Power and power quality are of most importance in modern energy systems. The traditional method for power measurement and calculation uses Fourier transform. With Fourier transforms power calculations are performed in the time and frequency domains. However, it requires suitable windowing functions along with lengthy and complex (trigonometrically, exponentially. etc.) operations. In this study, a new approach which based on wavelet packet transform for power calculations of both fundamental and all harmonics of signals is proposed. The method is fast, efficient and overcomes limitations of the current techniques as mentioned above. The accuracy of the derived analytical equations is demonstrated via simulations implemented with the designed graphical user interface program and experimental works. (C) 2009 Elsevier Ltd. All rights reserved.