Chris G wrote:
When powered up, even though it's quite a fast machine (quad core processor, 8Gb of memory, two big disk drives) it only takes 50 watts so the power supply isn't all that inefficient when it's working.
So how can I search for power supplies that don't take much power when completely idle? Typical 'efficient' 300 watt and 400 watt power supplies are better than 80% efficient but that means they may well still consume 20 watts when doing nothing. Efficiency *doesn't* necessarily equate to low 'off' power.
Yes, this is the issue with The "80Plus" certification, it doesn't take standby current into consideration and the lowest power state they even measure is 20% of rated load capacity.
I have seen various designs of SMPSU and a whole lot of them suffer from terrible inefficiencies like this, there are "nice" ways of doing it but all would add cost and have no marketable relevance until something like the "One Watt Initiative" sets it's sights on computer equipment manufacturers and OEM's
Even then there is an issue getting there, The soft power circuit on a mainboard draws very little on its own as it is really just pulling the green wire on the PSU ATX connector down to ground. However in addition to that you have cmos battery saver circuits (which provide the cmos battery voltage when the machine is plugged in but not powered up) and WoL (requiring the ethernet chipset to be powered up and holding a link) these will probably on their own require more than 1W of power before conversion.
Add to this mainboards that power usb devices even when the machine is "switched off", possibly so that they can make "wake on keyboard/mouse event" possible for USB HID's and you start to wonder why we left the 2 pole mechanical mains switch of the original AT design in the first place.
Of course the answer to that is convenience both in terms of PSU installation, features like WoL, and allowing the software to "almost" power down the machine completely. Although I think we should have stuck with the mechanical mains switch in addition to a soft power "standby" button (like on a TV set so it is clear that this isn't "power off" as many people currently think). I still think we need the soft power functionality both for operating systems that now like to install updates as they are shutting down and for IT departments that want to WoL machines in the middle of the night to do security updates or whatever. But I think that there should be a front panel provision for a proper "mains off" button as well.
All that said, one final point of consideration is that depending on your measurement equipment. You may be getting rather skewed results when measuring a SMPSU's consumption due to the slightly odd power factor profile of switch mode power supplies, this anomaly can be even more prevalent when a SMPSU is in standby mode, regardless of whether PFC is fitted or not.