This paper addresses scheduling and power management issues in energy harvesting embedded systems. We focus on a fixed priority based operating system such as one found in a wireless sensor node which is powered by a solar cell. We show that a classical fixed priority scheduling solution has to be reconceived. This is because new problems arise when perpetual operation is required with no deadline missing and no energy starvation being authorized. We provide a method to assign processing time and energy to tasks in a short-term perspective. The concepts of slack energy and slack time defined for dynamic priority systems are revisited for fixed priority systems.