Do I have to tell the others how much money I have or may I make this a secret and everybody has to calculate/estimate themselves?
Officially, Money is hidden.
See the FAQ for Funkenschlag (the game was originally published in German under that name.
As Chris notes, this can slow the game down quite a bit as
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: