Hmm how about having a 'vacation' mode in which you earn no income from anything but the energy regen is much faster lets say 6 hours to 1 hour and 3 hours to 30 minutre
Oh, and about the energy refill, if you make it so that you can refill it using in-game money try increasing the costs of it every time, for example if the first one cost $100 the second should cost $200 and so on, when you reach a $1000 or so increase it to $500 increments