Fifty years after the Apollo programme, humanity prepares to return to the surface of the Moon with greater ambition to stay longer, explore wider and eventually use the Moon as a staging post to further destinations. The short Apollo missions could rely on primary batteries to power the lunar module, but longer robotic and crewed missions will require other power system solutions. The plans of ESA to enable European roles in the frame of an international human lunar exploration campaign are described, along with the power and energy needs of the foreseen mission concepts. The performance characteristics of solar generators, secondary batteries and nuclear power sources presented in a way specifically tailored for lunar surface missions. It is shown that the characteristics of a long-term lunar surface mission lead to an optimum power system architecture comprising a radioisotope power system as well as a solar generator and secondary battery. New European radioisotope power technology based on americium-241 is advancing in maturity and will complement the established U.S. and Russian capability in plutonium-238 systems, thus enhancing access to these mission-enabling but scarce power sources. The ESA-led international collaborative lunar mission HERACLES will be the first lunar surface mission that is required to survive and operate over multiple lunar days and nights. This achievement will be enabled by the first use of European space nuclear power sources, in the form of Am-241 radioisotope heater units (RHUs).