Electrical Engineering - Home energy monitors

Author

Home energy monitors

Dave Martindale

2008-01-22, 1:25 pm

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/produc...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

Don Kelly

2008-01-22, 8:25 pm

----------------------------
"Dave Martindale" <davem@cs.ubc.ca> wrote in message
news:fn58hm$du9$1@swain.cs.ubc.ca...
>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/produc...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

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)?

--

Don Kelly dhky@shawcross.ca
remove the X to answer

phil-news-nospam@ipal.net

2008-01-23, 9:25 am

On Tue, 22 Jan 2008 17:18:46 +0000 (UTC) Dave Martindale <davem@cs.ubc.ca> wrote:

| 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) ?

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / spamtrap-2008-01-23-0808@ipal.net |
|------------------------------------/-------------------------------------|

LinkBot