In a method and computer program product for optimal operation of a power plant and a power plant optimising system an optimisation mode (1) minimises a cost function (J[u],J[P]) that comprises a deviation of lifetime of plant components (LTp(.tau.)) from a desired nominal lifetime trajectory (LTn(.tau.)). This is done by, at a given time, determining future values of input values (u(.tau.),P(.tau.)) such as control values (P(.tau.)) or process values (u(.tau.)) to the plant and simulating, in a simulation modulate (2), the behaviour of the plant up to a given future time. Corresponding lifetime values are determined in the simulation, and incorporated in an objective function. The optimisation module (1) minimises the cost function (J[u],J[P]) by varying the input values (u(.tau.),P(.tau.)). As a result, it is possible to operate the plant such that component lifetime (LTp(.tau.)) follows the desired trajectory (LTn(.tau.)).