Jim Elbrecht wrote:
mike wrote:

Jonathan Grobe wrote:
I have a traditional 220 volt electric hot water heater and would
like to estimate or measure how much electricity it is using.

-snip-
All electric water heaters use the same amount of energy
to heat the same amount of water. Differences are in
how much energy is lost thru less-than-perfect insulation.

And the temp differential in the room, and usage. . .
I just went thru that procedure using a current
clamp and a computer. I can tell you exactly
how much my water heater is costing me.

I think that's what the OP is asking. Tell us more about your
current clamp & computer setup.

Jim
If you buy a current generation current probe, they're typically
1mv/amp. Maybe usable for a water heater, but I wanted something more
sensitive.
The Triplet model 10 current clamp is designed to hook onto a 50
year old analog meter. It puts out 3VAC full scale from as little as 6amps.
And it doesn't need a battery.
Plug that into a Radio Shack DVM with RS-232 output.
Then it's just a matter of a simple visual basic program to
read the meter and log/graph/analyze the data.

It worked, so I intended to build a second generation using a
pic processor and bluetooth transceiver and do the acquisition
and analysis on a PDA. But then I realized that the data wasn't
much use anyway. What I needed was to use less hot water.
Didn't need any data for that conclusion.

I did need a way to measure consumption from things that I couldn't
easily get at. Air conditioner is the most obvious. Power factor
is an issue.
So, I sidestepped the whole issue and went straight to the source.
The power company meter on the wall puts out an infrared pulse
for every watt-hour consumed.

Turns out that the older PalmIII PDA's PALMOS 3.5 will let you trick
them into
reading that pulse thru their IR port. And there's a trialware basic
interpreter
available for free. It has limitations, but works great for this.

So, go to Walmart and buy a yummy single slice of cake.
It comes in a plastic container that's a press fit over the power meter.
Stick in the PDA, press it onto the meter and log yourself silly.
You're gonna have to use bigger batteries in/on the pda to get
sufficient run time.

With a little experience, it's easy to distinguish the signature
of the water heater, air conditioner, microwave, stove etc.

NO power factor issues or accuracy issues. You're measuring
EXACTLY what the power company is billing you for.

For about $25 on ebay, you can buy a "blue line innovations"
wireless remote reading power monitor that will also work with the
old-style power meters with the wheel that goes round and round.
They work well, but only accumulate total consumption.

I also wanted to monitor the air conditioner without a lot of
post-processing of data.
The same pda and almost the same program can be used with a
wind-vane/microswitch into the serial port to sense when the fan
is running. That's pretty close to the on-time of the compressor.
Also works with a gas heater.
That did supply some useful information that led to shading the front of
the house on hot afternoons and reducing electricity consumption
by about a third. My front door is no longer 185 degrees.

While I've got your attention, I'm interested in ideas on how to
reduce the attic temperature from 136 degrees. I tried blowing
air up into the attic with a 15" fan, but it hardly put a dent
in the temperature.
The math suggests that the insolation is pushing over 100KW into the attic.
A fan to push out that much heat costs more to run than the improvement
in air conditioner efficiency saves...according to the back of this
here envelope.
Ideas?

mike