What is the complexity of the algorithm to calculate power set of a set?
The running time of step 6 depends on the size of subset_of_subset.

The reason is that the final list needs to contain both the original subset and the subset with last_element added to it. This requires copying the subsets, which takes time linear in its size.

Consequently, the running time of the algorithm scales at least as the total size of all subsets, which is $n2^{n-1}$.