I wrote this for a discussion in another sub-forum, but decided to double the reward for my effort by plonking it 'ere as advice for anyone who's ever wondered whether it's worth spending the extra cash on buying a more efficient PSU when speccing up a build. THE NUMBERS IN FULL 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 Titanuim 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!