^ Nonsense. I don't undervolt and they're running fine on a 1250W PSU. It draws max 1240W from the wall on 4 OC'd HD7970's, AMD X2 and 4 GB of ram. With 91% efficiency, that's 1130W on the 12V side of the PSU. That's 120W of headroom @ full load, which is plenty.
No need for a 1500W PSU on 4 7970's.
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Might want to check your math. That's 10w of headroom.
The wattage of a power supply is how much it delivers, not how much it draws. If a machine with a 1250 watt PS is drawing 1240 at the wall but is 90% efficient, it is delivering about 1116 watts of power to the PC. So, if a 90% efficient 1250 watt PS is drawing 1240 at the wall it still has an additional ~134 watts or so of available power. Conversely, at maximum power delivery of 1250 watts, one could expect to see about a ~1375 watt load at the wall. I wouldn't run anything that close to the rated power though, but that's just me. Even within 10% is a bit too close, I prefer at least a 15-20% margin. Don't need anything going POOF! on my machines, ya know?