Home energy monitors

I recently bought a UPM EM-100 "electronic energy meter" (\$25 at Canadian Tire). It measures line voltage and current drawn by whatever
is plugged into it, calculates "watts", and records max current, max "watts", operating time, and the integral of watts. It will also convert watts to dollars if you tell it your energy rate. It looks like this: <http://www.upm-marketing.com/products/ProductDisplay.cfm?CFIDi51872&CFTOKEND108250&pt=%22%28%20%24%20%0A&rtRnH4%263O%5E%5F%3A29VU%5B%5ERP%3D%5BY6%226N%22W%5EL%20%0A&item=%22%290%3C%20%0A
Internally, it just uses a shunt to measure current; there's no current transformer. The circuit is battery-powered and connected directly to the line. Current range is 0-15 A with 0.01 A displayed resolution. I compared it to my best handheld meter connected in series with the load, and the EM-100 reading agreed within a percent or two of the "serious" meter.
However, it seems that it really measures volt-amps and calls that "watts". I plugged in an unloaded isolation transformer, which would have had a very low power factor (almost pure inductive), and the measured current was quite accurate, but the displayed "watts" was simply the product of current and voltage. So this unit should work well for high power factor loads, but it will be quite inaccurate for low power factor loads.
The most commonly recommended device for measuring appliance energy consumption seems to be the "Kill-A-Watt". It does distinguish between watts and VA in display, and can also display power factor. But is it accurate for low power factor loads?
Dave
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<http://www.upm-marketing.com/products/ProductDisplay.cfm?CFIDi51872&CFTOKEND108250&pt=%22%28%20%24%20%0A&rtRnH4%263O%5E%5F%3A29VU%5B%5ERP%3D%5BY6%226N%22W%5EL%20%0A&item=%22%290%3C%20%0A
That's useful information. How many other such devices are no better? Buyer beware.
The Kill-a-Watt which does display pf probably may do reasonably well at low pf because, if it can distinguish between watts and vars at normal power factors, I see no reason why the circuitry can't handle low power factors although greater errors at low pf because vars aren't measured -just watts and va.
However the brochure and manual "specifications" are really pretty sketchy and inadequate as well as the claim that one can Also check the quality of your
power by monitoring Voltage, Line Frequency, and Power
Factor.
Does one stand there watching the meter continuously to do this (and what has the pf to do with the supply power quality)?
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Don Kelly snipped-for-privacy@shawcross.ca
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| The most commonly recommended device for measuring appliance energy | consumption seems to be the "Kill-A-Watt". It does distinguish between | watts and VA in display, and can also display power factor. But is it | accurate for low power factor loads?
Has anyone found one of these for 240 volt outlets (US NEMA 6-15/6-20) ?
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| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |