How can you determine PSU needs for a video card?

Every time I get a bit dissatisfied with my current video card, I go looking at newegg. It’s the same every time: I search, and agonize over this feature or that feature - Nvidia or ATI, GT or XT, whatever - but when I get down to the card, I can’t pull the trigger, because I have no idea how to figure out the card’s power requirements. Sure, sometimes I get lucky and see someone mention in a review what a card requires, but that’s rare. So I slink off, saying to myself “someday I gotta figure out how to figure that out.”

Well, the day has come. Suppose I see a card like this one and want to figure out if my PSU is bulky enough. How the heck do I do it?

System Requirements: PCI Express based PC is required with one x16 lane graphics slot. available on the motherboard and one additional adjacent expansion slot. Connection to the system power supply is required: 450-Watt power supply or greater, 30 Amps on 12 volt rail recommended (assumes fully loaded system). For CrossFire: 550 watt power supply or greater, 38 Amps on 12 volt rail. 512MB of system memory. Installation software requires CD-ROM drive. DVD playback requires DVD drive and decoder software (not included).

I just googled for the card name and “power supply” and that turned up. I didn’t check other links, there’s probably more out there about it.

A rule of thumb:

If you don’t have an overclocked CPU and you’re only using one video card, then any decent-quality 450W power supply that actually has the PCIe power connector (almost all of 'em these days) should do just fine.

If you want to do SLI/Crossfire, figure a 550W.

If you’re overclocking your CPU, add another 50W or so.

Those are, of course, rules of thumb. Most of the hardware I test falls in the above rules of thumb and I almost never have power supply problems with a good 550W. It gets tricky when you start talking about SLI and overclocking, especially with multiple hard drives in RAID and stuff. You get into how many amps are available per rail in the power supply and which devices are on which rail, stuff like that.

On they often add info about power consumption for the entire test system in reviews of video cards. For instance, here is a nice overview for current high-end cards. This is for a system with these specifications.

For video cards the important thing to look at is how many amps you get on 12V. Cheaper PSU’s tend to divert more amps to 3V and 5V, and since Wattage rating just combines the total Amperage times the total Voltage you can have two 500W PSU’s where one gives out 20A on 12V and the other gives out 15A on 12V.

Thanks for the info, guys. Of course, I’m gonna find out my off-the-shelf Dell is too wimpy for much, but next time I do believe I’m a-gonna build a custom rig of some sort, and this info will be quite handy.

One additional question: if I’m trying to determine whether a specified power supply will work for a video card, do I just go by the PSU label (if it’s one I already own) to determine what amperage is available? You don’t go measuring stuff with a multimeter do you (and if you do, how do you determine anything)? I’ve been reading about PSUs here so I sort of get it, I kinda sorta think.

Considering that video card manufacturers, when they don’t just keep it simple and demand a “600W PSU”, just tell you they want so and so many Amps on the 12V rail, the PSU label should give you all the information you need.

Since it’s so hard to get solid information on PSU performance for the average user, anything more than this would just make customers confused I’m guessing.