Energy to confirm one block? What is the minimum energy (or CPU operations) required for someone to confirm one block on their own, to launch a double-spend attack?

How can I calculate this from the current difficulty or hashrate?
There is no minimum. In theory, you could find a valid block on your very first attempt. The best we can do is an average.

Here's the average energy required to find one valid block using CPU mining:


  average energy required = difficulty * 232 / 2*10^6 * joules


Or, at current difficulty, 1.018×10^14 joules.

Explanation:


  difficulty * 232


The average number of hashes to find a block.


  2*10^6 * joules


According to the Bitcoin wiki, the typical mining efficiency for CPUs is 2 Mhash/J.