What will happen is the large centralized for-profit mining operations will close down, sell of their ASICs and mining will become decentralized again (which I predicted a year ago).
Once mining becomes break-even with electricity costs on the most efficient ASIC designs in low electricity cost regions, mining will only be viable in "home" type setups where other costs such as space, cooling and support are free. "Datacenter" style operations will become money losing operations because by design their cost structure includes more than electricity and have to cover space, cooling and IT support.
What we are going to see are the large datacenter mining operations slowly sell of their H/W at lower and lower prices and close up shop.
There are only three variables that will matter for mining:
1) $/BTC price
2) Watt per GH/s
3) $ per Kwh
For a given BTC price, an optimal GH/s per Watt ASIC design, and cost of electricity, there is a difficulty where mining becomes break even. I ran this calculation and posted a year ago assuming $200 BTC, 0.7W per Gh/s and $0.10 Kwh and if I remember correctly the break even global hashrate was around 50,000 TH/s. The inputs need to be updated to get the new break even hashrate but we are getting close.
To see what will happen just look at the GPU era. Mining difficulty overshot and then leveled off and slowly came down as some miners shutdown their operations till the effort was breakeven. Then FPGAs came online which had a higher capital cost with ~1 year breakeven timeframe but miners bought these because of the focus on electricity efficiency. I am guessing we will see the same with the highest efficiency ASIC designs selling at ~1-2 year breakeven rates.
I might have this wrong, but 0.7W per GH/s equates to 16.8 Wh per GH/day. At 10c per kWh that's $0.00168 per day in electricity per GH/s. At 50 PH/s thats only $84k per day in electricity charges - well below the current mining reward. Assuming only 144 blocks per day and 25 BTC per block that's 3600 blocks. Assume $400 per BTC and that's $1.44M per day in mining rewards. The break-even for just electricity would be 17x higher at around 850 PH/s.
The capital cost for the mining rigs is the majority of where the mining reward is currently having to go - it's much higher than the operating cost. Operating cost only really gets interesting to decide when to switch the hardware off. If the difficulty levels out more then the balance will move much more towards the operating cost.
Thanks for double checking, your calculation is right. I was going off of memory and switched the break even hash rate with break even difficulty in my head.
Just checked and my assumptions last year were $200 per BTC, $0.10 per KwH and 0.8W per GH/s. This worked out to a break even hash rate of 375 PH/s at a difficulty of 52 billion. (I remembered this as 50 PH/s). If you take your assumptions and double the price to $400/BTC and use 0.7W efficiency I got a break even hash rate of 857 PH/s, so yes that's right.
What's interesting about this calculation is you can see how the difficulty should rise or fall once the ASIC market settles down.
1) Global hash rate should go up and down with BTC price. For example if you double the price of BTC the hash rate should double.
2) Global hash rate should go up and down opposite of the price of electricity. For example if the cost of electricity doubles, global hash rate should drop by half.
3) Global hash rate should go up and down with ASIC efficiency. For example if ASIC designs improve by 10%, global hash rate should go up by 10%.
Regardless of the exact number and break even date though, the basic point still holds. Once the hash rate rise to where the most efficient ASICs are break even with electricity, "datacenter" style operations are not profitable and lose cash flow every single month. At that point the only thing for them to do is sell off their hardware and exit. Who knows how it will happen, they might get decent market prices from home users or it might be a fire sale, but my guess is mining will become more decentralized than today.
Even 875 PH/s is not that far away, probably 6 to 12 months. Here is the log chart:
http://bitcoin.sipa.be/speed-ever.png. Each of those bars on the vertical axis represents a 100x increase in hash rate. We are at 50 today which is only 14x from break even using your numbers....