# Minimum System Power

#### anon8792

##### Distinguished
Hello,
I was wondering if having a power supply below the recommended system power but giving enough watts would still work. My computer has 400 watts and before I installed that it was 250. If the only thing changed was a graphics card that had a max power usage of 150 watts but needs a 450 watt psu, would it still work, or would it fry my computer even though it's technically within power requirements.

Edit: If it's a bit jumbled and long. computer uses 250 watts, and is has a 400 watt psu, so it has 150 it can use for whatever, but this card says it has a system power requirement of 450 but only uses 150 watts.

##### Titan
Moderator
Unfortunately, swapping power supplies isn't as easy as "well it ran on 250 watts, now just adding a 150 watt requirement graphics card, all I need to do is add 150 watts to my system".

We'll use This Antec Neo 400 watt PSU as an example.

There's a bit of math involved here so bear with me. On the side of the power supply, there should be a sticker that shows power outputs along each of five paths (more if your power supply has multiple +12v rails). If you click on the images of the linked power supply, you'll see the sticker.

They are:

+3.3v
+5v
+12v
-12v
+5VSB

Each of these rails has an amperage assigned to it which denotes the maximum output. In the case of the Antec Power Supply they are:

+3.3v @ 20A
+5v @ 20A
+12v @ 30A
-12v @ 0.8A
+5VSB @ 2.5A

To get the wattage for each rail, multiply the volts(v) times the amps (A) and add it all up. For a total of 528.9 watts... Hey wait a minute! This is only a 400 watt PSU! What's going on? Well that's because the values listed on the power supply are only what each rail is rated for. Not what it will actually output.

If you note on the sticker there is also a rated maximum output for the +3.3v and +5v rails. That says 120 watts. Subtract that from the rated wattage output for the power supply (400 watts) and it leaves you with 280 watts. Significantly more than is required for the card itself but also significantly less that the rated output for the +12v rail (360 watts). Unless it's really close, you can count the remaining two rails as negligible. You'll need to do the math on your own power supply.

In short, this power supply should run the graphics card without issue, but not all power supplies are created equal. Power supplies vary greatly in quality. This 400 watt power supply from Antec would probably run your system without issue while a lesser company's 600 watt power supply could have problems. In general, power supplies from Antec, Corsair, PC Power and Cooling, and Seasonic (and some few others) are considered top rated.

As far as trying it out, you shouldn't have any problems. If the power supply is not up to snuff, the computer will either fail to boot up (not enough power at all) or it will crash when utilizing more power (i.e. playing games).

Hope this helps.

-Wolf sends

#### anon8792

##### Distinguished
Thanks, I was just asking since I already upgraded my computer once, but the card I dropped in wasn't cutting it as well as I thought it would(was still really new and didn't want to buy something expensive and screw up) so now I'm getting a 550 ti which is well within power requirements, but was considering 560 which is on the border of my current power requirements since it has a significant boost. I think my PSU is decent, but in all seriousness since I didn't know much, I got one that seemed alright making sure it would fit in my computer and didn't waste power much. I'll probably keep with the 550 ti since it still will do what I need it to do and doesn't give me the feeling my comp may give out if I run it hard.

#### jjb8675309

##### Splendid
what is the make and model of your psu and your full system specs?

#### 4745454b

##### Retired Mod
Without reading Wolfs probably correct answer, I'd like to point out that I wouldn't try it. Yes, 250 + 150 = 400 and if you have a 400W PSU you have "enough". But based on that info alone I wouldn't do it as you'll be running your PSU RIGHT AT THE EDGE of what it can do. It's a bad idea to do that to a PSU. You'll want a new one. (possibly. We have no idea what's in your computer or what your PSU is.)

#### anon8792

##### Distinguished
This is my power supply.

http://www.newegg.com/Product/Product.aspx?Item=N82E16817256060

My computer is some old E-machine I bought a while back since I needed a cheap computer. I did a simple upgrade for 100 bucks a few weeks ago, but the graphics card is weak(Geforce 220 I think) so I'm upgrading to a geforce 550 ti since it's 100 max power with a 400 watt supply requirement and fairly cheap for what it's able to do. If you're wondering about the emachine, it's ET1331G-03w. It was very cheap when I got it and I'm not a hard core pc gamer so the ability to run the games and look decent is all I'm looking for.

#### 4745454b

##### Retired Mod
You'll be fine. Your computer has a AMD Athlon™ II X2 dual-core processor 235e, which is a 45W CPU. Lets toss in another 45W for the board and other parts and round that up to 100W. Your PSU has 25A on its 12V rail, giving you 300W. You are using 100W already, leaving you with 200W. Your PSU is a pretty good one, and you have two 6pin PCIe plugs so you could run a 6870 on it. Your CPU has no chance to keep up with such a card, so look at getting a 5770, 6770, GTX460, 6850, etc. 5750 or 5770 is probably your best bet from a balanced CPU/GPU stand point.

#### jjb8675309

##### Splendid
your fine with that psu, id probably stick with a 5770 until you get a better cpu or buy a nicer gpu but have the cpu upgrade coming in shortly after that

#### anon8792

##### Distinguished
Thanks for all the advice. I was considering getting a Zotac Geforce 560 ti, but since it seems my CPU will bottle neck it even if I can run it, I'll get the cheaper 460 which is almost as good and will still run decently.