Embodiments herein determine performance parameters for a balance of plant estimator of a power plant controller configured to control a renewable power plant comprising a plurality of power generating units, wherein the balance of plant estimator is configured to provide an internal power loss estimation of the renewable power plant. The embodiments include providing at least one first pair of associated first and second power values in a first power range, providing at least one second pair of associated first and second power values in a second power range, determining a representative of the first pair, and determining a representative of the second pair in accordance with predetermined rules, and calculating a first set of performance parameters using the representatives of the first and second pairs of associated first and second power values.