Monitoring heat-pump kWh usage
_matt_p
| Posted in General Questions on
Hi,
I am looking for a device to monitor the kwh usage of my heat pump. A whole-house monitor that breaks usage down by device or circuit would be good too. Any recommendations are appreciated. Thank you.
Matthias
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Matthias,
I just sent an email to Marc Rosenbaum, asking his advice on this issue. Here's how he responded:
"I use the SiteSage product and that is not a single point item. It’s costly to start and has a monthly fee. I use a lot of Hobo data-loggers, but I haven’t set one up with the ability to measure true power (which means current and voltage and power factor), but I know that set-up starts to also get into some $ (I think they use a product called WattNode as part of this set-up). There may be products out there I just don’t know about.
http://www.onsetcomp.com/products/sensors/kW-and-kWh
"Depending on how accurately the person wants the data, they can measure amp-hours. Knowing the voltage and guessing the power factor gets you in the ballpark. You can get a Hobo logger and a couple of CTs and do that, and there may be other ways that are better. One recommendation is for them to call Onset Computer Corp and tell them the application and see what they recommend.
"Let me know if someone else suggests something cheap and accurate!"
Matthias,
Marc Rosenbaum just sent a follow-up email, noting that Matthias "may be well served by looking into eGauge."
I use the greeneye monitor from brultech in my home. Using the remote dashbox server is not required, but makes things much easier. The system offers lost cost sensors to monitor temperatures and water usage as well.
http://www.brultech.com/products/
Thank you for the suggestions! Much appreciated.
Here's a store that offers a few interesting options:
http://www.powermeterstore.com/category/residential-power-energy-meters
The TED with the "spider" option lets you monitor the whole house electric consumption plus individual circuits. Or you could install a monitor meant for the whole house (TED or Efergy) on just the heat pump circuit.
I have an older TED that just measures the whole house, and have found it to be useful, convenient, and accurate. You can also get TED directly from their web site:
http://www.theenergydetective.com/
You can also find simple 240 V meters on E-Bay for around $20. But I'm not confident they are safe, accurate, or reliable.
If you're just trying to measure one thing, like a mini-split heat pump, then I recommend the Efergy True Power Meter at about $200 (http://efergy.com/us/products/energy-gateways/elite-true-power-meter-engage-hub). This includes an in-home display as well as no-extra-charge online/smart-phone app access.
If it's a traditional split system heat pump, then you're going to have separate circuits for the outdoor unit and the air handler. If you don't mind combining the data, then you might be able to run all the wires through one pair of CT's and still use the above-mentioned product...if not, then you need a monitor that can pull in data from multiple sets of CT's, such as Site Sage, eGauge or Curb.
From what I've seen none of these boxed systems seem to address the power factor issues the Marc is worried about. Is this the case, or do they have some sort of phase angle calculations based on actual voltage and current? Most of the quick research I've seen only include CTs and some of them say they monitor voltage, but not how they apply this to the metering.
Since P=IV or IE, the relationship between the peak current and voltage are important as in true circuits you are generally either slightly resistive or inductive and cause a shift in the phase angle. In residential systems I would assume this to be a smaller factor, but this can be much larger based on overall usage and actual power factor for a commercial or industrial entity. This slight shift could account for 10% change in usage calculations or more depending on the ratio of resistive vs inductive loads
versus unity.
Mike, I believe that all of the products described here, except the Hobo logger, do a true power measurement that would not be fooled by a low power factor. I know for sure that's true of the TED products.
Note that a low power factor can result from a non-sinusoidal current as well as from phase shift. The way the meters work is to measure instantaneous power at each point in time during the cycle, and then average over the cycle. That correctly measures power regardless of whether the current is sinusoidal and regardless of phase shift.
Charlie,
I would be much less worried about the shape of the curve as the phase angle. Most really sophisticated software that I deal with has a hard enough time dealing with harmonics within the sinusoidal curve that change the overall shape. I would think that a PWM controller on an mini-split might cause some of this, but your average load I wouldn't worry too much about.
The shift that I'm worried about is between the voltage vs current curve. If their peaks do not align, you are not in unity and the PF may effect the overall performance..
Many industries such as steel recycling have such effects on grid harmonics that they destroy step down transformers and effect grid stability. I have two mills that I consult with that have to have separate AC and DC furnaces to limit the harmonics near these sites since they are so close. I would not think any home loads could every truly influence this. These transformers are in the 50+MVA range and operate under bolted fault currents for nearly 3/4s of every hour.
I'm sure Dana may be able to chime in to see if the smaller grids would see these effects at all.
Matthias, you might want to look into the Sense home energy monitor. It's a new product that learns loads automatically. https://sense.com/
My Sense unit arrived today, apparently was in their second production run. More after I get it hooked up over the next few days!
Thank you for all those additional suggestions. Much appreciated.
Andrew -- Did your Sense ever recognize your heat pump? Or is the pattern of power consumption just too variable?
Hi David. Yes Sense did ultimately recognize my geo system.
However I also traded a number of messages with the developers to fine tune the process; sometimes it would miss. All that about a year ago now, has been working well since.
Here are some screen shots from just now and showing data from last week.
(The gaps in operation are in the case of last Friday an extremely warm day, and over this past weekend from daytime passive solar gains. No heat required those times.)
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One more graph, geothermal (ground source water-water heat pump) compressor usage, full year 2017.
January '17 had a number of above normal warm days, I did some gardening then! Also took a few months to hone our accuracy at determining when the geo unit was running.
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