The Power Grid is not a visible entity in the game. You can view the outer limits of your power grid by selecting Power Blocks and backing away, giving you a view of the sphere in which they are effective.
To the left is a picture of a single unmodified Power Block and its' 8 block sphere.
Place Turrets tactically within the power grid. Buy a second Power Block if necessary. At lower levels, you will need to buy many Power Blocks to power each Mine Shaft and/or Mining Drill and their associated defenses. As you add Power Source Mods to turrets, the Power Block will be less useful. The block installation pictured here is incorrect, the Power Block will draw attacks. Cover the Power Block in Wall blocks.