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So basically you just need to pay attention to the CPU and the video card?

M

micrometers

Diamond Member
In calculating the size of the PSU, basically just take the CPU + vid card's power consumption? And add like 20 watts for miscellaneous wattage from like the hard drive, ram, lights, and disc drive?

I'm wondering if a 380 watt PSU could handle a Pentium G850 + Geforce GTX 560. Under load the Pentium G850 is like 55 watts, and the GTX is like 150 watts, for around 220 watts, more than enough room, right?
 
I add up to ~50W extra for the other stuff. It's good to have a bit of headroom, but you'll never be using full power on everything. So long as your numbers are correct (& depending on 12V specs of your PSU)...

380W should be a good size.
 
GTX 560 needs two 6-pin PCIe power connectors, I'd just get a PSU that provides two by default so you can avoid using adapters...
 
be quiet! Straight Power E9 400W ATX 2.3 (E9-400W/BN190) is probably one of the best power supplies for such a machine.
80+ gold certified (for what it's worth) and a 135mm fan. Not cheap, but not really expensive either. At least in Germany they used to have a next-day-exchange service as well, for the Straight Power series. Not sure if that's also the case in the US, or if they changed something, but while I did have a few dead PSU's from them, that's mostly because I used a lot of their PSUs during a time.

Regarding the initial question: It depends. I run a 72W CPU and no GPU in one of my machines, but 12 spinning hdds, an ODD and two SSDs, as well as a TV-Card, dual NICs etc...which at boot (12 disks spinning up) can probably push the installed 430W PSU close to its limits.
 
It really depends on the other components. 50W might be enough for someone with 1 SSD and a low price motherboard but someone with RAID 0 for 2 drives and a high end motherboard and extra sound card wont be able to fit in that. You need to add it all up.

Secondly add it all up and make that the 80% point of the PSU absolute maximum. The reason is that most PSUs are most efficient between 20% and 80% of their load so you want to always be within those. Setting it up so that you are much closer to the top will produce considerably more noise and use more power than necessary.
 
When I checked the Geforce GTX 560 needs about 400w, I have two of those in my system so ended up going with a 1000w psu. I'd go with at least 500w to be safe. Of course they'll only use that much power when being pushed to their limit, but still best to have lot of power capacity than not enough.
 
Red Squirrel said:
When I checked the Geforce GTX 560 needs about 400w, I have two of those in my system so ended up going with a 1000w psu. I'd go with at least 500w to be safe. Of course they'll only use that much power when being pushed to their limit, but still best to have lot of power capacity than not enough.
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A GTX560 is physically incapable of using 400 watts. That's TOTAL system power you are looking at and yes it means your 1000w is overkill.
 
Red Squirrel said:
Of course they'll only use that much power when being pushed to their limit, but still best to have lot of power capacity than not enough.
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Emphasis mine.

They actually won't use that much power ever. The power connectors you used to hook them up aren't even specified to deliver that much power, even if the card would try to draw it.

When companies tell you to buy an X Watt PSU for their graphics card, there are two things you need to keep in mind.

1) That's the wattage for the whole system, not the graphics card.

2) They have to account for the fact that their recommendation has to be valid for systems with an unknown amount of additional parts AND the potential for crappy no-name PSUs that don't deliver the wattage they're supposed to. Those factors combine to make GPU companies way overstate the power needs.
 
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The problem with calculating the needed wattage rating of a PSU is that there are vast differences between how powerful PSUs with the same rating actually are. A low end 600W PSU can quite easily lose to a high end 450W PSU.

Usually the +12V rating of a PSU tells you how powerful the unit actually is, but that's more of a generalisation than a rule. If the +12V rating is near the overall rated wattage you've got a unit that can probably output its rated wattage continuously. Or you may have a PSU that can actually output a lot more than its rated wattage, this depends on where the manufacturer has chosen to set the overpower limit. Or you may have a PSU that technically can output its rated wattage but it will do so at voltages that are out of spec (thus a danger to components).

All in all a PSU calculator is reliable since it will recommend according to the lowest common denominator, i.e. the crappy PSUs are taken into account. But I prefer to either look at actual power consumption tests with hardware similar to what I plan to use, using graphics card reviews by Anandtech or Guru3D or similar sites. Then, to be safe, I apply a reasonable overhead (+30-50% or so) on the estimated load power consumption and call the result my PSU's rated wattage.
 
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DSF said:
Emphasis mine.

They actually won't use that much power ever. The power connectors you used to hook them up aren't even specified to deliver that much power, even if the card would try to draw it.

When companies tell you to buy an X Watt PSU for their graphics card, there are two things you need to keep in mind.

1) That's the wattage for the whole system, not the graphics card.

2) They have to account for the fact that their recommendation has to be valid for systems with an unknown amount of additional parts AND the potential for crappy no-name PSUs that don't deliver the wattage they're supposed to. Those factors combine to make GPU companies way overstate the power needs.
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Did not know those ratings were for the whole PC, so they just calculate it based on the average cpu and components that would be used in a PC that uses that card?

The wire sizes in a PC is something I've always wondered about though. 400w at 12v would draw 33 amps. You practically need like 10 gauge wire for that. 😵
 
Red Squirrel said:
The wire sizes in a PC is something I've always wondered about though. 400w at 12v would draw 33 amps. You practically need like 10 gauge wire for that. 😵
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Incorrect, for the short distance a computer PSU is running wire very small gauge wire is more than sufficiant even for large loads.

And OP i think you will be fine with a quality 400w unit with 2 PCIe connectors.
 
Don't forget the motherboard. It's the thing that uses most power in idle nowadays. So which one are you using? And which psu you got exactly? 380W, most likely an Antec Earthwatts? 380W should be enough but I'm not sure about the distribution of those Watts among the 12V rails. Might be wiser to get a psu with native dual pci-e connectors (or a vidcard that needs only one).
 
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