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If 1 trillion diff corresponds to $42k price for BTC, what BTC price of $110 corresponds to which diff?
That should give us the end-point of difficulty leveling off and should also tell us roughly when to expect that to finish levelling.
≃2.6 billion diff = ways to go.
That'll take about 14 difficulty adjustments at 30% each. I think we'll reach that by end of this year.
But I doubt it's a reasonable estimate as it would represent about 50x the current hashing power, or add another 27000+ TH.
That's a lot of hardware, even at 400GH/s per unit (≃67K units).
Remember that assumes free hardware. It is just electrical cost = value of Bitcoins mined. Any higher difficulty and one would be mining at an operating loss. Most people aren't going to buy a $5,000 box which lets them covert $100 into $100 in BTC. I mean imagine MtGox required a $5,000 fee just to open an account. Would you? That is essentially what we are talking about. So as difficult rises buying more hardware will be less attractive and at the break even point it is completely unattractive.
You could consider it asymptotic, I don't think difficulty will reach break even electrical cost but it does provide an upper bound. Obviously if break even electrical is ~2.6 billion it is kinda silly to project difficulty to reach 1 trillion. Miners would be mining for a >95% loss (convert $100 in electricity into $5 in Bitcoins). I don't think most people would do that given they can just flip the switch to off
Not sure what numbers were used for the 2.6 billion but I used an average GH/W value for the network based on all ASICs released/pre-ordered so far. It is only a guesstimate but without hard numbers it is difficult to know more. Another way to say it is that unless either efficiency significantly improves (i.e. 0.2 W/GH or lower) or Bitcoin exchange rate significantly rises (>$200 USD per BTC) it is highly unlikely difficulty will go beyond that.