The invasive Asian shore crab Hemigrapsus sanguineus is now the second most abundant intertidal crab in the North Sea after the native European green crab Carcinus maenas. To compare their respiratory performance and energy expenditures, we measured standard respiration rates of both species from around the island of Helgoland, North Sea, Germany (54°11′N, 7°53′E) in 2015. Oxygen consumption was recorded in a flow-through setup between 5 and 20 °C. At lower temperatures, H. sanguineus had similar respiration rates as C. maenas, but approximately twofold higher rates at higher temperatures. Numerical models for the calculation of individual respiration rates were established and applied to the entire intertidal populations around Helgoland to compare the energy expenditures of both species. Abundance and biomass data recorded in August 2014 showed that H. sanguineus reached values as high as 21 and 59%, respectively, compared to those of C. maenas. Based on these data and the numerical respiration models, the energy expenditures of both populations were calculated for the whole year 2014. The H. sanguineus population reached, depending on the assumed diet of both species (complete herbivory versus complete carnivory), 86–135% of the expenditure value of C. maenas. As population densities of H. sanguineus are likely to increase in the North Sea, the energy expenditure of the invader and thus its impact on the energy flux in the intertidal habitat will further increase. Whether this will lead to an impact on the local intertidal community depends on species-specific dietary preferences and remains to be investigated.