GTX 750 ti
Hi All, 

I am an altcoin miner who is just starting to understand the full capability of the mining rigs I have built,

I am using 3 H81 Pro BTC Motherboard with 6 gpu's each for a total of 18 Nvidia GTX 750 ti's which are known to be very efficient at hash/watt ratios. I want to find other ways besides mining to make theses rigs earn their keep and I am starting to become fascinated by networking and security. 

My question about hardware is this,  if the cards are efficient at mining it should follow that they will be efficient (not outright speed) at hashing passwords as well? If we are not pressed for time (relatively speaking of course) and we have plenty of GPU's on hand that are running concurrently (openCL/VCL) is it more efficient(GTX750ti)  than running the gas guzzling video cards?
It's not. Three GTX750Ti don't reach the speed of one GTX980, needing more power though. Four use roughly the same power than a GTX980Ti and lack a quite bit of performance. Plus you need more PCs to stuck them all in. Plus you cannot share easily tasks easily between computers.

But in your case where the hardware is already on site you can surely use it. The efficiency gap to the bigger cards is not that wide.
Ok Thankyou for you help. Humour me here as I am a bit of a novice as to the difference in cracking rigs vs mining rigs and I am trying to figure out why they are different.

Mining quark I am achieving around 6000kh/s per card  with the 750ti or around 36 mh/s per rig for a fuel burn of 400watts. So including associated infrastructure that's 66 watts per card

A 980 will mine quark at around 11900 kh/s not quite double the speed at  TDP 93.6% of 165 Watts which is 154 watts not including infrastructure. 

750 ti Cost AUD $180

980 cost AUD $700

3.88 cards x 6000 = 23 mh/s for the price of 12mh/s per 980 $ wise

power wise 2 x 66=132 watt for 12000 kh/s with 750 ti 
lets allow the 980 154 watts with infrastructure for 12000kh/s and its still behind hash per watt.

please don't get me wrong I am not trying to argue just understand why the two operations are different

If I have plenty of cards to play with I am speculating that the power cost should be slightly better in favour of the 750 ti and I can make up lost time by simply by multiplying the number of cards available. I am trying to make sure that I understand the exact differences in required hardware between each rig and how it impacts its role as either a miner or cracker. I want to build a rig that's running 980 cards just for the sheer power of it so I can mine harder when profits are up and utilize it as a cracker at other times, I thought that the 980 is a little thirsty but better miners are closing the gap.

And as you say I already have the 750 ti
The mistake in your calculation might be based on the bad scaling of that quark mining.

GTX750Ti=640 cores
GTX980=2048 cores

That's 3.2 more cores, so instead of 12000 it should be 19200.

But taking the "quark" stats from Crypto Mining Blog shows
for the GTX750Ti: 5272 khs
and the GTX980Ti: 15249 khs

Whatever goes on there, it's an anomaly of quark mining. Under Hashcat the scaling is pretty normal and the performance of the GTX980 will mostly be 3.2-3.5x over the GTX750Ti since it overclocks very well. The GTX980Ti will be around 4.5x.

But I agree about the prices, although you make the most common mistake nearly everyone here is making too: you compare perf/price ratios per card. That's naive fallacy. Cards don't run alone, they run in a system. The whole system brings the performance. So to run your 18 GTX750Ti you need three systems. The comparable system with GTX980Ti would be 1. Yes, you would always take the more expensive GTX980Ti over the normal GTX980, because you always minimize the number of systems and try to maximize the power. Taking a look at the prices here in Europe that's a no brainer anyway.

But since 6 GTX750Ti cost less then one GTX980Ti and you would need 4 of the latter to compare performance, they might have an mostly equal price/performance ratio. Since handling one system under Hashcat is way less of a hassle comparing with three systems you would always go for one powerful system then three weaker ones.