So I wanted to get myself a Kill-a-watt. Being who I am, I wanted information regarding its accuracy, especially at low power draws. I found a comparison with a industry grade equipment (Fluke is about the best out there in handheld electrical meters). It’s not encouraging, so I thought about a more proper meter, but it’s not easy to find an actual power meter that is accurate at low loads, isn’t a hassle to install and doesn’t cost an arm and a leg.
What do you use? Am I overthinking it?
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Am I overthinking it?
Depends. What’s the application you’re aiming to deal with?
If you want to get an idea of what you’re spending on electricity for a computer, I’d imagine that it’s fine. How much are you going to spend on this thing relative to how much are you going to save via optimizing any electricity use?
If you’re building a prototype for something that you’re going to build hundreds of thousands of units of and you need to size a power supply for that product, and care about knowing peak load, it’s probably worth getting something fancier.
Well, I’m an electrical engineer and I like tinkering. This wouldn’t be used solely to optimize my home server, I would use for other stuff around the house and projects I get into. So yeah, I’m willing to part with double what a killawatt costs if that means a better instrument.
Aight, well, then I’d say that you’ve answered your question as to whether it’d be worthwhile.
Oh, that was clear. Finding a more accurate killawatt is what eludes me.
Slight nitpick: Brymen handheld meters often have better specs in the handheld market, in particular when you are looking at a fixed price point.
You see a lot of Fluke meters around due to service agreement, as well as government and military contracts.
Don’t get me wrong, meters are fine, but there is no reason to spend that kinda money at home, unless the service manual of your washing machine explicitly states all measurements are to be done by a Fluke meter.
There was a youtube vid, testing multimeters, & there was a specific condition that produced wrong results in all the meters except Fluke, who had engineered to prevent that wrongness.
That was what decided me on trusting Fluke, in the future.
been years, no idea what channel it was on, sorry, but it should be findable for someone with patience, knowing that only the Fluke got it right, of the ones tested.
Do pay attention to the calibration-certificates, though:
Anybody paying for Fluke who ignores that their handhelds have no more than 2.5-digits of actual-accuracy, is foolish/incompetent.
( the cheap ones are sooo much worse… )
No way I’m spending over 2 grand to measure power. That was just a comparison to see how bad the killawatt is.
I personally make use of the sonoff pow smart plugs, with Tasmota firmware. Though any Tasmota compatible smart plug with power readings will work.
The key thing is that with Tasmota, you can properly calibrate the readings. I have a friend with a high quality power meter. I used that to calibrate my smart plugs, they seem to track within a few % of the expensive one, once calibrated.
Depending on if you have access to an expensive meter or not, this will either be the best bet, or completely useless to you. Your local Hackspace might also be a good option for getting your hands on an expensive meter for an evening.
I’ll look into it, sounds promising! Thanks!
I use Shelly Plus Plug S. Im not sure how accutate it is, but it does decent job for me. I feel like you are looking for something else, but wanted to mention shelly anyway
Are you looking to measure draw for one specific device or across circuits in your whole home. I’ve been considering two systems: IoTaWatt and Lantern Power Monitor. Both are open source and open hardware.
IoTaWatt uses entirely custom hardware for the actual device but serves its own webpage that can be accessed easily at home from any device and if you’re technically savvy can pass to other services. My reservation with it is if at some point in the future the hardware dies, how easy will it be to recreate if the main project goes dormant, even with the hardware open source? What if a chip manufacturer goes out of business or discontinues a part?
In contrast, the Lantern Power Monitor runs on Raspberry Pi with a relatively basic custom board you can solder yourself with minimal skill; I suspect you could even make it work with a breadboard. Recent shortages notwithstanding, Raspberry Pi seems to keep their models pretty available for a long time, so that inspires a little more confidence. On the flip side, the software primarily runs on a mobile app. There is an iPhone app but it doesn’t have as many features as the Android app. But what happens if the project is abandoned and the app is no longer updated to stay compatible with newer phones or operating systems? And the app by default sends the data to an outside server maintained by the main developer. He doesn’t charge for the hosting and it makes it easy to access from anywhere, but some people might have concerns about that. You can also configure it to self-host from the Raspberry Pi but then you need to have the know-how to make it accessible outside your home (if you want). Underneath it also relies on some other open source data analysis components and I assume if you really know what you’re doing you can pass it out to other software.
Of course, in the end I don’t have the budget to do either project right now.
One specific equipment at a time. Though I intend to delve into what you’re mentioning at some point.
It will be technically a bit more challenging, but I don’t think you need to use the lantern power monitor software with the device. The device seems to be built around two MCP3008 IC’s, which are basic 8 channel analog to digital converters. The values from these can be read and processed (like, sent to home assistant) using a python script on the raspberry. This is the kind of minimal programming effort someone with no experience could do using chatgpt.
if the draw is so low, does it matter?
Yes. Low power draws add up. 5W here 10W there and you’re already looking at >3€ per month.
I realize my situation may not be your situation but… Seriously?
The problem is that it’s not just 15W; I merely used that as an example of how even just two “low power” devices can cause an effect that you can measure in euros rather than cents.
You kind of have a point, but only to an extent. If the error in your measure is around 20%, are you really measuring anything?
If there is a problem at low power usage then you can easily solve it by temporary add more power. Lets say add a 40watt lamp or something, later remove it from the calculation.
While it sounds a bit hacky, I think this is an underrated solution. It’s actually quite a clever way to bypass the whole problem. Physics is your enemy here, not economics.
This is kind of like trying to find an electric motor with the highest efficiency and torque at 1 RPM. While it’s not theoretically impossible, it’s not just a matter of price or design, it’s a matter of asking the equipment to do something it’s simply not good at, while you want to do it really well. It can’t, certainly not affordably or without significant compromises in other areas. In the case of a motor, you’d be better off letting the motor spin at its much higher optimal RPM and gear it down, even though there will be a little loss in the geartrain it’s still a much better solution overall and that’s why essentially every low speed motor is designed this way.
In the case of an ammeter, it seems totally reasonable to bring it up to a more ideal operating range by adding a constant artificial load. In fact the high precision/low range multimeters and oscilloscopes are usually internally doing almost exactly the same thing with their probes, just in a somewhat more complex way behind the scenes.
Good point!
https://iotawatt.com/ is what I plan on using shortly
I mean if you want that kind of accuracy getting a Fluke setup is probably the way to go. The kill-a-watt is not bad for being $30 or so.
Tasmota nous sametplugs work great and have a tiny footprint. They integrate nicely in home assistant. They must be calibrated once with a resistive load eg old light bulb.
In terms of raw data, any properly calibrated equipment should be identical.
You could build one yourself or have a look at how to get readings from your actual electricity meter. Both can be found on the open energy monitor project page. If you self build i guess you could use a lower value CT that amplifies lower currents and just not use it at higher loads?
Mains power fluctuates a LOT though so youll definitely need averaging to some extent
Many residential tools can’t even be calibrated. Even if they are, calibration stability is a concern. The difference between a cheap instrument and a good one is short and long term repeatability.
Thanks for pointing open energy monitor, that seems overkill for what I need.
