Power

"For the very best Monkey Cities, where real estate becomes precious, you can upgrade one of your existing Watermills or Windmills to produce more power. Upgrade one now."

- "Power Spike" Quest description

Almost every building in Bloons Monkey City requires Power  to build. To get power, the player must build new windmills or watermills, or upgrade existing ones. Power is not like city cash, there is no maximum capacity and it cannot be spent. It simply gets used up. When the player buys a new building, more power will used.

Trivia

 * It is possible to demolish a Windmill or Watermill even if it would possibly make the amount of power consumption overflow.