Validation of combined Perez decomposition and sky diffuse models for estimating solar irradiance on vertical surfaces in tropical humid climates

This study evaluates the accuracy of estimating solar irradiance on vertical building surfaces in tropical‑humid climates using the integrated Perez decomposition (DIRINT) and Perez sky diffuse models. The study is distinguished by its focus on pure vertical irradiance, enabled by a laboratory‑developed sensor configuration that blocks ground‑reflected irradiance for both cardinal and noncardinal orientations. Two years of hourly global horizontal irradiance and vertical irradiance measurements were collected across four Indonesian sites representing equatorial, monsoonal, and submonsoonal climates. The decomposition results show that the DNI–DHI ratio is generally promising, although noticeable errors remain under humid‑tropical conditions. The Perez sky diffuse model exhibits clear orientation sensitivity, with east‑ and west‑facing planes producing higher RMSE values than north and south orientations. Its performance remained strong under uniformly overcast and very clear skies but declined under intermediate sky types. Climate‑specific differences also appear, with Pontianak’s convective equatorial climate producing the highest RMSEs and positive MBEs, Tangerang’s humid monsoonal environment yielding predominantly negative MBEs, and Tegal’s submonsoonal climate exhibiting moderate, mixed biases. The integrated Perez model achieved an overall mean RMSE of 30.6 W/m 2 across the main sites, and the additional Jembrana measurements with noncardinal orientations showed comparable accuracy.