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A simple approach in the design of digital algorithms for active power under non-sinusoidal conditions and wide-range frequency deviations is presented. In order to accurately estimate the fundamental component of the active power, the pure sinusoidal voltage model was assumed. Adaptive finite-impulse-response (FIR) filters are used to decompose out the fundamental components of the input voltage and current signals and to minimise the noise effects. The total active power estimation was carried out by applying adaptive low-pass FIR filters for averaging of the instantaneous power. A simple algorithm for simultaneous frequency and both fundamental and total active power estimation, in a wide range of frequency changes, with benefits in a reduced complexity and computational efforts is obtained. To demonstrate the performance of the developed algorithms, the results of computer simulations and laboratory testing are presented. The algorithm shows a very high level of robustness, as well as high measurement accuracy over a wide range of frequency changes. The algorithm convergence provided fast response and adaptability, thus, this technique provides an accurate active power estimates in about one period. It has been found that the proposed algorithm really meets the needs of online applications.