Notes the two kinds of problems associated with power measurements - selecting an instrument best suited to precisely measure electrical quantities under a particular type of distortion, and interpreting the results correctly. Presents the principle of operation of modern solid-state power analysers, their parameters and technical limitations. Describes an experimental investigation of the accuracy of three selected meters in adjustable speed drive (ASD) efficiency measurements. Discusses the pitfalls of the fast Fourier transform method used for harmonic analysis. Regarding theoretical interpretation of the measurements, reviews power definitions under non-sinusoidal conditions. Puts particular stress on reactive power, compensation of which increases equipment efficiency. Presents several definitions of reactive power and discusses and illustrates them by an ASD waveform example. Presents conclusions and suggestions for further research.