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i'm setting up a computer with an LGA1151 cpu socket, and i'm looking for the cpu that would use the least amount of electricity for this motherboard. the cpu will not be stressed in the slightest, and does not need to be fast. i already have a computer doing the same tasks, running on a 2-core 35W TDP i3-7300T with hyperthreading disabled, and cpu frequency locked to 800MHz, and htop reports the cpu usage as being roughly "5-8% per core"

going by TDP, it seems the 25W TDP Intel Xeon E3-1235L V5 is my best bet, but i don't know if TDP is a good indicator, because that's "power consumption during high-intensity workloads", which is not what i'll be using it for.. suggestions?

(also i'm confused at how the 4-core Xeon can use less power than all the 2-core / 1-core alternatives, going by TDP, but i guess that should be a separate question)

  • Xeons tend to underperform in single-threaded apps, and this is why. They're designed to go in server farms, where a 1w difference in TDP is often a 100w difference in a rack. However, it sounds like you're using hardware for something that might suit your purpose better (and using less power overall) with one single physical machine running virtual machines. – CDove May 15 '18 at 12:25
  • @CDove VMs is not an option, it controls some hardware on-location. it sure sounds like the perfect job for a Raspberry Pi, but the hardware requires a specific motherboard, sporting an LGA1151 socket. (hope to find some way to connect the hardware to an rpi in the future tho) – hanshenrik May 19 '18 at 10:41
  • This seems to be a bit of an XY problem. Why do you need whatever you're running to be on the 1151 platform? For something that requires this little processing power, it seems like the additional cost, complexity, and power draw introduced by a full desktop platform doesn't make sense. – JMY1000 Jun 23 '18 at 4:55
  • @JMY1000 i don't need 1151, but i need to control 18x RX580 GPUs, which have PCI-e connectors, this motherboard seems like the only board with the hardware to do it. the job does not require much from the cpu at all, and thus i'd like to save on electricity by using a cpu that use very little electricity. – hanshenrik Jun 23 '18 at 11:10
  • @hanshenrik I see. Are you mining? – JMY1000 Jun 24 '18 at 1:27
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TL;DR: Get a Celeron G3900 instead.

Although you are correct in that the Xeon E3-1235L v5 and Xeon E3-1240L v5 have the lowest rated TDP (both at 25W), I would recommend staying away from these processors.

Power consumption

One thing to understand straight away is that under most conditions, the actual power consumption of the process will not be equal to the rated TDP. The rated TDP only measures the maximum thermal output the processor is capable–that is, under full load. For example, the i7-6700K is rated for 91W, but only consumes 3.4W at idle. Note that this does not necessarily scale with rated TDP: the i3-3240 and i7-3770K have vastly different rated TDPs, but both consume roughly the same amount of power at idle.

One thing you've noticed is that L and T series processors can consume significantly less power, even at idle/low load (although I'm not entirely sure what Intel does under the hood to do this.) However, this difference isn't necessarily huge–especially given their lower performance due to lower clocks.

Given the nature of what you're doing, I wouldn't expect the CPU to be under full load most of the time, and therefore, I don't believe it makes sense to buy an L or T series processor given...

Cost

The cheapest L series v5 Xeon (the E3-1235L v5) has an MSRP of $250.00 (if you can find one at all.) Meanwhile, the Celeron G3900 is only $30.61 with a rated TDP of 51W. At the US average of $0.12/kWh, even at full load, it would take roughly 8 years of continuous usage to offset the cost differential between the two processors. It simply doesn't make sense to buy a more expensive processor.

Compatibility

The final nail in the coffin is that v5 Xeons simply aren't compatible with motherboards with consumer chipsets–including B250. While you could get a lower TDP consumer CPU, the price differential (a delta of $67.38 as of right now for the cheapest T series processor, the i3-7100T) probably isn't worth it.

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  • the G3900 seems great then, if TDP doesn't mean much for idle consumption (which is contrary to what Intel CS said :< ) - The final nail in the coffin is that v5 Xeons simply aren't compatible with motherboards with consumer chipsets–including B250 - gotdammit. i already ordered a Xeon E3-1235LV5 before you wrote this. – hanshenrik Jul 6 '18 at 13:14
  • However, this difference isn't necessarily huge - 1 significant difference if the usage is small enough, is that i don't need a fan to cool the cpu. my i3-7300T locked at 800MHz use this small heatsink, and doesn't go above 65c, and in fact, i can even lock it at 2GHz and run the Furmark CPU Burner for over 30 minutes, and it will hover around 80c. i'll be a bit disappointed if i have to cool the G3900 locked at 800MHz with a fan, but i'll check i'll check it out :) – hanshenrik Jul 6 '18 at 13:18
  • (but if i don't lock the speed, and run it at 3.5GHz, it goes over 90c in under 60 seconds, which is why i don't let the clock run unlocked - if some program hung up and started using 100% cpu and the clock wasn't locked, it would probably destroy my cpu) – hanshenrik Jul 6 '18 at 13:19
  • @hanshenrik You could always get a bigger cooler or run the fan at a lower voltage, both of which would decrease power consumption. Still, the power consumption of the fan shouldn't be above 1.5W or so, so it shouldn't really make a difference in the grand scheme of things. – JMY1000 Jul 6 '18 at 13:26
  • really? the intel stock fans use <=1.5W? – hanshenrik Jul 6 '18 at 13:28
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i asked Intel Customer Support about it, they said that TDP "should" also be a valid metric for low-intensity workloads, and that the Xeon E3-1235LV5 and Xeon E3-1240LV5 are the lowest TDP options for LGA1151... so the CPU i'm looking is probably 1 of those 2.

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