Two power meters, 5% apart, and neither one is wrong

Node0 measures its own electrical load twice, from the UPS units and from the PDUs, for the ordinary reason that both devices report it and both were already being polled. On 20260920 a routine cross-check put the two figures side by side at the same moment and they were 5 to 6% apart. That went onto the list as an unresolved discrepancy.

It was not one. The first thing to rule out was a timing artefact, since the two sets of numbers come from independently polled exporters and a lag of a few seconds between them would show up as a gap on a changing load. Reading both over a longer window with matched timestamps settled it. The offset holds steady at every moment, including while load moves, which is not what sampling lag looks like.

What is left is an instrument difference, and it is a reasonable one. The UPS side produces a coarse figure derived from a percentage of its rating. The PDU side has a dedicated metering circuit with its own stated accuracy class. They are also not reading the same point in the circuit. Two devices built that differently will never agree to the last watt, and treating the difference as a data-quality bug to be reconciled can absorb real time to no end.

The productive move, once the offset is confirmed to be consistent rather than random, is to characterise the relationship and alert on departures from it. Here that is a ratio band. The PDU total divided by the UPS total should sit between roughly 0.93 and 0.97, and a move outside that says one instrument has changed. If you run two independent power-measurement systems, expect a baseline offset, measure it, and write it down as the target rather than assuming convergence is correct.