The Heliosat-4 method developed by the MINES ParisTech and the German Aerospace Center (DLR), aims at estimating surface downwelling solar irradiance (SSI). It benefits from advanced products derived from recent Earth Observation missions, among which the cloud products are crucial for the assessment of SSI. The APOLLO cloud product provided by DLR includes abundant information about the cloud physical and optical properties. The performances of Heliosat-4 when using APOLLO product are evaluated for the period of 2004-2009. The estimated SSIs are compared to measurements made at six stations within the Baseline Surface Radiation Network. Extensive analysis of the discrepancies offers an in-depth view of the performance of Heliosat-4/APOLLO, an understanding of the advantages of this combination Heliosat-4/APOLLO when compared to existing methods and the identification of restrictions in both Heliosat-4 and the APOLLO product for future improvements.