sensitive to model specification. Irradiance and panel temperature were strongly correlated (r = 0.84), and including irradiance as a covariate, which the data supported (p = 0.043), changed γ to −0.40%/°C (−0.72 to −0.09). Eleven specifications differing in covariates, first-observation exclusions and temperature variable produced estimates from −0.33 to −1.08%/°C. In one campaign, measured power averaged 19% above the nameplate-based expectation, indicating an inconsistency in the absolute measurement scale. In an hourly PVGIS-SARAH3 calculation for 2023, each 0.1%/°C change in γ altered annual electrical yield by 0.94%. The measurements are compatible with the datasheet coefficient but constrain it only to between about ±0.3 and ±0.7%/°C, depending on the specification.