We came across an interesting story at VR-Zone about Haswell's power-supply requirements yesterday. According to that story, some PSUs won't support the chip's C6/C7 low-power states because they're unable to feed less than 0.5 amps through the 12V2 rail.
We can now confirm the accuracy of that story.
An Intel document seen by The Tech Report echoes the gist of that article: Haswell's C6/C7 states require a minimum load of 0.05 amps on the 12V2 rail, and some power supplies can't pump out that little juice. Haswell systems powered by incompatible PSUs can run into stability problems. They can also shut off when entering the low-power C6/C7 states, if the PSU's under- or over-voltage protection kicks in.
As VR-Zone reported, PSU manufacturers rarely publicize their units' 12V2 rail specifications, so it's hard to tell which units will support Haswell's low-power states. However, the PSU selector on Intel's Reseller Center website includes a list of power supplies, and that list can be sorted by support for a minimum 12V2 load of 0 amps. I count only 23 units with that capability: 19 Corsair models, three InWin units, and a single Seasonic. That isn't very many at all—though perhaps some unlisted units also have that capability.
In any event, folks without compatible PSUs shouldn't necessarily worry. We asked Corsair's Robert Pearce about this issue, and he told us, "I fully expect the [motherboard] companies to disable C6/C7 in the BIOS (though consumers could enable it if they chose to) as there are simply too many PSU's in the market space which might not work correctly." That said, Pearce added that Corsair is working to ensure that all of its power supplies support the C6/C7 states. "We believe they already do but are checking," he said.