<p>This work proposes a methodology to extend the range of marine unmanned surface vehicles (USV) for border surveillance missions. The typical small scale of USVs and their lack of in-board pilots make USVs an important tool for remote applications, such as border surveillance missions and for dangerous areas operations. However, also due to their small scale, their mission range is typically limited. In this paper, it is proposed a combination of a smart energy management system (SEMS) with electric propulsion and photovoltaic panels to find the optimal path and speed capable of extending the mission range. The developed SEMS is capable of planning a mission profile based on the predicted environmental conditions. To achieve this, one presents a new A-star algorithm with probabilistic behaviour to avoid local minimums and find alternative paths that would reduce the energy consumption in later hours of the mission. The developed system was included in a USV prototype and tested under real environmental conditions at the interface between the Tejo River and the Atlantic Sea, in Lisbon, Portugal. Experimental results showed that the inclusion of photovoltaic panels and the SEMS allowed for planning the mission including the time-variable environmental conditions, leading to an extension of up to 50% of the mission range.</p>