The same is true of bitcoin transaction fees, no? Miners set the minimum fee that they will accept, and people tack a fee onto a transaction, large enough that a miner will likely process it.
No, because there is a block reward too that subsidizes the miners, so you have to account for that too.
We don't really know the relationship between transaction rate and electrical usage in a mature BTC system, because mining is mainly used to prevent double-spends and the minimum required mining rate to support a given transaction rate is a game-theoretical concern and not a technical one. We can only really observe what has happened so far in the Bitcoin ecosystem.
