Well, when I returned to the game after a few hours of absence, money was at zero, all my power plants were disabled and my population was plummeting.
I don't remember if the energy was capped; even if it was, it didn't help because you still have to manually sell it to get money. Considering that you need money to buy resources, the real buying priority is always below the priority of selling. If you can't sell, you can't buy.
My point is that with this being an idle game, there is an expectation that when you return to it after some time you'll have more resources to buy upgrades. It's strange for an idle game to punish for idling with the loss of progress.