How to Measure input AC power through a coil using formulas If the voltage and current is known?

Thread Starter


Joined Feb 14, 2021
I am using LTSPICE to couple toghter two RLC circuits for frequency analysis. I need to measure the power flowing through the primary side and induced powered in the secondary. I am able to measure the voltage and current at different frequencies and need a formula to find out the power. This is my first dive into AC circuits and am a bit confused regarding RMS power.

Thank you for reading!.


Joined Mar 14, 2008
The power can be calculated when doing a transient analysis.
For example, the V2 supply average Power (RMS) is the average of the instantaneous voltage across a device times its current at the same point in time.

In LTspice plot the voltage across the component.
Right click on the plot name to open up the plot expression editor and change to V(part) * I(part) (first attachment below).
Then Ctrl-Left Click that plot to get the power average value (second attachment below).

The power dissipated in a resistance can also be calculated by measuring the RMS voltage across it, squaring that voltage, and then dividing that by the resistance.

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