A power plant controller (36) is described. The power plant controller (36) controls a power generation system (10) having one or more power generators (12) and an energy storage system (22), and provides a utility grid or transmission system operator with the capability to select the droop response provided by the power generation system. Accordingly, an operator can request a specific generator droop response in order to provide appropriate frequency and grid control services. The power plant controller (36) operates in real time determining one or more power characteristics of the power generation system (10). Based on these characteristics and an indication of a future predicted power output for the power generation system, the power plant controller can take the necessary steps to ensure that the power generation system is capable of responding with the selected droop response, or can advise the operator that a different droop is preferred.