View Full Version : If a 800Wtt PS dosent have a hevy load will it draw all 800 watts ??????

01-13-09, 03:58 PM
i was looking to upgrade my PS to a 800Wtt modular but i was concerned about how much juice its going to pull right now i have a 550Wtt and its ok but i don't want to use alot of energy it is 80 plus certified that is my only worry


01-13-09, 04:13 PM
it will only draw what it needs. But to be honest you are probably wasting your money if you don't need more power, unless you are trying to future proof yourself?

01-13-09, 04:43 PM
It all comes down to how much stuff you have running on what PSU you have now, and how much you're going to run on the bigger model.

When you're talking about power efficiency a power supply will generally have it's highest efficiency rating around 50% load. So if you have a modest setup like a dual core, 2GB ram, and a 9800GTX, it will probably run very efficient on the PSU you have right now. Let's say that pulls around 300W loaded. 300W is about 55% of your 550W PSU's capability. We'll say that your PSU isn't the most efficient, and only achieves a 78% efficiency rating at half load.

The 800W PSU that same load is only 38% of the PSU's capabilites. On an 80+ certified PSU, it might reach 85% at best (somewhere around half loaded). At only 1/3 of the load it might be only 82% efficient. So from the wall your current PSU might be around 384W, and the other PSU is 365W draw. This means you're about 19W less power draw between the two PSUs.

Now to convert that into cost. We'll just stick with a nice even 10 cents per Killowatt hour for power. You can get an idea what it costs in your area here. http://michaelbluejay.com/electricity/cost.html

19W would take 52Hrs to reach 1 full killowatt of power draw, costing you 10 cents. So if you gamed an average of 52 hours a week for a full year, the difference over the entire year would be 52 week x 10 cents a week, or $5.20.

My point is that on a normal computer, being 4% more efficient doesn't really account to a whole lot. If you had a completely overkill system, it really won't matter anyway because you're not exactly being power efficient in the first place. Even if the difference in efficiency is greater than my educated guesses, it still won't even come close to the need of a bigger PSU. If you need to buy a PSU to run other devices then you could look for an efficient PSU, but replacing one that already works isn't cost effective.

EDIT: I was pretty close on the PSU efficiency for that 800W unit. http://www.driverheaven.net/reviews.php?reviewid=638&pageid=4

According to DriverHeaven's review it's around 81% efficient based on my 300W load.

AXP Supernova 800W PSU, efficiency graph

Power load 30% 40% 50% 60% 70% 80% 90% 100%
Efficiency 80% 81% 82% 81% 81% 80% 80% 78%

01-13-09, 06:34 PM
well guys i am thinking of doing some upgrades from what i have now..... I am planning on replacing my MOBO to a 750I FTW i am going to overclock the CPU and my RAM Not much. I have 9600GT but thinking of getting me some thing BIG !!!!! I have a dinki CPU cooler wich i am going to replace i have a 250mm fan on my case heating is not an issue lol and i have all my usb port taken !! planning to put in a Blue-ray player ...and i think some time i push my card ??? so i thinking that way i would be cover and just in case i want to SLI in the future then i would be all set ?? Any opinion would be welcome Thanx.....oh i have 4 GIG of ram .

01-13-09, 07:03 PM
The main concern is what card or cards you are looking at. The rest of the stuff isn't going to be nearly as big of a deal. The biggest issue with just buying a power supply is the connectors have been changing quite a bit lately, so what used to work doesn't anymore.

The PSU sounds okay from the review I linked, but there are also some other units that might be a little better. Their rating of the 12V rails is just a little optimistic as it's rated for 1752W or 146Amps. :lol: Obviously they mean combined each rail could balance to what power it needs, but 6 rails seems a little excessive to me. You could easily pull enough power on just 2 of those rails to get the necessary output of the entire PSU. (792W of V4 and V5)

01-13-09, 09:11 PM
If you are going to SLi in the future I would probably go for a 1000W PSU just to be sure, you never know what the future cards are going to demand from PSU's and two 280GTX's for example will need a 1000W PSU. I like the Cosair PSU's, the 1000HX that I have has been great and they are priced really well.

Check out the reviews on HardOCP as they really push the limits of PSU's to give you a good idea how they handle under full load and in hot conditions.

01-13-09, 09:13 PM
If you are interested here is a link the a review for the HX1000


01-14-09, 04:45 AM
for now i am looking at a evga 260 216 cores and run my 9600 GT on physics on less some alse better come out for a low price !!

01-14-09, 09:16 AM
any program that can show me how much my system is drawing from the power supply (load what rail)


01-14-09, 11:12 AM
This will give you a general idea but just keep in mind you'll need 4 x 6 pin PCI-E connectors and a decent amount of power on the 12V rail for 2 GTX 260's.


They actually made it a lot closer than it used to be. It rates my pc in my sig at 520W. That's a little higher than I've actually been able to load the system, but it's not horribly over. I think I've seen about 550W from the wall, at 80% efficiency it would be about 650W from the wall using their recommendations.

IMO you don't need a 1000W PSU unless you're trying to go triple SLI. You should have no problems running 2 gtx 260's on a solid 700w - 750W psu.

01-14-09, 12:52 PM
PC Power & Cooling Silencer 750 Quad 750W Power Supply $65 after $35 MIR (http://www.slickdeals.net/permadeal/17375/PC-Power--Cooling-Silencer-750-Quad-750W-Power-Supply-65-after-35-MIR)

01-14-09, 03:15 PM
Base on the calculations and the way my pc is set up now 488Wtts...... base on the specs i am looking to built i would need another power supply Thanx Bman

01-14-09, 04:24 PM
Okay, no problem. :)