Comparison of End‐User Electric Power Meters for Accuracy

The small loads we used were a charger and a compact fluorescent lamp. Battery
eliminator with 1 W load is a normal small voltage transformer with resistive load
(24 Ω) connected to it. This corresponds to a typical very small load, such as
charging a mobile device. Compact fluorescent lamp (CFL) also has a low nominal
power (15 W)and a very difficult current waveform.
In the category of regular loads, 2x36 W fluorescent tubes have choke to limit
current flow, which makes their load bit inductive. Even more challenging target
was an older, non‐power factor corrected computer (non‐PFC ATX PC, AMD 1
GHz Duron). In the tests, a webpage was opened on full screen, simulating web
surfing, and screensaver and power saving modes were disabled . Therefore this
computer did not have major difference in consumption between different
computational loads. Parallel to the PC, we connected a 19” CRT monitor to test
meters capability to measure switching power supply load. This older computer
also had high power consumption, and the combined nominal power of the setup
was 170 W. This computer setup was used to perform the electricity consumption
measurement.
Incandescent lamps dimmed to operate at 200 watts (nominal 440 W) were used to
test meters capability to measure clipped current. The dimmer holds current from
flowing until the amplitude of voltage reaches the level set by the switch. Current
flows again until the next zero crossing of current occurs. This causes current to
have sudden changes and harmonics. The final load was 600 W resistor which
resembles an electrical heating radiator or a hot plate. With this kind of load,
current is in phase with voltage which should make it the easiest possible target to
measure. This test also reveals how accurately meters can handle higher currents.
3.3. Procedure
During the measurement, all meters were plugged one meter after another so that
they all measured the same phase (see Figure 1 for an illustration). Although the
phase remains constant, this produces some error since meters do not measure
their self consumption and meters’ power is taken mostly with direct capacitive
coupling to phase voltage. This capacitive coupling also creates some error when
measuring small inductive loads as meters’ capacitive connection cancels some of
the inductive current on load. Almost all meters measure current with shunt
resistor which means that on higher loads the voltage drop over each meter will
affect meters after it. These errors were considered negligible as the reference
measurements taken from directly before the meter stack and after the meters, just
before the load. Reference measurement itself did not affect measured signals.
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