During a discussion on a web forum, I put forward the claim that for heavy usage scenarios buying a power supply that delivers twice the wattage required can add up to energy savings. This was brought into question, so I devised a neat little spreadsheet with three simulated scenarios – the result of which I have reproduced below.

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 models compare the energy used by two otherwise identical power supplies, one of which is running at at near-full-load and the other one at half-load – and compare the energy savings that come from the fact that PSUs are naturally more efficient at 50% load. Actual figures for this increase in efficiency, taken from the official 80 PLUS certification requirements, can be found in the Assumptions section.

THE RIG

  • Radeon R9 290(X) TDP: 300W
  • Intel Core i7 4770K TDP: 84W
  • Motherboard, Fans and So Forth: 40W
  • Non-Green Hard Drive: 8W active, 3W idle
  • Total maximum system power draw: 432W.

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 393.4W rounded down to 393W for simplicity’s sake.
When participating in distributed computing projects like [email protected] or Litecoin mining, both CPU and GPU are 100% loaded, while the hard drive is mostly idle for a total power draw of 428W.
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 Titanium 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 – both minimum efficiency figures at 100% and 50% load respectively as required by the 80 PLUS certification. 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 £36. In other words, this use-case makes no financial sense. Additionally, however, the move will have reduced the environmental impact of the PC by preventing the emission of 8.49kg 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 £8. Not much, but it is a saving. Additionally, the move will have reduced the environmental impact of the PC by preventing the emission of 33.96kg of carbon dioxide into the atmosphere.

The Folder
This user has their system forming part of a distributed computing cluster. Perhaps they’re running [email protected] 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 £139. Hey, that’s enough to buy a replacement PSU! Additionally, the move will have reduced the environmental impact of the PC by preventing the emission of 110.94kg of carbon dioxide into the atmosphere.

CONCLUSIONS
If you only load your PC for 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!

One Thought on “Should you buy a more efficient power supply?

  1. Hi, coming to the end of this report I’m tempted to rewind: « Holy cow, where have the maths gone already !? »
    Other than that, article appreciated. Enlightened my view of « appropriated PSU » with the main *usage* factor. Just when we’re refurbishing older boxes into speedy desktops thanks to FOSS and some experimentation ;o)

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