I'm running 4 on sapphire 7950 OC, cpu 2500k at 4,4Ghz

I started each task manually, so I have almost constant gpu load around 90%, one task starting, one finishing and two in progress 40-50% apart.

auto fan control changing rpm all the time produced some vibration noise in lower speeds so I switched it to fixed 1870rpm (45%), gpu temp is 52-54°C

edit: btw with this setting I see each wu finishing somewhere in 90-120 secs that means one result under 30 secs avereage :)

No, it has nothing to do with efficiency, it only has to do with where you live. In the United States, outlets output at 115V at up to around 15 amps, in Europe you will find that your power outlets are running at 230v with a much lower amperage, but it equals the same maximum watt output. If you plug a 115V power supply into a 230V outlet it will likely explode, likewise if you plug a 230V power supply into a 115V outlet it will likely die. Some power supplies have switches to switch between the standards.

Running any more than the amount of tasks it takes to get your GPU load to 95% +/- is the problem. 4 maybe 5 on a 7970 would be the max. I would stick with 4.

The problem I have had with that is that they don't stay apart. The completion times are somewhat random, and so they will drift in and out of sync. WCG needs an automated solution to ensure that the CPU-intensive portions of the work units are not in sync. That would probably mean that one is paused for a short period of time, but that is better than not being able to run two at once.

If you've only got access to 115 V it's obvious you'll need to use this, regardless of efficiency. But, atleast in some non-residential installations in USA like example server-rooms you've also got the option of 230 V. If it's possible to cheaply get 230 V also in residential USA-homes I've no idea.

If both 115 V and 230 V is available, always buy a 230 V-capable PSU and run off 230 V, since PSU is more efficient running off 230 V than 115 V. Due to this efficiency-difference, some test-sites does their PSU-testing at both voltages, and report these. As an example, according to http://www.anandtech.com/show/6168/rosewill-fortress-platinum-450w/5 you've got:

load - 115V  -  230V
5% -  71.25% - 73.29%
10% - 84.97% - 86.85%
20% - 91.03% - 91.84%
50% - 91.64% - 92.42%
80% - 91.22% - 91.85%
100%- 90.79% - 91.08%
110%- 90.38% - 90.72%

As can be seen from the table, regardless of load, if you've got the option to run off 230 V it's always higher efficiency. As an example, if assumes your computer with GPU-crunching uses 200 W and this is with the 50%-load-numbers, the difference is 1.84 W and this means 16.14 kWh per year.


BTW, atleast around here it's 230 V at 10 A, meaning can use upto 2300 W while 115 V at 15 A is only 1725 W so it's not just the efficiency being higher at 230 V.