This article on PSU efficiency for a modern sandy bridge gaming PC follows on from 3 previous articles, about PSU efficiency for a medium power server, a medium use very high power gaming PC and a low power server or HTPC.
In this article I’m going to assume the system is operated from 4pm — 10pm weekdays and 10am — 11pm weekends at full power. I’m also going to assume the system is on and idle for the rest of the time.
The system in question pulls ~45w idle and ~247w under full load. I’m assuming an AMD Radeon HD 6870 (18w idle, 163w full) instead of the EN9400GT (11w idle, 23w full), the power numbers are adjusted accordingly. All of these estimates are deliberately on the high side. I expect a typical PC will spend more time off, more time idle, and wont be loaded as intensely as the example.
The figure for the generic PSU is based on the minimum requirement and is therefore a worst case. Even with a very poor efficiency PSU the AC draw from the wall won’t increase as the DC draw decreases. I have calculated a best-case yearly saving, based on a kWh charge of 10p.
| Make / Model | Efficiency at idle (45W) | Efficiency at load (247W) | Est. A.C. power draw (idle) | Est. A.C. power draw (load) | Saving |
| ATX v2.2 400w | 65% (at 45w) | 70% (at 247w) | 69W | 353W | |
| Seasonic X‑400 | 80.4% (at 42.6w) | 91% (at 248.4w) | 56W | 271W | £31.43 |
| Corsair AX850 | 77.3% (at 43w) | 90.4% (at 251w) | 58W | 273W | £29.70 |
| Enermax Modu87+ 500w | 77.5% (at 43.4w) | 90.5% (at 250.6w) | 58W | 273W | £29.88 |
| Corsair CX400W | 73.2% (at 43.2w) | 85.5% (at 250.6w) | 61W | 289W | £23.14 |
| Kingwin Lazer Platinum 550W | 83.2% (at 41.6w) | 93.8% (at 248.6w) | 54W | 263W | £34.89 |
| HuntKey Jumper R90 300W 80+ Gold | 88% (at 60W) | 89% (at 250w) | 51W | 281W | £32.59 |
The conclusion is no longer clear. The value of an upgrade has improved over the previous scenario with a gaming PC. However, even with a lot of heavy use, and comparing the most efficient PSU’s with the worst a budget ATX PSU can perform, a new PSU will still take around 4 years to pay for itself. A real world upgrade is unlikely to net this much saving.
If you are considering replacing an existing PSU my advice would be not to bother, unless you have additional reasons. If you are buying new then it is worth paying more for a better PSU, within reason. Of course if you want the efficiency for other reasons (low noise, low heat) then those factors may make the value difference irrelevant.
Update (25-Dec-2011)
Corrected slight error for Modu87+ and added the HuntKey Jumper R90. This PSU changes the conclusion for anyone with a low efficiency PSU. It produces savings of over £30, which is approximately the price of the PSU! A payback period of 1 year, with an expectation that electricity prices will continue to rise is well worth the investment.
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