What kind of power supply will I need?

I've tried 4 different power supply calculators and have gotten 4 very different numbers. I did a google search on power supply calculator and found a post on here, where I noticed you guys can do your own calculations. (would love it if you showed me how to do it for my own system)

CPU: i5 2500k
RAM: 8GB 2x4GB PC3-10666 DDR3-1333
HDD: 500GB Seagate Barracuda 7200RPM
SSD: Might buy one if I see on on sale.
Case: I know I need this to do a more accurate calculation, but I have figured out what case I want yet.
GPU: 2x Radeon HD6870
Other: Will need a wireless adapter, and not sure what else.

If you can do the calculations and also show me how, that would be a huge help. I haven't set any of these in stone yet, but this is the general direction I want to head in. But from the 4 different calculators I got 900W, 800W, 600W, and 400W (last one I'm pretty sure I forgot something, and second last one is the newegg calculator).
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  1. the 900w will probably be the safest option. you can get more qualitative information if u search for power consumption benchmarks for ur cpu and gpu's. they are gonna be ur main power users, rest is gonna be 150w max. make sure u have enough power to handle all those on max load, and pay attention to articles that dont give complete power use. generally i overclocking increases everything by the percentage increase of ur total voltage increase.
    on the top of my head id say ur cpu does 300w ish on max load, ur gpus do roughly 450-500w on max load
  2. Two 6870s will run on a 600-650W unit. Whats your budget for it? You can pick up a nice modular one for $69 after rebate

    Bigbangs power guesses are way off, an i5 2500K uses about 100W, each 6870 uses about 150W under full load, so about 300W for the pair, which puts you at about 400W for the CPU and GPUs, so about 500W of total power for the whole system.
  3. sorry the 6870s on crossfire will consume 400 on maxload, its weird all benches say diff things -.-
  4. right the first time it will draw 500w on max load
  5. My budget for the entire system is $1500 CAD, which includes OS/mouse/keyboard/3 monitors. What I have up there accounts for about ~$900 CAD. So I do have alot of legroom in terms of budget, the 3 monitors though I might buy them later so they aren't exactly included in the budget.

    So I have maybe ~$700 CAD, for PSU, case, keyboard, and mouse. I plan on taking advantage of a nice combo deal $99 for OS and 500GB HDD, and that is included in the ~$900 cost above.

    edit: Where are you guys getting your wattage numbers from? I can see that it's just simple addition, but I'm not sure how you get your wattage numbers.
  6. This is close to what yours would be
    The $1000 system is running a pair of 6850s which us slightly less than 2 6870s, the $2000 system is running a pair of 6970s which us a fair bit more, but thats also power at the wall so assuming about an 87% efficient PSU the $2000 one at stock clocks is drawing less than 500W from the PSU when under full load.

    Where i get my numbers from is many many benchmarks, but CPUs and GPUs also have power ratings given to them by their manufacturer, CPUs say it in the product description, GPUs tend not to but there are lovely wikipedia comparison tables for quick reference, the 6870 has a rated TDP of 151W, assume those are a bit low and round up a tad and you are close to real power consumption.

    Something very very important to remember when reading the power portion of any review or benchmark, power consumption is ALWAYS at the wall, which means that there is the efficiency of the PSU in between which makes the numbers at the wall higher than the actually power being pulled from the PSU.
  7. except that urs will be about 100watts higher.
    running the cards in sli does reduce their single usage in general,also makes them run cooler, so roughly 500w rather than 2x300w.
    these are worst case scenarios, so that when ur system is under full stress, u still wont crash. also take into account that ur psu wattage decreases year by year, and that u might want to add extra harddrives or fans thatll eat up more usage. also overclocking will be good use of ur components. so 900w or anything above that is gonna do u good.
    all the best
  8. i shall remind u hunter, that the toms hardware recommendation with a crossfire system was a 850w, for good reasons that u seem unaware of
  9. You seem to have missed the condition from the first one
    To measure power usage, a Kill A Watt P4400 power meter was used. Note that the above numbers represent the power drain for the entire benchmarking system, not just the processors themselves. For the 'idle' readings we measured the power drain from the desktop, with no applications running; for the 'load' situation, we took the sustained peak power drain readings at the end of a 30 minute OCCT power supply benchmark.

    The OCCT power supply benchmark loads up everything, not just the CPU or the GPU, so what that 329W at load represents is the power drawn by the CPU and the 5870 that they had in the system.

    and for the second one guru 3D puts the single card power consumption at about 141W per card which is a bit low compared to other things but about right, which would put the dual card power consumption at about 280-320W.

    Im aware that toms overestimates its power recommendations just like many other sites do, however, by plugging away at the math i know that the 650W unit i linked up above would be more than enough, and a good PSU made with quality components and good 105C rated capacitors wont lose much capacity over 5 years, and most of the good ones were capable of doing 15% higher than their rated capacity to begin with so it takes a long time before they cant do their rated power anymore.
  10. From an article on the guru3d website, was surfing around there for a bit.

    Measured power consumption two 6870 cards

    System in IDLE = 184W
    System Wattage with GPU in FULL Stress = 421W
    Difference (GPU load) = 237W
    Add average IDLE wattage x2 ~ 19W (ATI specified)
    Subjective obtained GPU power consumption = ~ 275 Watts


    Anyways the 421W would that change either of your guy's calculations?

    edit: nvm I don't think that bolded part means what i think it does. :(
  11. then i imagine around 700w-800w is what u need
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