How to Calculate RMS Watts

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Alternating current (AC) rises and falls following a sinusoidal rhythm, making it difficult to describe average values associated with AC electrical circuits. For AC circuit analysis, the concept of average is replaced by effective or equivalent values through the use of a mathematical procedure, the root mean square (RMS). RMS calculations compensate for the sinusoidal variations in AC electrical measurements and indicate what the direct current (DC) equivalent measurement would be. RMS wattage, which describes the power associated with an AC circuit, depends on RMS current and voltage, measured with a multimeter or oscilloscope.

    Use a digital multimeter in AC-enabled ammeter mode to read the RMS current value directly from the electrical circuit. Alternatively, determine the RMS current value by dividing the peak value of the alternating current by the square root of 2. The peak value is the maximum value or the amplitude of the sinusoidal current function, where the function describes how the current changes with time. AC current functions can be visualized as waveforms using an oscilloscope fitted with an AC current probe. Peak current is obtained by examining the pattern on the oscilloscope monitor and finding the value at which the curve reaches its maximum value.

    Use a digital multimeter in AC-enabled voltage mode to read the RMS voltage value directly from the electrical circuit. Alternatively, determine the RMS voltage value by dividing the peak AC voltage value by the square root of 2. The peak value is the maximum value of the sinusoidal voltage function. AC voltage functions can be visualized as waveforms using an oscilloscope. No special attachments are required for taking voltage measurements. Peak voltage may be read directly from the waveform on the oscilloscope monitor.

    Calculate the RMS wattage or the direct current (DC) equivalent of the AC power by multiplying the RMS voltage and the RMS current. The unit for the product is the watt.

    Tips

    • For greater accuracy, use an oscilloscope fitted with an AC current probe instead of a multimeter when current measurements are small. The oscilloscope should also be used in cases where the circuit cannot be opened to insert a multimeter for the purpose of taking RMS current readings.

References

About the Author

Pearl Lewis has authored scientific papers for journals such as "Physica Status Solidi," "Materials Science and Engineering" and "Thin Solid Films" since 1994. She also writes an education blog entitled Simple Science in Everyday Life. She holds a doctorate from University of Port Elizabeth.

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