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i've just bought a new Radeon HD5850. Seems to be working fine, etc. BUT, when I turn the computer on, it sounds like .. hmm .. maybe the HD's try to spin up 3 times during POST. And POST always has some weird error .. but if I continue, then it's all fine.

So i'm wondering if my PSU is not powerful enough to handle what i've got in my box, now that i've replaced my old 8800GTS with this new 5850.

PSU = Antex True Blue 480W
CPU = Q8400 @ 2.66Ghz (stock, not OC'd)
Ram = DDR2 PC2-6500 2x2Gig (single channel :( stock, not OC'd).
Mobo: P5N-EM HDMI (nForce 630i/GeForce 7100 ... onboard graphics disabled).
HD = 2 :: 1x500Gig sata and 1x80gig sata.
Dvd = 1 x pata
other: usb mouse, ps2 keyboard, usb external HDD 500Gig.

So .. is there any way I can see if i need a bigger PSU without having to buy one and then test that out?

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I think your symptoms are fair indicator that system needs more power. Also, the PSU calculators are not reliable. – Sathya Jul 24 '10 at 12:18
@Sathya I think it's more an indication his PSU might be failing, personally. Hard drives and power supplies are the most mechanical and failure prone bits in any PC. – Jeff Atwood Sep 25 '11 at 8:20
up vote 3 down vote accepted

This power consumption calculator is pretty comprehensive. It will tell you the total max wattage draw for your system. You can then look up how efficient your PSU is (most are 70-80%) and get a ballpark for how many watts your PSU can actually provide.

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Hmm. interesting. So that calculator is suggesting that my usage is around 286W .. and recommends a 336W minimum PSU .. so my 480W PSU should be able to handle this... hmm. maybe it's not the PSU then .. :( Do those numbers sorta sound right, for my rig? – Pure.Krome Jul 24 '10 at 4:00
@Pure - Over time PSUs deteriorate. Those numbers sound about right for a system with that config. There's a chance that if your PSU is older, it could be 50% efficient or worse. – MDMarra Jul 24 '10 at 20:11
hmmm. i might try and borrow a psu from somewhere to see if that helps.. ?? – Pure.Krome Jul 25 '10 at 23:49

Warning: A lot of the power calculators out there produce ridiculously overspec'ed PSU recommendations.

If you want to be precise, you need to look up the max TDP watts of

As these components will be by far the largest consumers of power in your system... unless you have an array of ten 10k RPM hard drives inside your PC, or something else extremely unusual. I would also add that two (or more) high end video cards in SLI definitely will require a massively larger PSU than almost any other type of build.

Anyway, start with CPU and video card peak power consumption watts -- honestly, everything else is almost a rounding error beyond that. Maybe 50w for all the other parts inside your computer (motherboard, dvd-rom, hard drive) speaking conservatively. I've built HTPCs that idle at 45 watts (2008) and 17 watts (2011), so once you knock out video and CPU, it just doesn't take that much power to drive all the other bits.

Also remember that it's mostly important to have a high quality PSU rather than one with a giant number printed on the outside.

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