Why do PSU calculator lie? Help!

[iloveToms]

I've had a lot of problems with computers over the years, but I never imagined anything quite as hard as picking a correct PSU!

I ordered some parts for a new computer. I was hoping to make it a cheap and very energy efficient machine with a killer GPU. Here it is:

AMD Phenom II X4 945 Deneb 3.0GHz 95W Quad (socket AM3)
ASUS M4A78T-E AM3 AMD 790GX HDMI ATX AMD Motherboard
G.SKILL 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666)
Sound Blaster X-Fi Xtreme (PCI sound card)
Run of the mill DVD-RW (SATA) and 750GB HD (also SATA)
and a VisionTek 900297 Radeon HD 5850 1GB 256-bit GDDR5 PCI Express 2.0 with one 80mm case fan and 4 USB devices (mouse,keyboard, scanner, and webcam)

So, idiot that was, I went to Thermaltake's PSU calculator to find out what I should get for a psu. I typed in everything above, and it said the max wattage draw should be around 314w

Well, excellent, I think to myself, I'll go to newegg and get something. I get a 380 Watt EarthWatts unit. I look on Wikipedia and see that the ATI 5850 uses as max of 151 Watts, with a 95 Watt CPU, that makes sense right? And I think to myself, there's over 70 watts of wiggle room, so I should be good!

Well, after I order the PSU I learn that you have to know about 'rails'. I look at the back of my PSU and it says this:

I read that the 12 V rail is the most important for graphics cards. If this is true, then PSU gives 17 Amps on one of the 12 rail, and that's 17 x 12= 204 Watts. That should more than cover the ATI 5850 which needs a max of 151 W right?

I need help! Can this PSU run my computer setup as is??? What do all those numbers on the back of my PSU mean???

Thanks!!

 

[iloveToms]

Hi Nowa,

Wow, great article, thanks for posting! They do a great job of breaking stuff down. I think I am pretty PSU savvy now thanks to the tips posted here and that article

As for the efficiency rates on power supplies the optimal load/sweet spot for maximising the efficiency varies depending on the power supply, but usually it is around 50-60% of the maximum load.

Thanks for that, that makes a lot of sense.

I just bought a really cool 550 Watt PSU that is guaranteed to be at 88% efficient at all loads!! 80+ Silver certified, yeah baby. I'm really excited as my current power supply purchased 4 years ago is (or was) 60% efficient, and that's without capacitor aging....yikes!

I hope to save a lot on my electric bill, what with this PSU and the components listed in the OP.

Thanks again everyone, this forum has been great to me 😀

 

[rawoysters]

You may not need 5 , but 1 x 80mm isn't going to provide much airflow to help cool your components.

I'm using an old 8600 GT I had lying around and my CPU/MB temps never get above 35C. So far, I see no need to buy an extra 4 fans!

I bought my components with energy efficiency in mind. The 95 W CPU does really well and doesn't give off a lot of heat

Let me know what you guys think about my current PSU! Thanks

You seem to be going for the absolute minimum with your thinking IE: one fan is plenty, minimum PS. This is a very dangerous way of thinking. Spend a few extra dollars on a better PS and a few more fans whether you think you need to or not. When you spend lots of money on other high end components and then go the cheap route with your PS and fans, you are risking major damge to your computer. Soend a few extra dollars, it will be very much worth the money.