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Building a low power PC on Skylake – 10 watts idle

Putting together a computer that has extremely low idle power consumption AND can handle some casual gaming is pretty tough. Just in case someone’s aiming to do it and isn’t sure where to begin, I figured I’d list some details about the machine I just put together so that you’ve got something to start with (or a basis for comparison).

CPU – Intel i3-6300

  1. Skylake architecture. Broadwell with Iris Pro was a consideration but lacked some DirectX12 and HEVC/VP8/VP9 improvements. Still waiting on AMD Zen and Intel Kaby Lake at the time of this writing.
  2. Best iGPU/frequency/TDP/price ratio of the Skylake processors (based on what’s important to me).
    • Top iGPU (HD 530 at 1150Mhz)
    • 3.8Ghz dual core (only the i5-6600 and i7-6700 match or exceed this at the 2-core turbo level)
    • 51watt TDP (looking at higher-specced processors, only the i7-6700T beats this with 35watts but at the expense of frequency)
    • Considerably less than the aforementioned i5-6600 and i7-6700(x) models.

    The i3-6320 has an edge at the list price with a bump to 3.9Ghz, but local price threw it out the window.

A final piece to note: a few testing tools I ran showed the iGPU only hitting 1100Mhz in Windows, despite Intel (and others) listing it as 1150Mhz. Not enough data on my end yet to determine whether I’m just seeing a bug or if something else is going on. Update: It does hit 1150Mhz, but very infrequently and only under specific loads. Think of it as a 1100Mhz iGPU that will magically touch 1150Mhz in those rare moments when the stars are all aligned.

Case & PSU – Antec ISK 110

  1. High efficiency PSU. Also rated in the 80w-90w max window, so low idle power (~10w or roughly ~12% PSU load) shouldn’t be too far out of it’s efficiency band.
  2. No fans – a standard “laptop brick” power supply and circuit board. Not that I care about the noise, but no need to waste an extra watt on the PSU fan if it can be avoided.

I went over the Antec ISK 110 in a separate review.

The other alternative I considered was a picoPSU with a separate case.

I wanted to avoid a larger PSU (anything nearing 200w or higher) because efficiency usually drops a bit harder at very low load percentages (very low wattages) and I didn’t want to start looking at shelling out money for something in the neighborhood of an 80PLUS “Titanium” model to retain efficiency at really small loads.

Hard Drive – Samsung EVO 850 SSD (500GB)

  1. Samsung is still owning it when it comes to SSD power consumption.
  2. Samsung electronics in general tend to be pretty good/reliable. They’re also one company that (so far) has managed to maintain a strong focus on remaining cutting-edge.

Alternatives here were a different SSD brand or possibly a lower-power laptop hard drive.

The i3-6300 and GB H110N inside the Antec Case (hard drive behind)

Motherboard – Gigabyte H110N

Not a lot to say here, except that Gigabyte’s been reliable for me long-term (all my Gigabyte motherboards from up to a decade ago are still working). Always decent BIOS settings, very few issues.

I would have preferred a step up to one of their wifi versions, but local stock wasn’t available. Also considered a Z170 board which would have allowed me to run the RAM at over 2166Mhz (the “H” versions of the Skylake chipsets cap it at 2166Mhz), but I wasn’t willing to shell out the large price increase for that alone.

Important: Installing the Realtek network driver on this motherboard reduces idle power consumption by 1 watt (compared to standard Windows 10 Realtek driver). I know 1 watt doesn’t seem like much, but it’s technically a ~10% savings since we’re in the 10 watt range. This was the last piece of the puzzle that got me from 11 watts to 10 watts idle.

RAM – 32GB Kingston DDR4 2400Mhz (2x16GB)

I’ve had a bad luck with many brands of RAM. Kingston’s still in my good books, and this was a cost-effective set.

Power Consumption and Quick Undervolting Tests:

This was measured from the wall, and are values using the Realtek Network driver (which shaved off a watt), in an otherwise clean Windows 10 install.

  1. Standard voltage settings:
    -10w idle
    -69w load (Prime95)
  2. Undervolt of -0.10v (BIOS):
    -10w idle
    -58w load (Prime95)
  3. Undervolt of -0.12v (BIOS):
    -10w idle
    -56w load (Prime95)
  4. Undervolt of -0.15v (BIOS):
    -10w idle
    -FREEZE (Prime95)

Undervolting had no discernible effect on idle power consumption. The real benefit was in reducing load power consumption which is nice not only for the savings there, but also because the 80-90w PSU it’s running on doesn’t leave a whole lot of overhead when we’re near the 70w consumption region.

In any case, every -0.01v drop in CPU vCore seems to equate to roughly a 1w reduction in load consumption.

Before anyone asks, I did try GPU undervolting too, but it also had no impact on idle consumption. Values of -0.10v to the GPU caused crashes when waking from sleep, and even -0.05 met with some periodic instability in 3DMark.

My gut feeling is that you’re probably not going to be able to bump the iGPU voltage down much (if at all), and will likely hit -0.10 as a realistic max undervolt on the CPU, possibly having to shave that down to -0.08 or so depending on stability.

9 Comments

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  1. What was your power meter in tests?

  2. I got 1 W off by undervolting memory. My memory is XMP-2400 @ 1,650 V DDR3. Have you tried undervolting memory?

  3. A little more than a year down the line, I’m looking into a similar build and, after some research, have come to the conclusion that the i5-7400T has the best iGPU/frequency/TDP/price ratio at this time. It may be important to note I’ve also thrown in passmark score as a variable.

    I’m curious whether you would agree or if the i3-6300 still takes the cake in your book?

    • Given the choice between the 2 today, I’d definitely go for the i5-7400T. However, keeping in the Kaby Lake line, if the i3-7300T happens to be widely available (and is cheaper) in your area, it has a bit more Mhz on the CPU (and iGPU) than the i5-7400T so could be worth consideration if your use case isn’t likely to exceed 2 cores. My first couple searches showed that the 7400T can be had for less than the 7300T though, so the i5 you were looking at is probably the way to go.

      • Anonymous

        I’m building a low power PC to use as a file server, music/photo/video streamer and TV server and would be grateful for your input. I was set to go for the i3 7100T, but then saw the i5 7400T on offer (although still way more expensive here in the UK than in the US).

        I’m planning to use the PC as a some time web browser, CD/DVD ripper, etc. (general home PC), to get the most use out of it.
        I don’t do any PC gaming.
        Is the i5 overkill for my needs?

        I don’t want to end up being hobbled by the CPU, but equally I want to keep costs down.
        I’ve bought an 80+gold Corsair CSM 550w PSU, as I read that this would help keep power consumption down.

        • The i5 would probably be a little overkill for most of that, although if the price difference between them where you are is negligible, I’d probably lean towards the i5 just because you get the extra couple cores which gives you a little more flexibility if you decide to repurpose the machine in the future. If the price difference is pretty large though, I’d likely hit the i3.

          However, if in addition to ripping the DVD’s you’re going to be doing a lot of *encoding* (ie through Handbrake), then I’d put a bit more weight towards the i5. Not that the i3 won’t do it, but the i5 will do it a bit faster (the 4 slower cores should outweigh the 2 faster cores + HT), which starts to matter if you’re ripping/encoding your entire DVD collection.

          • Anonymous

            Thanks for the feedback.

            The i3 is £108 here in the UK and the i5 (7500T now), costs £192, so a fair difference.
            However, I don’t want to buy the i3 if I then find it’s not powerful enough.
            My TV server will be Media Portal, which means I’ll have to install SQL or mySQL, plus I will install iTunes and I’ll also have a DVB-S2 HD TV tuner card in it.

            Would an i3 7100T have enough grunt? I really have no idea!

          • If it were me I’d likely be going with the i3 at that price difference, since it should easily handle that workload under the really common/standard use cases. An exception (beyond what I mentioned earlier) would be if you’re recording through your TV tuner and want to have it dump directly to H264 or HEVC through software that doesn’t support hardware-encoding – in that case depending on the software and the settings an i5 might make little difference, or might make all the difference. Some software’s efficient and some is bloated and terrible. Another exception would be if you’re streaming to multiple devices in the home since live-transcoding multiple streams tends to start gobbling a lot of CPU power pretty quickly.

            If you’re really uncertain, you might consider peeking around any forums for the specific media center and TV tuner you’re planning to use and see what the experience of others has been with different hardware. If not, the extra £84 may or may not be worth the extra peace of mind.

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