Power is the rate at which work is done (work/time). It is also the rate at which energy is generated or used. For DC (direct current) systems, power is expressed in watts: P=VdcAdc. For AC (alternating current) systems, the determination of power is more complex. The voltage and current in an AC circuit periodically change direction. In a purely resistive circuit, the voltage and current change direction at the same time (in phase). Power measurements are made by measuring the RMS current and voltage and applying the formula P=VrmsArms.
The Power Measurement Application Note (pdf) discusses crest factor, bandwidth, power factor, single phase circuits, polyphase circuits, energy measurements and power formulas.
The Power Quality Primer (pdf) is an introduction to non-sinusoidal waveforms, harmonics, harmonic distortion, TRMS and power quality measurements.
Power & Energy Monitoring in the Era of Green Energy (pdf) provides an overview of energy trends and measurement equipment. This presentation was originally given at the 2009 IRIS Conference.
See an overview of Power & Energy Monitoring Products.