Photosynthetic granular biofilms, i.e., oxygenic photogranules (OPGs), can remove pollutants from wastewater. OPGs are composed of cyanobacteria, microalgae and heterotrophic bacteria. The biomass yield in a phototrophic bioreactor is higher than in a strictly heterotrophic system as CO2 is incorporated in freshly grown biomass. It is therefore even more important to identify uses for the excess biomass. The produced biomass can serve as substrate for biogas production. The aim of this study was to find out the relationship between biological methane potential (BMP) and OPGs culture conditions. We hypothesized that the BMP of OPG biomass varies with operating conditions. Therefore, two sequencing batch photobioreactors were operated for four months with synthetic wastewater and an HRT of 8 hours. We changed the COD load over time favoring heterotrophs (ranging from 0.27 to 1.17 gCOD/L) and the photoperiod length favoring phototrophs (ranging from 2.75 hours per cycle to constant light). We benchmarked a numeric simulation of BMPs, based on near infrared spectra (FLASH BMP) with results from a traditional BMP test. FLASH BMP allows us to obtain a BMP faster and with less initial biomass. We found that operation conditions influence the potential energy recovery from OPGs. A decrease of BMP after three days of starvation for the two independent photobioreactors, from 450 to 360 mLCH4/gVS and from 518 to 322 mLCH4/gVS was observed. We compared BMP from OPGs and from biomass of cyanobacteria isolated from OPGs. Isolates had a BMP of on average 285 mLCH4/gVS, compared to the OPG average of 410 mLCH4/gVS. The elevated BMP of photogranules is therefore caused by the specific growth conditions within photogranules and is not simply related to the presence of cyanobacterial biomass. We did not find a relation between photoperiod and BMP, neither with biomass concentration of photobioreactors. We conclude that load conditions are a lever for steering the biogas potential of OPGs biomass towards higher values.