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Need clarification on PSU wattage needs please

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December 30, 2013 4:12:14 AM

Hi Everyone,

I'm planning on a future build and I'm still very nooby in the matter. I'me doing loads of researches on internet and I'm starting to grasp a few things but there are still some dark areas, so I surely will be asking a few more questions.

My first one is on the PSU. I want to figure out how much wattage I need. I don't want to just hand over my build and get an answer, I'd like to be able to figure it out myself, using the data from my choice of components. I know that the outervision calculator tool is widely recommended but I'd like to be able to understand the maths and back up the result with my own findings/understanding.

So from what I've read so far, all components will need power determined on wattage, volts and amps. It's all confusing to me since it's been 16yrs since I've learned the basics of electrical current at school lol. So for now I'll stick with basic wattage.
Also, assuming I'll be picking a 1000W high-end PSU for my build with a 80+ rating, it means that effectively and safely, I'll only be able to stack up 800W worth of components to safely use the build. Now, keeping in mind I might upgrade in the future, I will want to up the available wattage to say 1200W, meaning the PSU will have an optimum range of 480W to 960W (again from what I read during my own research).

But here's my question, how do I figure out the total wattage of my build without using an online tool?
Should I only take into consideration the TDP from each part, knowing it's not the maximum power they can come to use?

Finaly, I'm not dismissing online calculation tools. In fact I'd like to go from what they say, but I wanna be able to back up the result with my own knowledge.
Here's the dilemma I had using the outervision tool. Is the calculated wattage the actual usage of my build or the recommended wattage of the required PSU taking into account the 80% optimal margin?
For exemple, my build has an I7 4770K with a TDP of 84W. So to try out the tool, I made a calculation only including the processor. The tool start off with 34W which I assume is the minimum required for the PSU itself, so I figured out that the calculation should be 34 + 84 so 118W, but the tool gave me 164W and I can't figure out why?

Thanks for you help

More about : clarification psu wattage

December 30, 2013 4:30:55 AM

Did you have a motherboard type selected?
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December 30, 2013 4:31:44 AM

No only the CPU
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a b ) Power supply
December 30, 2013 4:49:34 AM

Welka said:
No only the CPU


First off! well written question and admirable self teaching right here.

It gives you that wattage because your TDP is essentially the most viable wattage, its like a safezone. Intels processors are ussualy arround 75 -85 lately. When you grab a benchmark from vantage for example, your power consumption will rise to atleast 120+ for CPU. Thats the reason people use vantage, because its so hard on the system. In worst cased scenario this is what you can expect and most probably not worse ever.

The rest of the wattage can be explained easily as overhead incase for example the PSU degenerates over time.
When a PSU degenerates your ofcourse lose wattage. As far as ive noticed thus far is that those calculators vary mostly between each other dependent on the amount of overhead calculated in the calculator. But a calculator will never give you exact wattage.

Your 1000 watt PSU should be able to handle 3 way SLI or Crossfire which is way too much. For 4-way you would need a 1250w Gold. Now for example, on paper your system with such a multi GPU setup will consume on average 900w, sometimes a PSU needs to up the power to give better and more consistent power.

900w Netto should be around: 1100w bruto, so a PSU of 1250w would be ideal if you actually do use so much power. Esspecially in this case, the amount of power coming through PSU accelerates degeneration of the PSU.


It sounds vague and i must admit, its hard to explain, hope ive helped you? let me know if you dont understand something.

To give a summary:
- an 850 seasonic gold rated PSU can support 3 way GPU's
- if going for 1000 watt: go for a seasonic 1250w gold.
Why? those kind of PSU's are really for 4 way crossfire for people who want to make their Litecoins/Bitcoins. (not a bad investment considering Litecoins right now.)
- CPU can go above TDP
- Overhead is often calculated dependend on the calculator, not much but just ok.
- Another rule: more heat = more Power consumption, what if you want to render a week straight some point in future? an 4770k is well capable of doing so, in this case you can expect that 160w to be reached.

Your question has mind boggled me as well, perfectly understandable, it kind wont get clearer on the matter? but i think we have come a decent way now. (atleast i hope so :p )

ps: here is a website with an entire PSU community! i know! lol! : http://www.jonnyguru.com/

Awesome people there as well, been part of it for a while regarding the same questions like you do and for the fact i am a heavy powerconsumer here.


In reply to the guy underneath this post, this topic explains the formula under section D: http://www.jonnyguru.com/forums/showthread.php?t=3263

Yes any PSU that high Wattage does have sufficient cables for GPU's, besides thats no problem at all sinds useless molex connectors are always present here, you can always use a Molex to 6-8 pin converter :D .
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December 30, 2013 5:09:56 AM

Here's a few thoughts:
As long as you get a good quality power supply (if you're a noob stick with either corsair or seasonic - you can trust every one of their units), I'd go by the following.
You only need to consider the 12V rail, as that is where your GPU(s) and CPU is powered, which is where the lion's share of your power is drawn.
80+ is a measure of efficiency, it doesn't mean you can only draw 80% of their rated output. It is a guide to let you know how much power is lost to heat by the power supply itself. Any good power supply will provide 100% of it's rated output, though of course you don't want to draw those loads on a regular basis.
I generally just look at TDPs of CPU and GPU, add 100W or so for the rest of the system, then add 20% or so to be safe. If you're overclocking (or want the option to in future), particularly if you're upping voltage, you'll need to look at reviews or other forums to see how much extra power you'll need. In some cases a 20% overclock can draw 50% extra power, or even more.
Bear in mind that under normal use, even under heaving gaming or video encoding, your CPU and GPU won't draw anywhere near their full power. The rough maths above usually gives you somewhere near 50% load for a heavy gaming session, which is the efficiency sweet spot for a power supply.
Remember from your high school maths that W = V x A, so if you're looking at the 12V rail, just divide the Wattage by 12 and you'll have the amps your power supply will need to provide.
I wouldn't get too caught up on what calculators suggest, and especially not on what video card manufacturers suggest for their units, they're often trying to account for the myriad of cheap PSUs on the market that outright lie about the power they are able to provide.
Also, check before you purchase that the PSU you want has enough PCIe cables (8 and/or 6 pin) for your video card(s). And if you care about a clean build (which you don't have to), pay extra for a modular unit.
Hope that's helpful and good luck.
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December 30, 2013 6:00:44 AM

Wow guys, thanks a lot for the swift replies. They will be put to good use.

Now here's a little explanation on my build. It will be mainly used for gaming and has to last me for a long time.
It has to be the most compact possible since the tower will be in the living room and plugged in to a 49" Bravia 3D. So I figured out the prodigy M would probably be the best fit for the job.
I first went for the prodigy but it's only M-ITX compatible. I wanted to go down this route but looking at what features the MITX mobos have, despite their quality versus size, I wasn't quite getting what I wanted. So I decided to go for matx build, probably asus since they seem to have good reputation for quality. I haven't looked at them in details but I need a mobo with integrated sound since I read that there's no need for sound card unless you have specials needs like editing, music creation... It has to have networking capabilities as I don't want to waste a PCIe for a network card and ideally it has Bluetooth but I haven't decide whether I will be using Bluetooth keyboard and mouse yet. Dunno if they're the best when it comes to competitive gaming.
A good 4770k processor isn't overly priced so I think I'm gonna go for this, and since it is only for gaming, overlocking isn't necessary right?
Since I'm not overlocking, liquid cooling isn't necessary (but it looks so good lol) and I'm still debating wether I should upgrade the stock heatsink and fan, and/or upgrade the case fans as well for extra lifespan/noise reduction.
As for the GPU, I'd like to go for the GTX 780, though it is really pricey unfortunately. Then again, no need for more than the stock fans.
As for the RAM, it isn't expensive unless you want quality. If you haven't guessed yet, I'd like a build based on quality and lifespan. So I think I'll go for 16gb (even if 8 is probably enough for gaming, but bare in mind I need this machine to last for a bit), probably Corsaire vengeance, some good RAM anyways.
For storage, I'll have an Samsung EVO PRO 250gb (246gb?) as from a few reviews, it is said to be very reliable and fast. I'll also stick a 3"5 HDD, probably black caviar or something along these lines.
It'll have a BD-RW drive.
And I think that's about it.

As you can see I'm still in the middle of my research for my build and I'm solving one point at a time lol.

As for the PSU, I'll go for a fully modular as I want the build to be neat inside.

So any comment?
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a b ) Power supply
December 30, 2013 6:11:34 AM

You dont need so much power but it will be an absolute beast though! seems your aiming high so ill give you this recommendation. Good pick by the way on the SSD, no doubt.

Motherboard: http://www.asus.com/Motherboards/MAXIMUS_VI_GENE/
Fun fact!: That audio is beast! you can do everything and anything with it (just about) AND supports multi GPU's !!.

PC favorite right here.

As for ram you could go 2400mhz but your tottaly fine with 1600mhz, as far as i know only battlefield 4 loves faster ram. If you really wanna go all the way then 16gb Ram would be nice. Darn mistake i made when i built my PC, i do heavy work on it and automaticly picked 1600mhz lol.

Anywayz that should be it right? as for PSU well as said, 850 seasonic gold is fine, if the price does not differ allot from the platinum version. Then why not platinum.

If you decide to go this build, no **** that PC crushes all the new consoles hahaha. You could do some heavy art/work on it as well, like rendering, editing and recording games!. Maybe youll want to make your Youtube channel in the future and become a famous youtuber ;) .

Goodluck with that beast mate.
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December 30, 2013 6:22:26 AM

Every MB will have onboard sound and LAN, though only a few have wireless LAN. You don't need Bluetooth as any wireless keyboard/mouse will come with a USB receiver to plug into the computer.
If you're not overclocking, just get the 4770 (non K), though for gaming the 4670 is just as good for cheaper. I suppose if you're trying to account for the future you could get the 4770, but all the K gives you is overclocking and 100mhz on the base clock... Which is nothing.
Again, if you're not OCing, a H87 mobo would be fine.
The 780 is a hell of a lot of muscle for just 1080p. You'd save yourself money in the long run buying a cheaper card and upgrading in a couple of years when it's not cutting it anymore, but if you want to build it and leave it alone, again, you could make a case for a 780 I guess.
Good choice with your storage. I have found the WD Blacks to be quite noisy when reading and writing. If that bothers you, might be best to look elsewhere.
Nice build. A Seasonic 520W would do you nicely btw... No need for higher.
Good luck.
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December 30, 2013 6:22:45 AM

Cheers mate, you've been really helpful. I'll make sur to come back for more questions haha but tbh, you can find most of your answers by simply searching.
One last thing on the PSU, using the outervision tool, should I consider the calculated wattage as the maximum the rig will need, or do I need to add a safety margin on top of it?
If it were your build, would you upgrade the cooling system or would you stick with stock fans?

For the RAM I was looking at the 1866Mhz Vengeance Pro. It seems that faster than that, the price doesn't justify the small gain in speed.

I don't really need a beast for gaming but I'd like to keep my rig far as long as possible and be able to play say BF7 in a 5 or 6 years time lol. As for now, I'm conscious it will be way too powerfull for what today's games require but if I make a build which will be strecthed out to be able to play, not only I will need to replace it sooner but the lifespan won't be as good since it'll be doing more work.

That's how I see it anyway. I might be wrong!
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a b ) Power supply
December 30, 2013 7:08:20 AM

Welka said:
Cheers mate, you've been really helpful. I'll make sur to come back for more questions haha but tbh, you can find most of your answers by simply searching.
One last thing on the PSU, using the outervision tool, should I consider the calculated wattage as the maximum the rig will need, or do I need to add a safety margin on top of it?
If it were your build, would you upgrade the cooling system or would you stick with stock fans?

For the RAM I was looking at the 1866Mhz Vengeance Pro. It seems that faster than that, the price doesn't justify the small gain in speed.

I don't really need a beast for gaming but I'd like to keep my rig far as long as possible and be able to play say BF7 in a 5 or 6 years time lol. As for now, I'm conscious it will be way too powerfull for what today's games require but if I make a build which will be strecthed out to be able to play, not only I will need to replace it sooner but the lifespan won't be as good since it'll be doing more work.

That's how I see it anyway. I might be wrong!


On HD settings it WILL last very very long, esspecialy with the Motherboard i recommended, adding the fact you could add another 780 to it later on when the price will drop. The other person might be right about the other i version but i disaggree entirely about going for a non K version even if you wont overclock. The simple reason is...the resale value will be allot higher having a K version if you would like to sell it later. It will be more attractive later on, so you get a return on it.

As for the cooler by the way, i recommend a cheap Noctue L9i which comes with good performance 20 dollars worth (about) of paste which lowers temps decently. The L9i is silent as well, as the stock cooler will sound like it will lift off.

But as said, your build will last 5 years easily, but hey if you decide...(5 years is a long time) you want a youtube channel or something later on and start editing videos. Well you can do that just fine with a 4770k, but as said its not a crime to go with the lesser version proposed by the other poster.

As for the ram 1600mhz is absolutly fine believe me, as many oppose 16gb ram i support it. It has more benefit esspecialy combined with an SSD with will do your SSD well.
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December 30, 2013 7:20:44 AM

Thanks everyone for the input. As for cooling, would you recommend a Corsair H series or is it an overkill?
In fact, is there a way to find out what model can cool what CPU?
For the mobo, the gryphon seems to be quite decent, same grade as the gene but slightly cheaper
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a b ) Power supply
December 30, 2013 7:43:31 AM

Welka said:
Thanks everyone for the input. As for cooling, would you recommend a Corsair H series or is it an overkill?
In fact, is there a way to find out what model can cool what CPU?
For the mobo, the gryphon seems to be quite decent, same grade as the gene but slightly cheaper


Unnecessary indeed overkill, the recommended L9i Noctua is the way to go for sure, this is mainely to get rid of noise and give just that bit of extra temprature headroom.

Check out on youtube...haswell CPU's on stock are NO JOKE XD......its an airplaine! i was shocked by the shear noise it makes compared to the older stock coolers which were already horrible!. (just for fun do check out what i mean).

ps: i noticed that post about a 550wat psu... thats fine but like said before, that PSU wont cut it when you will go dual GPU later, meaning you will lose money and a good PSU high wattage of about 850 gold like mentioned before wil last. This part and the case are your reusable parts for in the future. You save money overal. What if a friend of yours will want to seel a 780 cheap to you in 4 years because he wants a new GPU. You could take his GPU for a couple of bucks and outperform his newer GPU.

I know its kind of a stretch but i hope you get the point!.
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a b ) Power supply
December 30, 2013 8:03:01 AM

Cool, thanks for the solution pick! much appreciated, i didnt even know i needed one more to become power supply expert. anywayz goodluck with your PC! got more questions or problems feel free to PM me.
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December 30, 2013 8:09:27 AM

No probs dude!

Oh nearly forgot, one question still remains unanswered. From the outervision estimate, do I take it it's the total power needed for the build or does it also include a safety margin takin into account the efficiency range of a PSU?
Dunno if I make sense lol
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a b ) Power supply
December 30, 2013 8:16:44 AM

Welka said:
No probs dude!

Oh nearly forgot, one question still remains unanswered. From the outervision estimate, do I take it it's the total power needed for the build or does it also include a safety margin takin into account the efficiency range of a PSU?
Dunno if I make sense lol


Your System will use an estimate of 400-500 watt - with second gpu this adds approx 300 watt, hence the good 850wats PSU.

I am not quite sure what you mean? your tool is showing the ultimate max that, that component can use.
As said before the power consumption on load is quite accurate: 120-160 could be reasonable and it could include a bit of overhead.

I have not seen in this topic what your system will use according to outervision in total?
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December 30, 2013 9:08:09 AM

You've sort of answered my question anyway, thanks again.

Say using this tool, if the calculated build equals 500W and it's the maximum possible from the build, I'll add 20% to stay within the optimum operating range of the PSU even at max usage. So I'd go for 650W and will be safe no matter what. Then if I chose to OC or SLi/Crossfire my build, then I'll need to look at a more powerful PSU.
That's if I understood well lol
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