Did I break your brain? Awesome. I am in the USA, and while power isn't free to me, I could probly get away with running a few 6990s for a few months before noticing any power increase but since I only have one PCIE slot to play with my options are to get what I can afford or attempt to build a FPGA or a PCIE exter or something.
Ah, you'd be surprised at how quick someone will notice spikes in power usages/bills. A Radeon 6990 will easily draw 350-400 watts at full load and hash somewhere in the neighborhood of 750 mh/s depending on overclock. Now assuming you can get a few of them, which will run you $300-$400 each, even for used ones on eBay, two of them in a a hashing machine is going to probably use up 800-900 watts for about 1.5 GH/sec.
Now let's talk to our old buddy math.
If your power rate is 5 cents/kwh (crazy low)
- (900 Watts * 24 Hours * 30 Days /1000) = 648 Kilowatt/hours * .05 = $32.40 Power Cost.
10 cents/kwh (Low to average US):
- (900 Watts * 24 Hours * 30 Days /1000) = 648 Kilowatt/hours * .10 = $64.80 Power Cost.
20 cents/kwh (High in most of the US, Low in California)
- (900 Watts * 24 Hours * 30 Days /1000) = 648 Kilowatt/hours * .20 = $129.60 Power Cost.
None of those are particularly bad because at current rates, you'll be producing about 16 BTC a month worth about $175, making a profit of anywhere from $142 to $45 after you pull out your power costs. Then again, you still have at least $600 worth of video cards you need to pay off, so in the absolute best case scenario, you're looking at 4.2 months before you even break even.
You also can't forget that difficulty will probably go up every two weeks and you'll be making less as time goes on and that 4.2 months could easily stretch out to 5 or 6 months. And I don't know about you, but I'm sure any landlord/parent/boss is going to notice a $100 jump in a power bill really quick.
Finally, if you consider that there's a very good chance that within 2-3 months ASIC's will start arriving and those two 6990's that were producing 16 BTC could very soon only be producing .5-.8 BTC a month while still racking up the same power costs, you might want to rethink _any_ GPU purchase at all.
All in all, at least do some calculations to figure out if you are at least making back what you are spending in power with an eye towards the fact that what you are making today will not be what you are making in a month, 3 months or 6 months time.
