The Stock Companies is compatible with all Power Grid maps and power plant decks.  It modifies the game by allowing the players to buy and sell stock in the power companies that build the power grids and supply electricity to the cities. 

The expansion can be used two ways: in the basic variant, players try to collect shares in the most profitable companies.  In the advanced variant, players must sell shares in their own companies to raise capital, and can buy shares in their opponent’s companies to collect a share of their profits.

The new expansion was designed by Power Grid’s creator, Friedemann Friese.  It requires a copy of either Power Grid or Power Grid Deluxe to play.  With the expansion, the game is for 2 - 6 players, ages 13 and up, and plays in about 2 hours.  MSRP is $29.95.