Energy-neutral real-time systems harvest the entire energy they use from their environment. In such systems, energy must be treated as an equally important resource as time, which creates the need to solve a number of problems that so far have not been addressed by traditional real-time systems. In particular, this includes the scheduling of tasks with both time and energy constraints, the monitoring of energy budgets, as well as the survival of blackout periods during which not enough energy is available to keep the system fully operational. In this article, we address these issues presenting E n OS, an operating-system kernel for energy-neutral real-time systems. E n OS considers mixed time criticality levels for different energy criticality modes, which enables a decoupling of time and energy constraints when one is considered less critical than the other. When switching the energy criticality mode, the system also changes the set of executed tasks and is therefore able to dynamically adapt its energy consumption depending on external conditions. By keeping track of the energy budget available, E n OS ensures that in case of a blackout the system state is safely stored to persistent memory, allowing operations to resume at a later point when enough energy is harvested again.