An estimation unit can estimate, on a real-time basis, a maximum power generation capacity of a single cycle or combined cycle gas turbine power plant. For example, the actual power output and the maximum power generation capacity can be calculated relying on a mathematical process model. Subsequently, the calculated actual power output can be compared with the measured power output yielding a model-estimation error. Based on the model-estimation error, a correction signal can be deduced, to correct the calculated maximum power generation capacity. A controller can maintain a specified power reserve. The controller can use an estimate of the maximum power generation capacity as a reference, subtract a load offset, and apply the resulting signal as upper limit of the load set-point.