Efficiency testing of a power supply produces two figures from one test setup: consumption at rated load and consumption at no load or in standby. They differ by two or three orders of magnitude, and both are normally reported from the same instrument.
That single requirement is what makes standby measurement difficult, and it is why the instrument specification should start at the bottom of the range rather than at the top.
The scale of the problem
A 100 W adapter operating at rated load draws roughly 0.45 A at 220 V. The same adapter in standby may draw under 0.3 W, which corresponds to a current in the low milliampere region - three orders of magnitude below the primary measurement. An instrument selected for the full-load figure alone will display the standby current as a value sitting in the last few digits of its range, where uncertainty is largest.
This is the reason a power meter intended for efficiency work is judged on its minimum measurable current rather than on its maximum range, and why the accuracy statement must be checked specifically at the low end rather than assumed to apply across the whole span.
What to confirm before ordering
- The minimum current at which the stated accuracy still applies, not merely the minimum current the display can show.
- Whether the low-current range is a genuine measurement range or a scaled interpretation of a higher range.
- Whether automatic range switching is used, and how the reading behaves at the moment of switching.
- Whether power factor remains meaningful at very low current, since the ratio becomes sensitive to small phase errors.
- Whether the instrument accumulates energy and elapsed time, because standby consumption is normally reported as watt-hours over a period.
Why the range floor differs between instruments
Instruments designed for general electrical testing optimise for robustness across a wide range of loads. Instruments intended for low-power and efficiency work add finer current sub-ranges so that the same instrument can resolve both the standby figure and the full-load figure.
The difference shows up in the specification as a lower minimum current on the same maximum range. It is a design decision, not a marketing one, and it is the first thing to compare when two instruments otherwise look identical.
Practical consequences for the test setup
Because the measurement spans such a wide ratio, the wiring and the settling time matter more than they do for a single-point measurement. Allow the reading to stabilise after a range change, keep the current path free of parallel leakage, and record the operating condition alongside the figure so the result can be reproduced.