If I I mostly (not always of course, otherwise it'd be illogical to buy such) use a PC for simple internet surfing, coding and office tasks, will my electricity expenses be higher if I buy a powerful Core i7 PC with a 800W PSU (and a correspondingly cool video card) compared to a humble Core i3 with some humble PSU (and on-board video)? Won't a more powerful system just slow-down significantly on a low-demanding task and consume approximately same power as a natural-born slower system on the same task?

I bother because at the same time I am going to invest into a new PC and to move to another accommodation with fully counted and paid electricity (currently I have virtually unlimited electricity and water included in rental price).


The 800W rating on the PSU is the maximum the output it can provide.

The PSU will only draw what it needs to from the mains, and power management nowadays is pretty good at reducing the power to components when not in use, so a "high powered" machine might only consume about the same as a "low powered" machine when both are idle. It might not, depends how good the components are at reducing their individual power consumption when idle.

That said, if you have extra components (a proper graphics card vs on board, extra hard drives) or connected devices (USB devices can draw power, for example), you should expect higher idle consumption.

Also note that PSUs have an efficiency rating (often in the range of 60-80% IIRC) so have to draw more energy than the computer requires, and this efficiency tends to be worse when they are under less strain (so using your 800W PSU to power 50W is going to be less efficient than using it to power 500W).

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    @DMA57361 - Do you have any thoughts on the other part of his question, though, regarding a more powerful system drawing more power? – JNK Sep 9 '10 at 12:43
  • @JNK I'm just editing at the moment... – DMA57361 Sep 9 '10 at 12:45
  • @DMA57361 - Excellent! Not pushing, I just value your opinion from other posts you have made. – JNK Sep 9 '10 at 12:48
  • ...and a +1 from me for the edit! – JNK Sep 9 '10 at 12:49
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    +1 for mentioning the efficiency rate bell curve. Using over-rated PSUs on systems that do not need such capacity will waste electricity. It is best to get a PSU that will allow the machine to idle at around 60% of its rating, for optimum efficiency. – paradroid Sep 9 '10 at 13:18
  • Even if you have 800W Power supply unit , The Computer System will take power only as much needed. It doesn't mean it will always take 800W.

  • Additional chipsets like graphic cards, sound cards etc consumes additional power.

For your reference as you mentioned about i7

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The PSU will not continually draw 800w, it will only use what it needs. However, if power bills concern you, make sure you get a green/80%+ certified devices. If you are a heavy user, these can pay for themselves very quickly.

An Intel Core I3 Has a max TDP of 73w, whilst an Intel Core I7 has between 82w and 130w

Windows and the processor has some smart features including the ability to "park" / Shutdown cores which are not currently in use in order to save power.

But to be honest, I am not sure how to answer this - a task will take longer on a Core I3 than an I7, I wonder if it is quicker to do a 20 second task on an I7 then shutdown, or the same task that may take 25 seconds on an I3 then shutdown!

I would personally go with the I7, these are all maximum values, and I would prefer the speed. In When you consider that we are talking about 20w, the cost of leaving the machine on 24x7x365 will be a matter of less than £5 a year.

  • if a system can park cores, will it usually do so when one core is enough but there are many threads running (as when a PC is used for simple office/web tasks)? – Ivan Sep 9 '10 at 14:33
  • I use to work and watch a video simultaneously (using 2 screens), and my current one-core 1.6 MHz Centrino laptop manages pretty well. – Ivan Sep 9 '10 at 14:37
  • @Ivan - Core can be parked seperately, whether they are or not depends on how your OS allocates tasks to them, I guess. I'm not sure how Windows manages it, but I've a feeling you can check the current status of cores in Windows 7 under "Windows Resource Monitor" somewhere. Not at home to check at the moment... – DMA57361 Sep 9 '10 at 15:26
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    @Ivan - Windows scans and checks hundreds of times a second, and can switch cores on and off very quickly, If the processor is at 2% usage with 20 threads, there is no need to start another core for and additional thread. It is very smart and does what is best at any given moment. – William Hilsum Sep 9 '10 at 15:28

Yes, quite possibly.1

For tasks like web browsing and "office", your CPU and GPU load, and consequently net power consumption will be very low, most likely around 50W or even less. My Yorkfield-generation desktop with dedicated GPU (GTX650Ti) and 2 solid state disks uses 75-76W for such "mostly idle" tasks, but modern CPUs are a lot more more energy-efficient.

A 800W PSU would thus run at less than 10% of its maximal output rating. Probably closer to 5% than to 10%.

Yes, efficiency on modern PSUs has improved, and their load-efficiency graphs gain a lot steeper and have a much greater plateau than they used to have, but even certified PSUs are no panacea for loads in the single-digit percent range, let alone a cheaper PSU.
Even a very expensive 80+ "gold" or "platinum" certified PSU will not perform optimally at such low loads (the certification gives a guarantee for 20% load, not for 5%).

A computer with a weaker CPU/GPU and a weaker overall PSU (which maybe runs at 25-35% net power) may thus very well be more energy efficient and factually pull less electricity out of the wall plug.

My more recent Skylake system which has a 450W 80+ gold rated PSU idles at about 35W with integrated graphics only, and 45W with a nVidia 1060-6G, respectively (2 SSDs, 64GB RAM).
With the CPU under full load, it draws about 120W, with GPU also under maximum load, highest ever measurement was 280W. All measurements on the wall plug, thus these are overall (gross) values. 280W < 450W, no worries there. For a 800W PSU, that would be 35% of the PSU's capacity at 100% load.

1 One has to assume PSUs of similar quality with similar/identical efficiency certification, of course.

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