How to measure power consumption of a mini PC

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WARNING: Electricity can be dangerous if you don’t treat it with respect. If you follow the circuit, it will work but because we can’t control how you use it, this info comes with no warranty whatsoever – you use it at your own risk.

Quite often, the simplest way to measure power consumption of a mini PC is to simply throw an AC power meter onto the power brick feeding your device and take a look at the reading.

The problem though is that it won’t be accurate – that’s because the AC power meter has to also take into account the conversion process from AC to DC, which results in some loss of power through the process, depending on the circuit inside the power brick.

So the guaranteed way to accurately measure the power consumption is to make your own USB breakbox cable and to use the basic circuit we’ve shown here to measure the actual current flowing through the USB cable.

The basic rule is that DC power (in watts) equals DC voltage (in volts) multiplied by DC current (in amps). So we need to find out the voltage feeding to the mini PC and the current its using.

The other basic rule is that you measure voltage ‘across’ a device and the current flowing ‘through’ it. So in our circuit, you can see two digital multimeters (DMM1 and DMM2). You set one to the 10A DC range, connect the positive lead to the VCC pin on the input USB socket (the one connected to the power brick), the negative lead to the VCC pin on the other USB socket connected to your mini PC. It’s important to use the 10A range because you want the meter to provide a very low impedance so that it doesn’t affect the reading.

The other multimeter is switched to its 20V DC range/setting and you connect the positive lead to the VCC pin on the output USB socket, the negative lead to the GND pin. Here, you’re measuring the DC voltage at the USB port your mini PC is connected into.

When you’ve recorded the two readings, multiply one by the other and that will be the power consumption of your mini PC in watts.

HOWEVER.

The big ‘however’ here is that power consumption will vary considerably depending on what your mini PC is doing. If your DMM has an ‘average’ setting, use it to measure the current consumption and use that figure to calculate the power, in which case, the result will be the average power consumption, but only relative to the function the mini PC is performing.

If it doesn’t have an averaging function, watch the DMM for a little while when measuring the current and gauge the fluctuation. You should be able to see a general upper and lower limit in the current range – use those figures and you’ll get an upper and lower power consumption figure.

If you don’t have two DMMs, that’s fine – just do the measurements one at a time, but you’ll have to make sure you’re doing the exact same thing on your mini PC to get the most accurate result.

In reality, the DC voltage to the mini PC shouldn’t vary much – if it’s being powered via USB, it should be within +/-5% of 5VDC – that’s the USB-standard voltage requirement. The current draw will be what varies considerably more depending on what you’re running on that mini PC.

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2 comments for “How to measure power consumption of a mini PC

  1. Michael Thompson
    August 27, 2013 at 10:40 am

    Great site, love the articles!

    I myself have a CX-919 / U2C-D combination with an external hard drive and an Ethernet adapter I just set up so I’ll be breaking a cable soon to take some measurements too.

    • Darren Yates
      August 27, 2013 at 11:19 am

      Thanks, Michael.
      I’d expect you’d get somewhere around 300-400mA with the CX-919 just idling on the Android desktop.
      Cheers,
      Darren.

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