The possibilities of using curly-leaf pondweed for energy purposes were analyzed. This plant contributes to overgrowth of water reservoirs, causing their eutrophication. The plants examined were from two different water reservoirs: Lake Winiary (Gniezno) and Lake Rusalka (Poznan). On the basis of the investigations, it was determined that it is possible to use curly-leaf pondweed for energy purposes, both in the combustion method and in the biomethane fermentation process. Studies were performed to assess the suitability of the plants for combustion as a solid biofuel and studies on the use of pondweed as a fermenter feedstock. The calorimetric study showed the possibility of obtaining more energy for the curly-leaf pondweed coming from Lake Rusalka. The heat of combustion of these plants was 13.95 MJ·kg−1 (Winiary pondweed) and 9.10 MJ·kg−1 (Rusalka pondweed). On the other hand, the calorific value of these plants was 12.60 MJ·kg−1 (Winiary pondweed) and 7.80 MJ·kg−1 (Rusalka pondweed). In the case of biogas yield studies, significantly higher biogas production was observed for Lake Rusalka pondweed than for Lake Winiary pondweed. The total biogas yield for these plants was 8.05 m3·Mg−1 for Rusalka pondweed and 3.19 m3·Mg−1 for Winiary pondweed. Differences in the chemical composition of pondweed originating from different lakes were also found, which translated into differences in the amount of energy that could be obtained from plants from both stands.