Hardware Corsair Obsidian 250D Review

Bit confused at what you're saying, there: You're saying that a 900W PSU running at 50% load is around the same efficiency as a 450W PSU running at 22% load - with the 450W rated at 80 PLUS Titanium and the 900W rated at the lower 80 PLUS Gold. Is that right?

I'm not sure how that relates to my argument, which is that any given PSU running at 50% load is demonstrably more efficient than the same PSU running at 90% load?

 

Ah, I understand. Well, yes, buying a more efficient PSU will mean an increase in efficiency that may equal or exceed the increase you would get from buying a higher-rated PSU and running it at 50% load - but you can get even higher efficiency gains by getting a higher-rated PSU that's as-efficient or more efficient than the 100%-load PSU. Case in point: rather than, as you suggested, comparing a 91%-efficient-at-50%-load 900W PSU and a 92%-efficient-at-100%-load 450W PSU, compare the 92%-efficient-at-100%-load 450W PSU with a 96%-efficient-at-50%-load 900W PSU.

So, to answer your question, both are important: there's no point buying a higher-rated PSU if its overall efficiency is lower. But, then, I never suggested that: the two PSUs I quickly chose for my price comparison were, very deliberately, identical in terms of design efficiency which removed that variable from the equation.

 

Just over 46 years. Assuming the PSU lived that long, of course.

Like I say, the efficiency concerns really only affect people running the rig heavily-loaded for most of the year - 8,760 hours, for example, to your 730 hours. (Two hours a day? Wish I only spent that long in front of a screen!)

 

Were there no options that would have got you improved efficiency but not cost £100 extra? The example I posted up-thread - switching from a 450W PSU to an otherwise identical 850W PSU from the same manufacturer and model series - had an upfront cost of just £44 and a break-even for eight hour a day usage of 4.5 years, six months ahead of the warranty ending.

Like I keep saying, spending the extra doesn't make sense for everyone - but it certainly makes sense for anyone who uses their system more than eight hours a day at full load.

Yeah, never understood that myself. My system goes into S3 standby every time I'm getting up from the keyboard for more than a few minutes. Free cash, innit?

 

See later posts: anything equal to or more than eight hours a day at full load means switching to a power supply with a 5% increase in efficiency at your given power draw equals savings within the warranty period. That's not considering that energy prices are rising at around 10% a year, either...

But yes, for people who game a couple of hours a day there's little point beyond potential environmental savings. Even then, you're probably better off just donating the extra £44 to an environmental charity of some description.

 

I'd be interested to see your analysis of the error in my calculation, 'cos I believe I've proven otherwise.

My apologies - my use of the word 'switch' upthread was vague. I was talking about switching from planning to purchase one PSU to planning to purchase another; you're absolutely right that there's no point switching an existing PSU out for a slightly more efficient one.

 

Pro-gamers, like the interview I linked up-thread.

Gaming loads the GPU heavily and the CPU reasonably heavily (to heavily, depending on title.) Drives are a tiny, tiny fraction of overall power draw - unless you're talking a file server or something, of course.

Re-read: I'm not saying to buy a $50 PSU - I'm saying to spend $50 more on your PSU than you would have otherwise done. In the example I posted, it was a £118 PSU instead of a £74 PSU - a spending of £44 that you need to save.

You did see the post where I point out that I'm talking about buying a better PSU for a new build, and not replacing an existing PSU? By buying a more efficient PSU in the first place for a new build, you're actively helping the environment - not harming it as you seem to be suggesting here.

Read my earlier posts: as well as cryptocurrency mining, there's also Folding@Home and other distributed computing projects which are massively popular. There's a dedicated sub-forum to it, and two pages dedicated to the topic in every Custom PC Magazine each month, in fact.

Spending around £50 on a more efficient PSU when speccing a new build system will save money and help the environment for anyone using the PC at load for more than eight hours a day. This is not an opinion, it is a fact. Scroll upthread and check my calculations, if you want. I'm pretty sure you'll find them correct.

 

Decided to break this out into a fresh post, to bump the topic up and increase the chances of the nay-sayers seeing my AWESOME SPREADSHEET SKILLS, YO.

THE NUMBERS IN FULL
Yeah, so, I decided to double-check my calculations with the creation of a spreadsheet simulation. Using several assumptions, I built three models: a gamer who plays two hours a day, a pro-gamer who plays eight hours a day, and a Folder or miner who has the system fully loaded 24 hours a day. Each of these users is building a new rig, the specifications of which are given below. In all three cases, it's a completely new rig: no existing parts are being used.

THE RIG
Radeon R9 290(X) TDP: 300W
Intel Core i7 4770K TDP: 84W
Motherboard, Fans and So Forth: 10W
Non-Green Hard Drive: 8W active, 3W idle (based on a 2011 Ars Technica post)
Total maximum system power draw: 402W.

ASSUMPTIONS
When gaming, the GPU is 100% loaded and the processor 60% loaded (two cores versus all four cores, plus overhead), while the hard drive is mostly idle for a total power draw of 363.4W rounded down to 363W for simplicity's sake.
When participating in distributed computing projects like Folding@Home or Litecoin mining, both CPU and GPU are 100% loaded, while the hard drive is mostly idle for a total power draw of 398W.
Electricity currently costs on average 15.32p per kilowatt hour (KWh), based on figures from the Energy Saving Trust. From the same page, generating each KWh of electricity causes 0.517kg of carbon dioxide to be emitted into the atmosphere.
The cost of electricity is rising at 7 per cent annually, based on an average of the most recent price rises listed on USwitch.
The PSUs in question have a five-year warranty, and thus five-year worst-case lifespan. All calculations, therefore, are based over a five-year period.
The two PSUs under comparison are both 80 PLUS Platinum rated, one at 450W and one at 900W. As a result, at the system's peak load the 450W offers 91 per cent efficiency, and the 900W offers 96 per cent efficiency. Buying the 900W PSU costs £50 more than the 450W PSU.

With that in mind, let's run the numbers.

The Gamer
The gamer works in an office all day, during which time his or her PC at home is powered off. On average, the gamer manages to get in around two hours of gaming every day - some days there's no gaming at all, but on a weekend it might be an eight-hour marathon. At all other times, the computer is switched off or in an extremely low power mode.

Result of Simulation:
Over a five-year period, paying the extra £50 for the 900W PSU will have cost the user £37. In other words, this use-case makes no financial sense. Environmentally, however, the move will have reduced the environmental impact of the PC by preventing the emission of 7.84kg of carbon dioxide into the atmosphere.

The Pro-Gamer
The pro-gamer works at gaming all day. Eight hours a day, seven days a week he or she is hammering the system, honing skills and pwning the opposition. Outside the 'office hours,' the PC is switched off or in an extremely low power mode.

Result of Simulation:
Over a five-year period, paying the extra £50 for the 900W PSU will have saved the user £3. Not much, but it is a saving. Environmentally, the move will have reduced the environmental impact of the PC by preventing the emission of 31.36Kg of carbon dioxide into the atmosphere.

The Folder
This user has their system forming part of a distributed computing cluster. Perhaps they're running Folding@Home or BOINC for scientific research, or renting their system out as a renderfarm, cracking passwords and generating rainbow tables, or perhaps they're trying to mint the latest cryptocurrency. Whatever the reason, the system is at full load - CPU and GPU - all day, every day. Hey, on the plus side: at least their room is nice and warm.

Result of Simulation:
Over a five-year period, paying the extra £50 for the 900W PSU will have saved the user £126. Hey, that's enough to buy a replacement PSU! Environmentally, the move will have reduced the environmental impact of the PC by preventing the emission of 103.17Kg of carbon dioxide into the atmosphere.

CONCLUSIONS
If you only load your PC by a couple of hours a day, don't bother speccing it with a PSU capable of delivering double your wattage requirement. You'll never recoup your investment, and the environmental impact is minimal. If you're a pro-gamer, it could be worth doing - especially as you'll be able to claim the cash spent on the PSU as a business expense against tax, something I didn't take into account in my calculations. If you're a folder, though, absolutely go for maximum efficiency - it has a real-world environmental benefit and gives you the cash you'd need to replace said PSU once it's out of warranty. Win-win!

 

I haven't ignored anything; I just made three simulation models based on the assumptions provided.

Most of the official efficiency requirements from 80 PLUS are only given at three load levels: 20%, 50% and 100%. It's to be expected that any given PSU meeting an 80 PLUS certification standard will exhibit efficiency along a bell-curve, but how this is weighted is variable: one PSU might only hit its maximum efficiency at 50%, dropping dramatically at 49% and 51% respectively, while another might hold its highest efficiency value right the way through to 30% and 90%.

An important thing to bear in mind before reading any further: the more power you draw, the bigger impact any given difference in efficiency has; the less power you draw, the smaller the impact. In other words: a 5% efficiency gain in a 20MW solar plant equals big bucks, but the same efficiency gain in my phone charger ain't going to make me a millionaire.

Now, to answer your question I'm going to have to pull some numbers out of my behind. Let's start with the following assumption: at idle, the computer in my model draws 50W instead of ~400W. Seems like a reasonable figure: taking the monitor out of the equation, my APU-based desktop with no discrete GPU draws ~30W at idle. You mileage may vary, naturally. That's running the 450W at 11% load, and the 900W at 6% load. While most 80 PLUS certification levels bottom out at 20%, 80 PLUS Titanium requires 90% efficiency at 10% - meaning both PSUs would be equally efficient at driving such a small load.

The result: the figures don't change in the slightest. If the pro-gamer, or the casual gamer, ran their system at near-idle for the entire time they're not loading it - in other words, they never bother to turn it off - they'd still lose £37 and gain £3 respectively by switching to a 900W PSU.

Naturally, in the real world, there may be a measurable efficiency difference between 11% load and 6% load - but even if we assume there's a giant 10% difference in efficiency in the favour of the 450W, which I'm reasonably certain there wouldn't be, the fact that you're only drawing 50W means two hours a day at idle is equivalent to £2.83 over five years - leaving the pro-gamer still 17p (!) better off having bought the bigger PSU.

In short: the difference in efficiency at idle load is nowhere near as important as the efficiency at full system load.