Only about 20% of that capacity is generally available during peak demand periods (about 5:00pm), while average generation during off-peak time averages about 40% of capacity.
If traffic congestion can be compared to water flowing through a sink drain, then a congestion toll is like the handle on the spigot -- it increases during times of peak road use, decreases during off-peak times, and is adjusted in real time to keep traffic flowing smoothly.