Do I have to tell the other players how much money I have?
Officially, Money is hidden.

See the FAQ for Funkenschlag (the game was originally published in German under that name.


  Hide the amount of money 
  
  Question: Are you supposed to hide the amount of money you have?
  
  Answer: The rules are not explicit on this point. You should play with hidden money, to prevent AP in the last (couple of) turn(s).


As Chris notes, this can slow the game down quite a bit as


People try to keep track mentally of everything
People may spend extra time figuring out their move by obsessively calculating how that would affect other people's turns.  (The common term for this is analysis paralysis)


So if you are playing in a tournament, expect that everyone's money will be hidden.  At home with your friends, if y'all are not overly competitive or wish to avoid analysis paralysis then feel free to keep money public for everyone.

Since shuffling money back and forth in Power Grid happens frequently, you can take advantage of declaring that money will be public by getting rid of the paper money completely:


Use a whiteboard or computer spreadsheet to track money for each player
Use poker chips.  They tend to be much easier to move around quickly and can be easily counted if everyone leaves them in stacks.