SOLARSPECTRA, a new physical transposition model to estimate all-sky spectral tilted solar irradiance

Accurate estimation of solar irradiance incident on tilted planes is essential for reliably characterizing photovoltaic production. Conventional transposition and sky radiance models offer fast computations of plane-of-array irradiance (or global tilted irradiance) using only broadband inputs and solar geometry, but their empirical nature restricts their spectral characteristics, adaptability, and generalization across climates and configurations. In this work, we present SOLARSPECTRA: a physical transposition model based on look-up tables (LUTs) of pre-computed sky radiances and horizontal irradiances. SOLARSPECTRA provides fast and accurate estimates of spectral plane-of-array radiation across the full solar spectrum, under all-sky conditions and for any orientation or tilt. It relies on modeled or measured inputs, including broadband global and diffuse horizontal irradiances, solar geometry, aerosol and water vapor content, and surface optical properties. The model is evaluated using 1-min measurements from two stations located in contrasting climate regions. Broadband results show performances comparable to established transposition models across ten plane-of-array configurations, with mean biases and root mean square differences (RMSD) lower than ± 1% and 6%, respectively, for non-vertical surfaces. Spectral validation, against measurements of direct irradiance and global irradiance on one horizontal and three tilted planes, demonstrates agreement within instrumental uncertainty, with mean biases within ± 1.5% and RMSD values between 11.2 and 13.8%.