Autonomous sailing robots have virtually no duration limit in operation because the main source of energy, for propulsion, is the wind. This paper focuses on the IBOAT3, a 1.8 meters long sailing robot ship developed at ISAE-SUPAERO. The onboard electronic consumption (actuators, GPS, ...) is supplied by a Li-ion battery. A 100 W photovoltaic panel mounted on the roof is used to recharge the battery. Unfortunately, shadows of the sail decrease the amount of energy given by the sun. This paper deals with the forecasting of the harvestable electric power taking account the trajectory, the wind and different factors that may have an impact on the journey across the ocean.