I have made an important discovery - my case has a ledge for the power supply to sit on, and at about 60mm deep it will partially obstruct a large fan on the underside of the power supply. The shelf cannot be removed (but I may be able to bend it down to give 5-10 millimeters clearance).
The only PSU on the shortlist without a large fan on the underside is the Antec Earthwatts 380W, so decision made? Not yet.
Older computers rely much more on the 3.3V & 5V rails than current computers and I have been concerned from the begining that most current power supplies are too weak on the 3.3V & 5V rails.
I haven’t been able to find the information that I need to calculate the load for the 3.3V & 5V rails properly (despite spending the weekend looking through every relevant technical document I could find on the internet), but by making reasonable assumptions and filling in some gaps using an example computer configuration in an AMD technical document I have come up with an estimate of around 150W. Most of this load is on the 5V rail (I estimate 100-120W).
[B][I]EDIT : This figures are based on the CPU being run from the 5V rail. I belive this to be the case but I have been unable to find confirmation (enquiries are ongoing). If the CPU is being run from the 12V rail then 45-51W can be deducted from the totals and so the following would not apply.[/I][/B]
The Antec 380W Earthwatts specification is 20A/+3.3V and 20A/+5V, but they don’t give the all important combined maximum. But it won’t be any higher than the 500W Earthwatts which has a combined 3.3V+5V maximum of only 136W (and 24A on each). And the excellent Corsair VX450W that was looking like the best option earlier has a combined 3.3V+5V maximum of only 130W (20A each). In fact the only PSU on the list that appears to have have adequate 3.3V & 5V rails is the FSP 400W ATX-400PNF which has a combined 3.3V+5V maximum of 150W and a 5V rail rated at 28A. But this PSU has a large fan that would be partially blocked by the shelf. There are even 500W to 700W PSUs that can’t supply enough 3.3V+5V power.
For comparison the specification of my current FSP 250W PSU (model FSP250-61GN(A)) is 15A/+3.3V, 25A/+5V, 7A/+12V and combined 3.3V+5V maximum of 150W. I didn’t have this information earlier because the label was partially covered by that annoying shelf and I had to remove the PSU to read it.
This makes me think that a) my estimate is about right, and b) I could really do with a PSU that can supply a bit more power on the 3.3V & 5V rails combined. Although I am giving my PSU a high load the 3.3V & 5V voltages are well within the ATX specification (3.26V & 4.95V) and just as importantly stable. The 12V voltage has always been low (11.80V) even with all drives disconnected, but this is still well within specification and stable. But I still wish to replace my current PSU as it is certainly noisier (and therefore probably running hotter) than it used to be.