A method and an arrangement in a forest work unit, comprising storing power produced by a primary power source in an energy storing means in at least one first phase of a working cycle of the forest work unit by controlling a hydraulic pump-motor to convert at least some of the power produced by the primary power source into hydraulic power and by supplying it to the energy storing means to be stored therein. At least one second phase of the working cycle of the forest work unit comprises supplying stored power from the energy storing means for driving at least one tool.
