Hello all,
I have both a Kill-A-Watt and a Watts Up? Pro power meters. To check how close they're against each other, I connected them in series with the Kill-A-Watt going into the plug, then the Watts Up? Pro and the load.
They should read about the same or the Kill-A-Watt should read the power consumption of the load plus the Watts Up?(no more than a watt or two). Comparing the actual meaasured values, the Watts Up? Pro consistently give a value about 10% higher than the Kill-A-Watt with inductive and non-sinusoidal loads and not quite as much difference with resistive loads. With the computer I'm using to write this message connected as the load, Kill-A-Watt is reading 174W and Watts Up? Pro is reading 190W.
Both devices agrees within a reasonable degree against a known good DMM for voltage. DMM: 120.3V W: 121V (does not resolve to 100mV) K: 120.4V
Current do not agree with eachother: DMM: Unable to measure, my DMM is not true RMS capable W: 2.37A K: 2.19A (w/ no load, device reads 0.02A, 0.0W)
PF: both devices reads 0.66
Here are the differences in construction:
Voltage measurement:
Watts Up? Pro: An isolating transformer drops the voltage used for both measuring the voltage and powering the device.
Kill-A-Watt: It is directly powered from the AC line through a series R- C circuit and a separate resistive divider is used for voltage measurement.
Current measurement:
Watts Up?: Current transformer.
Kill-A-Watt: Shunt
Which setup is likely to have an inherently better accuracy?
Product information:
Kill-A-Watt: