Idle power consumption


I was thinking about building a sensor network based on small wireless routers, like the TP Link TP-WK703N. The OpenWRT page states 0.5W while idle, but the measurement is made for what goes into the router, not what is drawn from the network. So, I wanted to see how much the mains power consumption is, and while here, measure some idle power for other devices around the house.


At first I thought about building some energy meter, there are a lot of designs out there, starting with Atmel’s app note for a real meter and ending with simple DIY ATtiny meters. But then I remembered I had one of those simple plug energy meters. It is rated for 5W to 3.6KW, not suitable for my needs, so time for hacking.

I opened up the meter and found two boards inside: one seemed to take care of the display, the other to do the measuring. The display board contains two crystals, one for time keeping and the other for regular use, i suppose.  The connection between the two is power plus another pin, indicating that the metering is probably sent as pulses. So, the easiest way to make this more sensitive is to simply swap the current shunt with a larger one. Two multimeters and one amp later the shunt proved to be 1.5mΩ.

Nothing simpler, I would upgrade that to 150mΩ and make my device 100 times more sensitive, capable of measuring 0.05W to 36W. I built the new shunt out of three 0.1Ω, 1W resistors since this is what I had available.

I tested the new meter using some resistive loads, somewhere around 0.1W, 1W, 5W which is the area of interest, plus 25W. A multimeter provides the exact resistance and mains voltage, computing the power which is the compared to the meter. As I expected, lower powers are underestimated, 0.1W is underestimated by 40%, while 1W just by 9%. 5W and 25W loads are correctly measured. A little work gave me the correction curve for loads under 3W, higher than that needing no correction. The correction formula is as follows: corrected_value = original_value * (-0.107 * ln(original_value)+1.1419), only if original_value < 3W.


I couldn’t test the 703N router because I have accidentally ordered its smaller brother, the 702. Specs are similar, so results should be similar. It turns out it draws under 1W out of the grid while idle, which is good enough.

Next, I grabbed the Raspberry Pi and did some measurements on this, since most available data is just current drawn by the Pi, not total mains consumption.

Power consumption with Ethernet, mouse and keyboard topped at 3.25W, showing insignificant variations with various activities, mostly because there’s no CPU frequency and voltage scaling. There’s a small power reduction by removing the monitor and some by removing the mouse and keyboard. Leaving just the Ethernet connection reduced consumption to around 2.75W.

After that I started measuring all sorts of power supplies and standby appliances from around the house, plus I bugged some neighbors. Data is in the table below.

Most adapters are SMPS, I could only find a couple using classical transformers. All switch mode adapters except one stay below the 0.3W limit while idle, with a few of them going below 50mW which is the lower limit of measurement on the hacked meter, so nothing is read. On the other hand, devices which have some sort of standby mode will be close to 1W, probably because they might need to keep some circuits running.

The adapters using classical transformers, indeed burn more power unloaded. I was surprised to find one that uses 2.7W and that a 10VA PCB mount transformer with just a LED drawing 0.2W draws close to 2W from the mains, no wonder they get so hot unloaded.

The table below gives more data on the measurements, the ‘Corrected Power’ column contains the corrected measurement, based on tests with known loads.


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  3. Dell monitors have the yellow light on when they are not getting a signal from the PC. I wonder what the power consumption is when this yellow light is on. It it substantially higher than when it is off? Another question I have thought about a number of times is how much power does a motion sensor on a security light draw? If I’m shutting off a 10 watt security light, and the motion sensor draws a few watts 24/7, then it’s not a cost effective solution.

    Thank you for the very useful information. I think I’ll have to investigate a way to replace the transformer (analog) adapters. Problem is that the switching power supplies make a lot more interference than the old transformer adapters. If I have AM radios running, often there are stations that are completely blocked by the interference from the switchers. I’ve found that looping the power cord a few times through a ferrite toroid will reduce the interference. But not completely, and often not enough to get the problem fixed. The only solution is to shut the power off to the adapter. And this still doesn’t address the problem with the sensitive circuits being fed from the switching adapter.

    • On my monitor there’s almost no change in power consumption between off with the yellow light and off without. See above table.

      I don’t know how much a PIR sensor like that consumes, but i would guess that it is around 1W like my remote control sockets, the power supply and the relay should be about the same.

      There’s nothing much you can do for AM radios, that’s just the way it is: smps can operate in the AM band frequency…

      • You sound like the FCC: “Tthis device must not generate interference, and you must accept interference from other devices…” or words to that effect. 🙂 LOL! If it was a perfect world, I guess everything would tolerate every other thing, but it’s not. So I try to filter out the noise from the RFI generators, and I figure that at some point, the signal will get through. Probably not very likely. I guess if I wanted to be in an RFI free zone, I could move back to Green Bank, W. Va., where the FCC’s RFI cops make people turn off their Wi-Fi and cell phones. I tried this link and apparently the storm Sandy has affected them adversely, I couldn’t get it.

        • Some smps have the output cable as coaxial. Maybe if you ground the shield it could help?
          (most don’t have the shield grounded because they don’t have a plug with ground connection)

          Anyway, i guess the regulations just kind of say that your device should not burn, explode or whatever in the face of interference. They don’t say the device should also work….

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  5. Great experiment! Disappointing about the Ikea LED supply.. .

    • It kind of is, especially since both the LED adapters and the 20W halogen transformers are designed to have the power switch after them, so they stay plugged in and powered all the time.

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