How do home energy monitors work? - Page 1
Forum section: electronics / projects

author: number33
author_rank: Contributor
timestamp: August 15, 2013, 11:26:18 pm
content: It seems from the EDMI Smart Meter Teardown subject that there are some power measurement savvy people here. So can anyone please tell me how mains power monitors like the Owl work.http://tinyurl.com/ne8zcgdThey're battery powered and only connect to the mains via a clip-on current transformer, so there is no way to measure the voltage or sense its phase. I thought perhaps they just assume the current is sinusoidal and multiply it by the nominal voltage but they appear to be much more accurate than that. I have a similar one, an Electrisave, which doesn't seem to be available any more, and it is reasonably accurate even when connected to electronic loads such as a CFL light bulb which surely must draw a very reactive load current.number33 - Malvern - Worcestershire - England

author: richard.cs
author_rank: Super Contributor
timestamp: August 16, 2013, 12:57:23 pm
content: Capacitive voltage sensing perhaps, not really good enough for measuring the voltage magnitude due to the variability in cable sheaths, etc, but probably good enough to sense the phase between I and V

author: lewis
author_rank: Frequent Contributor
timestamp: August 16, 2013, 01:32:37 pm
content: Those energy meters do not sense voltage at all, only current. My Efergy elite requires that you program the voltage into the unit (in 5V increments!). Since the current transducer fits around the meter tail at the main supply point to your house, they can be reasonably accurate due to the averaging out of the power factor of your appliances. Some will be capacitive, some inductive, and they sort of cancel out as far as the energy monitor is concerned. Not only that, but big inductive or capacitive appliances will almost certainly have power factor correction which negates the measurement problem to some degree anyway.Another thing to remember is that most of the power hungry appliances in your home will be resistive in nature - heating elements in washing machines and dishwashers, electric showers, immersion heaters, fan heaters etc. Resistive loads mean that measuring the current alone and 'knowing' the voltage will give fairly accurate results averaged over time.I've tested the accuracy of my Efergy unit and it's fairly accurate for total loads above 2kW. It over-reads slightly when the aircon is on in the workshop, and it doesn't have enough effective resolution to tell if an energy saving lightbulb is on or not. It's OK for giving an impression of how much juice an appliance is using, but don't assume they're bang on even though it does have a couple of digits after the decimal point.

author: number33
author_rank: Contributor
timestamp: August 17, 2013, 06:06:48 pm
content: Yes, that all makes sense. They could sense the phase with a capacitive sensor and just assume the voltage is a nominal sine wave or maybe not even bother but just calculate the RMS value of the current waveform and multiply it by the nominal voltage. I don't think that there is any way to change the voltage but I seem to have mislaid the instructions. Now that I think about it mine isn't as accurate as I thought, I was confusing it with a plug-in power monitor I use regularly. In fact it only resolves to 10W and seems to have about 20-30W hysteresis.I now have a second household energy monitor which is actually the remote display unit for the utility company's smart meter and the two monitors often differ by one to two hundred watts. The smart meter is probably much more accurate but it is rather tedious to test with anything other than a large resistive load, so many things to unplug.The Maplin plug-in power meter.

author: steve30
author_rank: Frequent Contributor
timestamp: August 18, 2013, 04:02:15 pm
content: I have an OWL energy meter. It only senses the current (the voltage is set manually), and the quoted accuracy (if memory serves), is around 10%.Its fine for getting a rough idea whether devices use lots of power, or little power, but it isn't much good for accurate measurements.