^ 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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No need for a 1500W PSU on 4 7970's.
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PSU loses around 3% of its power/year because of aging capacitors. Also, you should be running PSU at around 85% of its max power. The closer you go to maximum, less efficient it becomes.
my2c
Assuming what you say is true (as you post no source whatsoever), you can run it at least 3 years before you reach it's rated sustained maximum output. Running 4 7970's on a 1250W psu is running it on 90% of it's maximum - completely acceptable.
Efficiency does not deviate significantly from it's norm as it reaches its maximum. Most gold-standard PSU's have an efficiency of ~92% @ 50% load and an efficiency of ~90% @ full load (source). Not significant at all.
