I spent a lot of time thinking about the title of the thread and eventually it boiled down to what I chose. Having chosen it, it does look a bit click-baity but if you read what I have to say then you will see that the title accurately describes what I am going to be talking about.
First of all, in my experience, the overwhelming majority of people don't know anything about PSUs. In most cases the PSU is pretty much the last thing that people buy with the pennies they have left over from buying the bling in their system.
To start off with, 650 Watts on a PSU (and I am going to be using 650 Watts continuously in this post) refers to what the PSU can supply to your system, NOT what it can maximally draw out of the wall socket. This is important to note for later.
Another thing which I have seen time and time again is that people think that because they are building a gaming rig they need a "gaming class PSU" and this would mean 850 or so Watts. This is not only false, it is just a colossal waste of money in the overwhelming amount of cases.
I know, I know, if you look at the specs of my machine you will say, "You are a hypocrite", look at the PSU you are using in your system. Well I had a reason for using the PSU I did over and above the Wattage rating of the PSU and also, when I was building my rig, you just could not get Platinum rated PSUs in the 650 watt class which didn't cost a lot more than the one I chose (and I got a really good deal on it). I think back in the day there was only one Platinum rated 650W PSU and that was from Seasonic and it was not available for sale in the UK. So with postage and VAT added it would have cost over twice as much as the AX860 I chose. This is now the second system the AX860 has been running in.
One thing to watch out for like a hawk is whether or not the PSU is rated for 650 Watts continuous use or if the 650 Watts is the peak value. If it says "650 Watts peak" then technically it can be called a 650 Watt PSU but in reality it is more like a 500-550 Watt PSU. If you go for the ultra cheap, what I would call an IED (Improvised Explosive Device) instead of a PSU, unit then whatever you see written on the box is basically a lie - it will not have the energy efficiency it claims and will not be able to output the wattage it is advertised as, never mind that it is completely unsafe to use.
The PSU is the single most important component of your computer build that you will buy. And when I say single most important component I mean by a long way.
The PSU is the only component of you computer build which can destroy other components in your computer if you cheaped out on it. And I am not just talking about power surges, you can have the system running for a time without any power surges and the cheap PSU will still destroy your components. I use the word "will" instead of "could" advisedly.
So the PSU is the FIRST thing you should buy when building your computer and not the LAST thing you buy as an afterthought.
The efficiency of the PSU is simply the amount of power in watts it needs to draw from the AC wall socket to convert it to a given DC wattage your system needs.
As a rule of thumb all PSUs will be at their peak efficiency at half their rated wattage +/- about 8% (let's for the sake of convenience call it 10% for the later discussion).
You can easily run a normal gaming rig off a 650 Watt PSU. If you look at the specs of my machine below I could have it running full whack and still have headroom left over if I used a 650 Watt PSU instead of the 860 Watt unit I have in it. Now obviously if you have a custom cooling loop with pumps and all the rest and are running two graphics cards then you would need to step up to something like an 850 Watt unit.
With the CPU boosted to 4.4GHZ, under load, my system documented in the signature below will draw 509 Watts from the PSU. So I would have quite a lot of headroom if I were running it off a 650 Watt PSU.
If you take a look at the prices of the PSUs here on the site you will see that a Platinum rated 650 Watt PSU costs exactly the same as a Gold rated 750 Watt PSU.
With my system under load I would be at 78% of the capacity with a 650 Watt PSU, with the Gold rated 750 Watt PSU I would be at 68%. In both cases I would be beyond the sweet spot, but I would still be in the same category of measurement (20%, 40%, 60%, 80%). I have the efficiency figures for both of the EVGA PSUs from JonnyGuru and it works out as:
750 Watt Gold = 90%
650 Watt Platinum = 90.5%
As you can see, at full load of my system the 650 Watt Platinum PSU is still 0.5% more efficient than the 750 Watt Gold PSU.
The 750 Watt Gold PSU would be drawing 566 Watts from the wall socket
The 650 Watt Platinum PSU would be drawing 562 Watts from the wall socket.
It may not be a lot, but considering that you are paying exactly the same amount of money for the two PSUs it means that from the minute you start using your system you will be saving money on electricity with the 650 Watt Platinum PSU.
When both of the PSUs are in their sweet spot (around 299 - 351 Watts for the 650 Watt Platunum and 345 - 405 Watts for the 750 Watt Gold) the efficiency is
750 Watt Gold 90.5%
650 Watt Platinum 91.6%
They overlap in their highest efficiency at 350 Watts so the draw from the wall socket would be:
The 750 Watt Gold PSU would be drawing 387 Watts from the wall socket
The 650 Watt Platinum PSU would be drawing 383 Watts from the wall socket.
It's not much, but under all circumstances the 650 Watt Platinum PSU would be saving me money if it were running in the system you can see my the signature below rather than the 750 Watt Gold unit.
This is based on the 115V AC power used in the US. In the UK where it is 240V AC the efficiency would be even higher.
The main thing to consider (and in the above example both the 650W and 750W models are equal) is ripple suppression.
Ripple is the measure of how cleanly the power is supplied to the hardware components and a PSU which has excessive ripple can damage your system. Especially the graphics card is sensitive to excessive ripple.
Above and beyond everything else, the ripple suppression of the PSU is the mark of its quality. If you are choosing a PSU look to see if there is an independent review of it and then pay particular attention to how well the PSU does at ripple suppression.
Bad ripple suppression will (not can) also lead to games crashing or even your system experiencing BSODs when under load.
post edited by Nec_V20 - 2017/07/21 12:38:52