> At the end of the day, it's a very simple calculation for them: electricity costs in, Bitcoin value out. No matter where their Bitcoin reward comes from, they

> At the end of the day, it's a very simple calculation for them: electricity costs in, Bitcoin value out. No matter where their Bitcoin reward comes from, they will not (for long anyway) spend more on BTC then they are rewarded. True, but the hypothesis in your original comment, that energy expenditure will equal Bitcoin total market cap, does not follow. As other commenters here note, with each block reward halving, the energy required to mine a new block becomes more and more decoupled from the price of Bitcoin itself. In the distant future when block rewards are minimal and fees make up the majority of miners' rewards, only miners who can utilise the cheapest sources of energy will be able to mine profitably, which in the "negligible-block-reward" era will be for the lowest fees. And people making transactions will not pay more fees than they need to, so the existence of miners able to mine profitably for lower fees will in turn bring down average fees. I think all of this should be uncontroversial; it simply follows from the Bitcoin protocol and the work of Adam Smith. So, given these incentives and Bitcoin's difficulty adjustment mechanism, in the future the only miners capable of making profit from mining will be those with access to the cheapest forms of energy. Anyone who can mine for marginally lower cost is going to push out other miners. This will likely mean the majority of the energy used for Bitcoin mining will come from otherwise wasted energy - such as flare gas, remote hydro not economical to transmit to where people actually live, excess wind and solar supply when demand don't line up, that kind of thing. (And, as an aside, it's probably not going to involve burning lots of oil and gas, since these are more useful to humans in other ways and will therefore cost more.) Obviously more efficient hardware will have the same effect, so there will also be an incentive there. In the end it will have very little to do with Bitcoin market cap and far more to do with the availability across time and space of cheap energy.