1) Here is the graph showing the Grundlast - Mittellast - Spitzenlast concept from the German Wikipedia:

2) I find that while Mittellast (c. intermediate load) does seem to lack from English totally, there is a Wikipedia article titled Load following power plant:

A load following power plant is a power plant that adjusts its power output as demand for electricity fluctuates throughout the day. Load following plants are in between base load and peaking power plants in efficiency, speed of startup and shutdown, construction cost, cost of electricity and capacity factor.