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I'm trying to measure the power consumption of a circuit which utilizes a buck converter. If I know the circuit in absence of the converter utilizes 5.125V @ 142ma, then I introduce a 12V battery source with a buck converter to drop the voltage to 5.125V, should I see ~0.061ma going into the converter (assuming 100% efficiency)? In other words, is it right to assume conservation (if 100% efficient)? If the converter is not 100% efficient, does that mean I should see a higher current flowing into the converter, where the difference between that measured and the theoretical conservation is the power loss across the converter?

Overall, I'm trying to understand where I should be measuring the power consumption of my system. I have 2 devices connected to a battery in parallel (i.e. 12V going to each device). Both devices have buck converters, which drop the voltage to ~5V. Currently I measure the battery at ~12V then put my multi-meter inline with the hot lead of the battery and measured ~300ma, which I'm assuming means my system utilizes ~3.6W. The one circuit/device I mentioned above uses ~0.72W, which means the other must use ~2.88W, but the datasheet says that it should be 1W. I'm trying to determine if the datasheet is publishing power consumption values not representative of full load or if I'm testing the system incorrectly. A discrepancy of 1.88W is a lot and I'm not sure how to explain it.

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The method you're proposing is correct. Efficiency is (power out) / (power in).

The efficiency for the buck regulator varies with load, and may be fairly low at light load. The regulator datasheet will have an efficiency vs. load characteristic. Check that and see if this correlates with what you're seeing. If it does, you have your answer. If not, you may have some other problem with your design.

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