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Hey dudes. I sold my small GPU farm off, except for 2 7950's, which I have in my main rig.  I have my PC on all the time so thought I would mine with them still, even though this is not a dedicated mining pc.

I was wondering if anyone knows, or has a guess, as to what Intensity mining rate is most power efficient. Does anyone know if it basically it is the greater intensity, the more the power usage, the greater the hashes --- or if instead, it is something like using 60% of GPU is better use of Watt-to-hash than using 90% of GPU, for example.

I have them mining at an Intensity of 14 but not sure if there is a lot of point, because power isn't super cheap here. 

Intensity is for performance control.  Currently, Higher the intensity the more reasonable performance as The Display function will not be used much.

My current test with HD7750, 900/1250, Intensity 16 can reach to 160Khash/s with Auto-Control including autotune 15%.

If I=14, in my current case the performance is only 150Khash/s, which is standard. At I=14, Consuming 120W including 4 Core CPU semi-idle (running 2 P2Pool), two fans (Normal fan and one with Radiator: Seidon 120v), one usb device (Keyboard and mouse), LAN connection and one HD7570.

After I have set as I=16, Still using the amount of energy, that means higher intensity the better.

Obtained from killer-per-watt (model used should be started with B, Deutschland made) in order to see how much does my rig consumes exactly.

Note that I usually consider at the worst case, Which should be USD $0.25 Per Kwh if reached some limitation in order to do efficiently.

If you think your card is not working enough, then Try higher intensity first, then considering other factors like liquid cooling and etc as Fans Consumes some of energy to stabilize the environment that may be the reason why you should use liquid cooling if the fee per Kwh is very critical like you.

Also try funneling cool air from outside. It would help you with the cooling. Just make sure to monitor your temperatures so you will know when to add the liquid cooling. It will cost half the card but it is worth an investment.

Liquid cooling is only needed if you want to do insane overclocking, which means very high voltage. At that stage, the efficiency is very low.

There is bigger factors that affect energy consumption : voltage, core & mem clock, PSU efficiency, other component consumption like CPU and RAM.

If the intensity affect the energy consumption, the ideal intensity factor could be different for everyone because every rig is different.

Yep, this. The only thing that can affect my power consumption is lowering the actual settings of the GPU.

Lowering the voltage: Your #1 method of cutting power use. Also helps a ton with temps! The lower the voltage, the less power you use. Problem is you need a voltage unlocked GPU. Also, the lower the voltage, the less you can OC the GPU, so you have to find a balance.

OverUnderclocking the GPU Core: Your GPU Core is the biggest draw of power on the GPU. Reducing that draw makes a huge difference. It's not exact, but typically, at the same voltage, a 10% drop in core speed will also be a 10% drop in the core's power consumption. Also, a lower core clock will let you undervolt farther, reducing consumption even more.

Memory Clock: LTC mining uses high memory clocks, and lots of memory activity. BTC Minging (when it used to be on the GPU) used extremely low memory clocks, and almost no memory activity. Because of this, LTC mining uses quite a bit more power. I think there was a 40-50W difference on my 7970, but I can't remember the exact numbers. Anyways, lowering the memory clocks will also reduce the memory usage, which isn't a huge deal, but still a factor.

PSU: PSU inefficiency is also a HUGE factor. For a 4x 7950 GPU rig, a 88% efficient PSU and a 93% one can be a difference of 50W or more from your draw at the wall. That's also 50W of heat that your PSU doesn't have to dissipate, heating up your case temps or your ambient temps.

And lastly: Rig hardware: You don't need a huge gaming CPU for a mining rig. I think it's crazy when people throw an OC'd i7 or a beefed up AMD FX 8-core CPU on a mining rig. there can easily be a 100W difference between an OC'd FX-8350 with power savings off, and an underclocked Sempron 140 with all the unnecessary BIOS features turned off (sound, firewire, 2nd SATA, wifi, etc).

Tried, the higher the more power efficiency that is, provided you don't lose hash increasing too much...

That's the problem, high intensity get you errors

What type errors are you getting?

I set my Intensity at 20 for my Sapphire HD 7950 and I have been mining few weeks without error.

Some of my settings.
Engine clock: 955
Memory Clock : 1250
Hash: 597KH/S

See if I am able to help you out with your erros and settings.

What leads me to HW errors are certain combinations of intensity and thread-concurrency. So I suppose, I=20 is fine for some and not for others

I have I=20 and I don't get any errors, what errors are you getting?