Since no one has replied, I figured I would reply to my own post so anyone new who has similar questions can see the answer (whether they knew they had the question or not)! This answer is based on my impressions after playing part of a single game of Power Grid with a time control of 1 day + 2 hours per move.
In the example given above, 3 days + 2 hours per move, I think time is indeed added to every player's clock every time they are asked to make a decision, and if a player uses less than 2 hours, their total time left increases, EXCEPT that their total time left will never increase above the original start time--in this case, 3 days.
