PSU wattage and safety margins

CPU
219.99
Intel Core i5-3550 Ivy Bridge 3.3GHz (3.7GHz Turbo) LGA 1155 77W Quad-Core Desktop Processor Intel HD Graphics 2500 BX806237I53550
http://www.newegg.com/Product/Prod [...] 6819116505

Motherboard
79.99
ASRock B75 PRO3 LGA 1155 Intel B75 HDMI SATA 6Gb/s USB 3.0 ATX Intel Motherboard
http://www.newegg.com/Product/Prod [...] 6813157314

RAM
42.99
Patriot Gamer 2 Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model PGD38G1600ELK
http://www.newegg.com/Product/Prod [...] 6820220558

GPU
349.99
ASUS HD7870-DC2-2GD5 Radeon HD 7870 GHz Edition 2GB 256-bit GDDR5 PCI Express 3.0 x16 HDCP Ready CrossFireX Support Video Card
http://www.newegg.com/Product/Prod [...] 6814121624

PSU
79.99
Antec EarthWatts EA-650 GREEN 650W ATX12V v2.3 SLI Ready CrossFire Certified 80 PLUS BRONZE Certified Active PFC Power Supply
http://www.newegg.com/Product/Prod [...] 6817371044

SSD
74.99
Mushkin Enhanced Chronos MKNSSDCR60GB 2.5" 60GB SATA III Asynchronous MLC Internal Solid State Drive (SSD)
http://www.newegg.com/Product/Prod [...] 6820226247

HDD
99.99
Western Digital Caviar Green WD10EARX 1TB IntelliPower 64MB Cache SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive
http://www.newegg.com/Product/Prod [...] 6822136939

DVD-ROM
22.99
LITE-ON 24X DVD Writer 24X DVD+R 8X DVD+RW 12X DVD+R DL 24X DVD-R 6X DVD-RW 16X DVD-ROM 48X CD-R 32X CD-RW 48X CD-ROM Black SATA Model iHAS424-98 LightScribe Support
http://www.newegg.com/Product/Prod [...] 6827106335

Case
79.99
Corsair Carbide Series 300R Black Steel / Plastic ATX Mid Tower Computer Case
http://www.newegg.com/Product/Prod [...] 6811139011


So for the build listed above, I have spec'd out a 650 watt PSU, which I think may be overkill. The newegg.com calculator doesn't provide estimates for the 7870HD GPU.... I ran some calculations in Excel using estimates from Scott Mueller's upgrading and repairing PC book and came out with a worst case number of 582 watts (I chose high for all estimates).

Couple of questions:
If the power supply is oversized and the components don't draw the full amount does it still waste power?

Is it ideal to have a smaller supply to the system?

Can someone describe the whole bronze, silver, gold, etc certifications?


Thanks for any input!
30 answers Last reply
More about wattage safety margins
  1. 1) A psu will draw only the power that it needs.

    2) It is not wrong to overprovision the psu a bit. A psu operates most efficiently in the middle third of it's range.
    The Antec you picked is a fine choice.

    3) The efficiency certifications indicate how much power is lost when converting wall power to PC usage.
    Bronze starts at 80% efficient, going up through silver/gold/platinum at 90%
    It also depends on 110v vs 240v. More efficient psu's will generate less heat, and be quieter as a rule.
    Read more here:
    http://en.wikipedia.org/wiki/80_PLUS

    Here is the AMD recommendation for a 7870 card:
    http://www.amd.com/US/PRODUCTS/DESKTOP/GRAPHICS/7000/7870/Pages/radeon-7870.aspx#/2
  2. ^ +1.

    I always like to do my own calculation when it comes to PSU's.

    roughly the sum of the following, plus a 25% head room.

    CPU TDP
    # of power connectors required for graphics card x power draw of each
    75w for board graphics power draw
    100w for overheads (fans, lights, ocing w/e)

    For your build that would be,
    77 + 75x2 + 75 + 100 = 402 + 25% = 502.5W.

    So a 500W should be plenty.
  3. Here's how I approached mine (again all worst case; I googled a couple of devices to get a more exact #):

    From book: (units are W)
    mobo 50-75
    Processor 25-150
    Ram 5-15
    integrated video 5-15
    vid card 25-250
    pci card 5-15
    pcie card 10-25
    HDD 15-30
    optical drive 15-35
    fan 3-5
    USB (used port 2-5)

    My system
    mobo 75
    Processor 77 (from google)
    Ram 2x15
    integrated video 15 (think this is not running because of the dedicated gpu, but still draws power)
    vid card 250
    pci card 30
    pcie card 0
    HDD 30 ( I think the SSD uses less power than HDD, but assumed both were HDDs)
    optical drive 35
    fan 4x5
    Usb 4x5
    Total 582

    The book recommends multiplying by a factor of 1.5 for overhead, but I think that using the maximum values for each achieves the same effect.
  4. That makes sense (psu is more efficient in middle third of range). So, if I decide to downgrade the video card to the $250 7850, which would drop the price of the build to ~$950, a potential upgrade would be a silver or gold certified PSU. One of the other goals of the build is to make a quiet running system.

    Does most of the noise from a system running come from the power supply fans? Secondary fans I guess would be on the GPU.
  5. Primary noise source under load is the GPU fans.

    As a noise source a quality PSU won't intrude much on a quiet system, even under load. If your goal is a quiet system a higher wattage rating might make more sense than a Gold or Platinum efficiency rating. The higher wattage rating would delay the onset of cooling ran speed up, probably more so than the higher efficiency rating alone would do.
  6. HD 7850 gaming load sound levels:
  7. The most noise typically comes from the CPU HSF and the GPU HSF; they use the most power, generate the most heat andrequire the most cooling. The PSU fans won't start running hard unless you are pulling a lot of power with your system. Additional power draw makes the PSU work harder to provide the power...and causes more heat. The EA-650D is more than sufficient to power your PC; it is actually sufficient to run 7850s in CF. A good quality 500-550W PSU would be sufficient to run either the 7870 or the 7850.

    It is not necessarily bad to have a slightly over-spec'd PSU unless you go way overboard and drive the PSU to operate at loads where it is less efficient. That being said most good quality PSUs maintain efficiencies over 80% even at very low loads and low-loads mean the PSU will typically operate more quietly. If you've already bought the PSU, then don't worry...it is sufficient and should operate quietly in your PC. If you haven't bought the PSU, then you might be able to save some money by getting a good quality 500-550W PSU and that will allow you to put that money into other components like an SSD or 7870 instead of the 7850.
  8. Rugger said:
    The most noise typically comes from the CPU HSF and the GPU HSF; they use the most power, generate the most heat andrequire the most cooling. The PSU fans won't start running hard unless you are pulling a lot of power with your system. Additional power draw makes the PSU work harder to provide the power...and causes more heat. The EA-650D is more than sufficient to power your PC; it is actually sufficient to run 7850s in CF. A good quality 500-550W PSU would be sufficient to run either the 7870 or the 7850.

    It is not necessarily bad to have a slightly over-spec'd PSU unless you go way overboard and drive the PSU to operate at loads where it is less efficient. That being said most good quality PSUs maintain efficiencies over 80% even at very low loads and low-loads mean the PSU will typically operate more quietly. If you've already bought the PSU, then don't worry...it is sufficient and should operate quietly in your PC. If you haven't bought the PSU, then you might be able to save some money by getting a good quality 500-550W PSU and that will allow you to put that money into other components like an SSD or 7870 instead of the 7850.


    Yeah, based on the graphs, the 7870 is pretty quiet under load - for the types of games I'll be playing initially (SC2, D3) on max settings this would be the equivalent of running the 7870 in the middle third of it's range as I think it's overpowered for those applications.

    In terms of the power supply do you have any recommendations? I need to try to drop about $60 from the build, while not compromising quality (I think the PSU that I have selected is a pretty good one)? Another option is to buy a PSU with less output (e.g. 550 w) but I still would like to target running in the middle third of the range, as well as maintain the high efficiency... gee I hope this all make sense lots of variables to be optimized :)
  9. WR2 said:
    Primary noise source under load is the GPU fans.

    As a noise source a quality PSU won't intrude much on a quiet system, even under load. If your goal is a quiet system a higher wattage rating might make more sense than a Gold or Platinum efficiency rating. The higher wattage rating would delay the onset of cooling ran speed up, probably more so than the higher efficiency rating alone would do.
    http://desmond.imageshack.us/Himg181/scaled.php?server=181&filename=hx750noisechart.gif&res=landing



    It sounds like something in the 600 w range would be ideal then and stick with the 80 plus certification.
  10. EarthWatts EA-650 is a good choice in PSU.
    That Corsair HX 750 chart was just an example of a PSU noise/power curve, and not a recommendation.

    Case choice and GPU choice are the best options to carve out an extra $60 budget savings. Checking for combo deals might turn up some savings too.
  11. Rugger said:
    The most noise typically comes from the CPU HSF and the GPU HSF; they use the most power, generate the most heat andrequire the most cooling. The PSU fans won't start running hard unless you are pulling a lot of power with your system. Additional power draw makes the PSU work harder to provide the power...and causes more heat. The EA-650D is more than sufficient to power your PC; it is actually sufficient to run 7850s in CF. A good quality 500-550W PSU would be sufficient to run either the 7870 or the 7850.

    It is not necessarily bad to have a slightly over-spec'd PSU unless you go way overboard and drive the PSU to operate at loads where it is less efficient. That being said most good quality PSUs maintain efficiencies over 80% even at very low loads and low-loads mean the PSU will typically operate more quietly. If you've already bought the PSU, then don't worry...it is sufficient and should operate quietly in your PC. If you haven't bought the PSU, then you might be able to save some money by getting a good quality 500-550W PSU and that will allow you to put that money into other components like an SSD or 7870 instead of the 7850.


    Also regarding the CPU, I think that the i5 3550 (less power requirement, fast speed) helps compliment the goal of low noise. I don't intend to OC. I'm not sure if the ivy bridge processor runs with just a heat sink or heat sink + fan; my computer that I'm on right now I purchased in 2001 and it just uses a heat sink :) with small fan. The graphics card just crapped out last week and I'm running off the integrated chip on the 2001 mobo :(
  12. WR2 said:
    EarthWatts EA-650 is a good choice in PSU.
    That Corsair HX 750 chart was just an example of a PSU noise/power curve, and not a recommendation.

    Case choice and GPU choice are the best options to carve out an extra $60 budget savings. Checking for combo deals might turn up some savings too.



    Thanks WR, yeah I thought that the Earthwatts is pretty solid based on reviews and I don't really want to skimp on the quality of the PSU (I think that it happens a lot when people build a computer). Also the case I have selected has a $10 rebate that is not listed in the 79.99 price, although saving that up front is always better.
  13. 3550 uses a lightweight stock Intel heat sink fan. It's pretty quiet at most workloads. And in gaming it's probably on par with the GPU fan noise.

    CPU/MB combo deal saves $10
  14. Antec Three Hundred Illusion Black Steel ATX Mid Tower Computer Case $40 after rebate and $5 promo code. $30 savings.
    http://www.newegg.com/Product/Product.aspx?Item=N82E16811129066
  15. The 7870 only requires a 500W PSU with (1) 6-pin 75W PCIe power connector. The Corsair CX500 is sufficient for your build and is only $40 after MIR: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027
    The EA-650 is sufficient for (2) 7870s in CF. If you want to step up the quality a bit more, then the Seasonic S12II-520 is more than sufficient for your build: http://www.newegg.com/Product/Product.aspx?Item=N82E16817151094&Tpk=s12ii-520 and costs $10 less.
  16. WR2 said:
    3550 uses a lightweight stock Intel heat sink fan. It's pretty quiet at most workloads. And in gaming it's probably on par with the GPU fan noise.

    CPU/MB combo deal saves $10



    Excellent, any recommendations on the case? I think that's the next item I could achieve the savings on.

    This might be a dumb question but I noticed that the Earthwatts doesn't come with a power cord... They make these things idiot-proof by designing the slot that it can only accommodate the correct cord/plug right? I would be able to use any cord that fits into the back slot of the psu (e.g. my 11 year old computer power cable?)
  17. WR2 said:
    Antec Three Hundred Illusion Black Steel ATX Mid Tower Computer Case $40 after rebate and $5 promo code. $30 savings.
    http://www.newegg.com/Product/Product.aspx?Item=N82E16811129066



    With this case, would I lose the USB 3.0 functionality that the mobo has? The case only has USB 2.0 ports on it
  18. Rugger said:
    The 7870 only requires a 500W PSU with (1) 6-pin 75W PCIe power connector. The Corsair CX500 is sufficient for your build and is only $40 after MIR: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139027
    The EA-650 is sufficient for (2) 7870s in CF. If you want to step up the quality a bit more, then the Seasonic S12II-520 is more than sufficient for your build: http://www.newegg.com/Product/Product.aspx?Item=N82E16817151094&Tpk=s12ii-520 and costs $10 less.



    Is Seasonic a better quality mfg. than Antec? By dropping down to the 520, would I still be operating in the middle third of the PSU range (to minimize heat and fan noise)? Factoring in 3.99 shipping, it's really only a $6 savings and I lose 130 w of capacity. That is why I ask about the quality of the mfg. comparison.
  19. You would need to use the rear USB 3.0 ports for USB 3.0 devices.
    On the front panel you would lose USB 3.0 ports.
  20. WR2 said:
    You would need to use the rear USB 3.0 ports for USB 3.0 devices.
    On the front panel you would lose USB 3.0 ports.



    Yeah, I'd like to keep them, if possible they are convenient on the front.
  21. The upgraded Antec Three Hundred Two has USB 3.0 ports on the front.
    But it's the same price as the Corsair 300R after rebate.

    Did you find a HD 7870 with a good rebate?
  22. HIS IceQ Radeon HD 7870 $349 before $20 rebate and free shipping. About $28 savings over Asus model.
  23. WR2 said:
    HIS IceQ Radeon HD 7870 $349 before $20 rebate and free shipping. About $28 savings over Asus model.



    Are there any quality differences between the manufacturers to be concerned about?
  24. NZXT Tempest 210 Mid Tower Computer Case $45 a/r and free shipping
    1 x USB 3.0, 1 x USB 2.0 on front panel. Front 120mm fan & top 140mm fan.
  25. WR2 said:
    HIS IceQ Radeon HD 7870 $349 before $20 rebate and free shipping. About $28 savings over Asus model.



    Also, regarding the case and the PSU - Is it better to have a top mounted PSU in the case? I would think this improves airflow and reduces dust getting into the fans (since it is not sitting on floor)
  26. It's hard to quantify that. The same GPU and board components, course.
    The differences would be in the fans/cooling arrangements and even those aren't much different.
    HIS has a reputation for making quieter graphics cards.
  27. I think you'd be hard pressed to find a modern gaming case with a top mounted PSU.

    The heavy CPU in the bottom of case gives better stability.
    And a bottom mounted PSU is often flipped so the intake is from inside the case and exhaust to the outside of the case. Minimizes dust issues.
  28. WR2 said:
    It's hard to quantify that. The same GPU and board components, course.
    The differences would be in the fans/cooling arrangements and even those aren't much different.
    HIS has a reputation for making quieter graphics cards.



    Yeah, true depending on the size and space I could always put the computer on the desk to minimize dust build up.
  29. machine67891234 said:
    Is Seasonic a better quality mfg. than Antec? By dropping down to the 520, would I still be operating in the middle third of the PSU range (to minimize heat and fan noise)? Factoring in 3.99 shipping, it's really only a $6 savings and I lose 130 w of capacity. That is why I ask about the quality of the mfg. comparison.

    I doubt that your PC will pull more than 300W at load in real world usage. PCs use much less power than most people think. Take a look at this article on power consumption from 3 modern PCs at SPCR: http://www.silentpcreview.com/article1245-page1.html
    They don't go over 400W until using an SLI config and a power hungry CPU. Your PC will use less power than the 2500K/6870 PC and it only used 271W at full load running Prime95/Furmark. nVidia and AMD already factor in sufficient extra power in their PSU recommendations to put total system usage in the PSU efficiency sweet spot of 40-70% load.

    Antec is a great PSU mfr, but the S12II-520 is higher quality than the Antec EA-650D. The Seasonic is 80+ Bronze and has a 5 yr warranty while the EA-650 is 80+ and has a 3yr warranty.
  30. CPU + Mobo combo
    289.98
    ASRock B75 PRO3 LGA 1155 Intel B75 HDMI SATA 6Gb/s USB 3.0 ATX Intel Motherboard bundled with
    1x Intel Core i5-3550 Ivy Bridge 3.3GHz (3.7GHz Turbo)
    http://www.newegg.com/Product/Comb [...] cvhvvq32ql

    RAM
    42.99
    Patriot Gamer 2 Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model PGD38G1600ELK
    http://www.newegg.com/Product/Prod [...] 6820220558

    GPU
    349.99
    HIS IceQ H787Q2G2M Radeon HD 7870 GHz Edition 2GB 256-bit GDDR5 PCI Express 3.0 x16 HDCP Ready CrossFireX Support Video Card
    http://www.newegg.com/Product/Prod [...] 6814121624

    PSU
    79.99
    Antec EarthWatts EA-650 GREEN 650W ATX12V v2.3 SLI Ready CrossFire Certified 80 PLUS BRONZE Certified Active PFC Power Supply
    http://www.newegg.com/Product/Prod [...] 6817371044

    SSD
    74.99
    Mushkin Enhanced Chronos MKNSSDCR60GB 2.5" 60GB SATA III Asynchronous MLC Internal Solid State Drive (SSD)
    http://www.newegg.com/Product/Prod [...] 6820226247

    HDD
    99.99
    Western Digital Caviar Green WD10EARX 1TB IntelliPower 64MB Cache SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive
    http://www.newegg.com/Product/Prod [...] 6822136939

    DVD-ROM
    22.99
    LITE-ON 24X DVD Writer 24X DVD+R 8X DVD+RW 12X DVD+R DL 24X DVD-R 6X DVD-RW 16X DVD-ROM 48X CD-R 32X CD-RW 48X CD-ROM Black SATA Model iHAS424-98 LightScribe Support
    http://www.newegg.com/Product/Prod [...] 6827106335

    Case
    54.99
    NZXT Tempest 210 CA-TP210-01 Black Steel / Plastic ATX Mid Tower Computer Case
    http://www.newegg.com/Product/Prod [...] 6811146084

    Total is 1015.91, however there are $30 in rebates, and free shipping on everything.

    Final cost 985.91

    Here's the updated build: The combo deal on the mobo and CPU shaved off $10, and the HIS IceQ 7870 had free shipping plus a $30 rebate. Also changed the case, I lost 1 USB 3.0 port on the front, but kept 1. Price was reduced by $25. Also decided to stick with the Antec PSU, because based on reviews they seem to be a pretty solid mfg.

    Thanks for the link to the discounted 7870, WR!
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