The main objective of this thesis is to investigate gradual approaches and methods developed to evaluate the power needs for an aircraft trailing edge flap control system during early phases of the aircraft development. The aircraft configuration definitions, aircraft level requirements, system architecture, and system level requirements either do not exist or are not mature enough in early phases. However, there is a need for the estimation of power from early phases to approximately size the secondary power sources. Sizing the secondary power source is important as it has important implications for engine selection. Power estimation is also important in getting appropriate power drive unit (PDU) supplier involved early in the process. The initial method of this study involves utilizing data from various aircrafts to logically estimate power for flap system under consideration. Power is approximated by evaluating the trend of PDU power with respect to maximum take-off weight (MTOW), wing area, flap area and full flap air speed for different large commercial aircrafts. The latter method elaborates on the process of calculating aerodynamic loads considering mechanical architecture, flap kinematics, and gearing, in order to size the overall needed power. Numerical results are provided to illustrate the presented methodologies. The results of a case study evaluated by both methodologies present similar power estimations. This study demonstrates that the two approaches investigated to evaluate power in early phases of aircraft development are reasonable and can be used for early power estimation and gradual validation.