The numbers on the generators are how much power is globally in the system. The generators can't generate power if the global capacity is full. Each generator increases the global capacity. The substation provides a large amount of capacity, and also a larger display, so it can display the power quantity even when it's a very large number. All substations and stirling engines display the same global power quantity.
The generators generate an amount of power every time the orange fuel bar ticks down (the panel displays the fuel level in the generator as a number). The amount of power generated depends on what is being consumed as fuel.
Power is consumed by structures when a recipe is finished, and the amount of power consumed depends on the recipe.