When having overflow in power production, the Production power display bar in the Electric network info window shows incorrect.
For example, I have three Power Plant buildings, and I know for a fact that it is able to produce around 74 MW all together.
Let's say 60 MW is required for the satisfaction bar to be full (the current total power requirement). That would mean the Production bar should be 60/74 full (my power plants produce 60/74 of what they can). But the Production bar shows as somewhere around 1/40 full, as if my power plants are only producing a fortieth of what they can!
When the satisfaction (power usage) is above ~74 MW, the production bar shows correct.

This is because the factory buildings have a much larger maximum energy output than the steam engines, as well as a power buffer that refills from inside faster than it gets used outside. The exact same thing also happens with vanilla accumulators when they have a filled energy buffer. The power grid displays how much energy those accumulators could output at exactly that moment, even if the accumulators would drain faster than they are resupplied.