Hi all, I recently got a Refoss EM16, whole-home energy monitoring. In Canada where I live, homes are on a split phase 240v system, which means that to monitor 240v appliances we need two probes each. This quickly eats up probes from the kid (e.g., electric hot water tank, range, ev charger, AC, etc etc).
Online, many people say to just measure one leg of the two and multiply it by two to get the same result. Of course, electricity is not always so cooperative and this only works for systems that are balanced across the two rails. Some systems may be quite unbalanced leading to pretty inaccurate readings.
Since I am motivated to free up my probes for other circuits, I did a quick test. after 5 days of regular use, I did a comparison by downloading homeassistant’s history graphs, and doing excel magic to align them and compare them.
Given that its winter and we had -35c this week, I haven’t tested the AC yet
Hot water tank: doubling vs summing was generally within .1W, so over time I think doubling ~= summing.
Induction Range: doubling vs summing was ±2w, which is larger but not crazy. It seemed to be highly variable based on how the range was being used. Not sure about using doubling here.
EV charger - doubling overestimated 5-15w regularly, with a lot of variation. so this is pretty inaccurate.
So I’m going to do doubling on my HWT to save a probe but not sure about the others.
Most 240V devices have some low voltage circuits running in them. The better devices step down the 240V for these circuits. It is obvious to me that your EV charger uses one leg to get ~120V and then steps that down thus the 5-15W error which would be the control computer. If I am right, putting your CT on the other leg would lead to an underestimate.
But step back for a minute. How are you going to use the data? In my case, I’m using the data to tell me how to modify my lifestyle to reduce my energy usage. I also use the data to tell me if a device is malfunctioning. You’ll find that parasitic losses are much more than your errors in measurements. (I’m seeing about 200W, ~5kW/day! ironically 50W from running HA ;). Your energy monitor is at least 15W and the 50 sensors you have around your house add up to at least another 50W, )
I would like to see if my electric utility provider is accurate with its measurements, but that’s a losing battle because it can claim its meters are NIST Traceable and mine are not.
I recommend an energy monitor because it makes one aware of one’s energy usage. That alone will precipitate a positive change.
You’re absolutely right of course! And the underlying motivation for me with all of this is simply that it’s fun, and a hobby.
I make the same assumption about the EV, with some of the electronics on one of the 120v rails. I was reading online (not sure how reliable of course) that some EV chargers do rotation over the rails for the charging as well, but I’m not knowledgeable enough on this.
I’m still trying to determine what level falls under “I don’t care” and what matters to me. If I feel the numbers are way off it’ll irk me for sure. If I sum up the error on the EV over those 5 days and it tells me 10kw - not an accurate integral without considering the windows, but it is some measure of an order of magnitude.
Honestly I with my kit just came with 50 probes
Edit - I meant to add that my “media center” which includes my always on tiny Dell running Prox+HA (among others), TV/stereo on vampire powr, etc., draws ~30w. Way less than expected.
My current untracked consumption was 22kwh yesterday, so I have more 120v plug in sensors on the way
I’m not sure how the Refoss works but keep in mind that there are 3 different types of power measurements - reactive, true, and apparent.
reactive power (KVAR) is the power that creates magnetic and electric fields and is never truly “consumed”. It just gets sent back to the grid.
true power (KW) is the power that actually gets used up doing the work that we want the appliance to accomplish.
apparent power (KVA) is the algebraic sum of the true power and reactive power. It is dependent on the power factor of your home - either leading (capacitive) or lagging (inductive). it’s hard to actually know at any given time what your true power factor is without a PF meter. A standard clamp on ammeter shows only apparent power. the standard PF for at least industrial environments is assumed to be around .8 lagging due to all of the inductive loads (motors and what not). A well balanced system with both capacitive and inductive loads (or a purely resistive load like a heating element - think toaster) will result in a PF of 1 which is desired. With all the modern electronics (which typically seem to be capacitive) in our homes it’s hard to guess what yours might actually be.
your city meter in all likelihood is a true power meter. it has circuitry to remove the reactive power component of the current going into your home and just measures the true power.
so depending on how the Refoss system works and the power factor of each device you are measuring. what your actual power usage is (true power) vs your displayed power (apparent power) might be as much as 20% off depending on the type of load you are measuring.
Hmm. I’m familiar (in passing) with the terms and types of power but don’t have enough knowledge to have considered it in this case.
The Refoss is 100% clamp on transformers. It reports power, power factor, voltage, current, and its own counter of Wh. I feed the power into a template for nice naming/features, and then into a sensor:integration to give me energy from the power. I have whole-house clamps on the service wires, as well as a bunch over my panel on other circuits.
Interestingly, because I have solar I have a 2-way meter. The meter does tell me when I’m providing reactive power vs true power, although I assume the metered value is only the true power… but again, not an electrician.
Given the hoops I have gone through I didn’t compare what the refoss integration gives me compared to my own calculations, I think I should go do that!
For a straight 240v load (2 hots and a ground) it doesn’t matter which leg you put the meter on. what ever current goes in one leg it will come out equally on the other…unless you have a leakage current to ground. but if that was the case and it was significant enough to cause measurement issues then you have more problems than just what a meter sees.
if you have a 240v supply that also has a neutral (2 hots, a neutral and a ground) then that implies it supplies 120v internally. In that case not all current going in on one 240v leg will come out on the other. some of it will travel thru the neutral from the 120v portion of the circuit.
If that’s the case then you can get total current in the system by measuring current on the highest current leg. But that will throw off your power readings a bit since power = v*i. and you need to assume that the current in the 120v portion is also 240v so the power reading will be erroneously high. that will then only require one clamp but with the tradeoff in a slight inaccuracy in power readings.
to get a completely accurate power reading you need to put a clamp on the lowest reading 240v leg (to capture only the 240v power) and one on the neutral (to capture only the 120v power). But that requires 2 clamps instead of one but you’ll get more accurate readings.
Again all that is said not knowing exactly how your measuring system is set up and assuming you can tell the sensor what the voltage is for the current that it’s measuring.
