Computing stuff tied to the physical world

Power consumption puzzle

In Musings on Jun 17, 2013 at 00:01

Recently I had to go abroad for a few days, and given a new access path I just added to my HouseMon setup, it let me track things while away from home. Is that good? Eh, sort of…

The nice thing is that I can now check the solar production in the evening while away from home. And obviously I can then also check on the energy consumption – here’s what I saw:

Screen Shot 2013-06-15 at 19.03.26

Very odd. Something was turning on and off all the time! Here’s a close-up:

Screen Shot 2013-06-15 at 19.00.26

As you can see, I did figure it out in the end – but only after this had been going on for two days, on the phone with Liesbeth to try and identify the source of this behaviour.

It was a bit worrying – that’s a 300..400 watt power draw, every 45 seconds. Going on for days. This is enough energy to cause serious heat problems and my worry was that it was some sort of serious malfunction, with a thermal safety constantly kicking in.

Yikes. Especially when you’re away, knowing there is no one at home during the day…

The answer to this riddle isn’t 100% conclusive, because I did find a way to stop the pulsing, but turning power on again did not cause the pulsing to resume:

Screen Shot 2013-06-15 at 19.29.32

This is crazy. The only explanation I found, was the laser printer: I had printed a few pages before going away (and as far as I can remember, I did turn the printer off after use). The printer was behaving erratically, and I had to turn it on and off a few times.

So my conclusion is: the laser printer (which has a toner heater built-in that can indeed easily draw a few hundred watts) got stuck in some weird mode. And kept me worried for a few days – all just because of me being a geek and checking my graphs!

I don’t know what lesson to take home from all this: stop monitoring? stop checking? stop traveling? – or perhaps just stop worrying when there’s nothing you can do about it ;)

  1. Ignorance is bliss… Last summer holiday, being far away, my home automation sent me ‘probably fire’ alarm. Once tripped, my fire alarm script did not silence. It gave alarms for every 0.1 °C over 35 °C for every room node, up or down. Appeared to be a hot summer’s day. Now the alarm threshold is set at 45°C, and every 10 °C…

  2. Interesting to read, as I am experiencing the same issue: spikes in power usage (from 230 Watt up to 1500). Still can’t pinpoint the device causing the spikes. Could be my refrigerator, but need some more gear and time to test that. Will let you know if I find out what it is.

    • From what I can see in my three-phase energy data (taken with OEM emontx) fridges tend to draw a small power peak when starting and then a slowly falling power ‘shoulder’. In my case (side-by-side fridge-freezer) the power drawn is ~120W falling to ~100W, duty cycle is ~20min ON and 25-30min OFF. The deep-freezer is very similar with 110W to 80W, duty cycle 15min ON and 35min OFF. Duty cycles varying only slightly (with ambient temperature and freezer settings). Sharp, high power peaks mostly come from heating elements like in coffee machines, laser printers etc. (during standby).

  3. My laser printer is a Xerox ColorQube which uses solid ink sticks that are melted and then applied to the paper using an ink jet method.

    Each colour ink stick has a small reservoir of molten ink which is held at a constant temperature even when the printer is in standby mode. About 20 watts in standby.

    I could turn the printer off completely but the molten ink shrinks when it cools so upon power-up around 12% of your ink is ejected as waste. It’s more expensive to turn the printer off than it is to leave it on.

  4. Often the most useful “signature” information is in the first few power-on cycles and/or from the load power factor. Unfortunately, most monitors fail to capture this information – either not measured at all for power factor or insufficient timebase resolution.

    Interestingly, many ‘Smart’ meters do have cycle by cycle capability, but from published specs, do not usually make this granularity available externally. The potential for device recognition is there at least internally (though of course, as more loads get overlaid, the signature recognition problem gets more difficult).

    Privacy advocates might get hot under the collar about this potential breach. Personally, I have no problem with some outside agency detecting that I have opened the ‘fridge door to extract a cold beer ;-)

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