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How Do You Calculate Your Needed Wattage?

D

DrakonSan

Limp Gawd
Joined
Feb 8, 2005
Messages
340
Doing my research before clicking the button and plunging into my purchase for a SFF system, I've come to learn that the Silverstone 450w PSU that comes bundled with the one version of the case can be a whining, performance killing dud that can't be trusted over 300w. The alternative Silverstone 300w bundled PSU is said to be pretty good, but pushing either of these units over 80% of their rating isn't a good idea.

Rather than have you pick choose for me, I'd like to know how to add things up and determine which rails they go on, without resorting to some "magic" scripted web calculator. Newegg recommended 497w for my system, OuterVision eXtreme said 337w, with a minimum of 287w. Numbers numbers numbers, but where is the math behind them?

Build link: http://secure.newegg.com/WishList/PublicWishDetail.aspx?WishListNumber=17449986
 
You can look up reviews of the gpu and see the test specs of the pc used and how much wattage was pulled at full load for more real numbers.

450 will power your build. But, I do not know if quality of that Silverstone psu is good or bad.
 
Usually, I go by the TDP of each component, subtract about 5-10%, and add ~50 watts for peripherals (motherboard, fans, and hard drives). Overclocking on the CPU with air cooling, I would say they all max out at 150 watts for the ~100 watt TDP CPU's and ~180 for the 125+ watt TDP CPU's. For the GPU's, I would add ~50 watts for the 200+ watt ones, and ~30 watts for lower than 200 watts. Of course, overclocking depends greatly on the degree of overclocking.
 
Mostly experience, but more detailed estimates can be put together using power usage values from various reviews. Most are not measured with very much accuracy so you have to take them with a grain of salt, but some websites like Xbit Labs and Techpowerup have setups that allow them to directly measure the DC power draw of certain hardware, and those numbers can be very useful.
 
Zero82z said:
Mostly experience, but more detailed estimates can be put together using power usage values from various reviews. Most are not measured with very much accuracy so you have to take them with a grain of salt, but some websites like Xbit Labs and Techpowerup have setups that allow them to directly measure the DC power draw of certain hardware, and those numbers can be very useful.
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Pretty much this for me.
 
The Xbit Labs and Techpowerup tip helped out.

It's a little disappointing to just have to "feel" how much power a setup might require. It would be nice if the power draw of components became a standard thing included in the product description.
 
It would, but the problem is that power consumption depends on the type of load, and "peak" power draw is usually not representative of actual power draw.
 
They also need to compensate for the fact that many uninformed people buy cheap power supplies that cannot put out power at their rated specs, so they have to estimate higher than actual usage. Additionally, many people are too lazy to add up individual component power draws, so they just go by the simplest thing, usually the power requirement rating put on the graphics card box.
 
I just buy the biggest power supply Corsair makes and figure I'm covered.
 
shawnoen said:
I just buy the biggest power supply Corsair makes and figure I'm covered.
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You're not covered if you're running 4 GTX 580's overclocked with an overclocked i7 9xx processor :p

Still, that just seems like an unnecessary waste of money, since the ax1200 is more than double the price of a Seasonic x750 when it's on sale, and the x750 will run most SLI/Crossfire systems.
 
Tsumi said:
You're not covered if you're running 4 GTX 580's overclocked with an overclocked i7 9xx processor :p
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Beg to differ.

That said, when I ran my rig through a power supply calculator, it gave me a power level of 9001. :eek:
 
Zurginator said:
Beg to differ.

That said, when I ran my rig through a power supply calculator, it gave me a power level of 9001. :eek:
Click to expand...

Let me correct that. 4 overclocked GTX 580's and a overclocked i7 9xx processor :p along with massive watercooling and a couple TEC chips thrown in the mix just for laughs.
 
Zero82z said:
Mostly experience, but more detailed estimates can be put together using power usage values from various reviews. Most are not measured with very much accuracy so you have to take them with a grain of salt, but some websites like Xbit Labs and Techpowerup have setups that allow them to directly measure the DC power draw of certain hardware, and those numbers can be very useful.
Click to expand...

Good advice, I just round up what the manufacturer says for TDP. If the proc is 130w TDP calculate 150w..etc
 
Brilhasti said:
Good advice, I just round up what the manufacturer says for TDP. If the proc is 130w TDP calculate 150w..etc
Click to expand...
The TDP is usually pretty far off from actual power consumption though, so you can use it for ballpark estimates, but it's not particularly accurate. Real power consumption is usually below the TDP.
 
shawnoen said:
I just buy the biggest power supply Corsair makes and figure I'm covered.
Click to expand...

Someone correct me if im wrong but doing that isnt usually a good idea cuz if i put a 1500watt psu on my system it would be bad cuz all that 80 plus gold and stuff is at 20% 50% and 100%. So if its below i thought it was bad cuz it doesnt transfer or convert the ac to dc or something like that. cuz its overkill
 
Aus10 said:
Someone correct me if im wrong but doing that isnt usually a good idea cuz if i put a 1500watt psu on my system it would be bad cuz all that 80 plus gold and stuff is at 20% 50% and 100%. So if its below i thought it was bad cuz it doesnt transfer or convert the ac to dc or something like that. cuz its overkill
Click to expand...

A quality power supply is pretty even despite a low or high load. If I put a 1500w power supply on a budget system that pulls 300w max it would be just fine. Its a waste of money, but it would work.
 
Aus10 said:
Someone correct me if im wrong but doing that isnt usually a good idea cuz if i put a 1500watt psu on my system it would be bad cuz all that 80 plus gold and stuff is at 20% 50% and 100%. So if its below i thought it was bad cuz it doesnt transfer or convert the ac to dc or something like that. cuz its overkill
Click to expand...

A power supply loses efficiency dramatically below 20% load; how much it loses depends on the manufacturer and the components used. Some lose only ~2% efficiency, some can drop up to 10% efficiency. Other than that, there's really nothing wrong with using a high-wattage power supply to power a low-end system, besides it being a waste of money. And at the low-end, a few % difference in efficiency translates to ~20-30 watts at most, so it's not really a significant difference.
 
Oh well see im corrected lol. Just seems like a waste of money now...
 
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