Residual load: the demand left after wind and solar

How residual load shapes the hourly price

Wind and solar run whenever the weather allows, and their fuel costs nothing, so they are used first. Residual load is what is left for everything else to cover: total demand minus the wind and solar generated in the same hour. Follow that line across a day and the shape of the electricity price follows with it, because the last plant called on to meet residual load is usually the one whose cost sets the wholesale price for the hour.

Demand climbs through the morning while solar climbs faster, so residual load sinks towards a midday trough. In the late afternoon solar fades while people come home, and residual load runs up to its evening peak within a few hours. That evening ramp is the expensive part of the day: it calls on plants started for a short window and priced to match. It is why charging an electric car overnight asks far less of the grid than charging it at the evening peak.

Residual load can fall below zero, in hours when wind and solar exceed the whole demand of the zone. Those are the hours behind the counts on Negative Prices: with more generation than the zone can use, somebody has to be paid to take it away, to store it, or to stop generating.

Read the curve as a gap to be filled. Imports from a neighbour, a battery discharging, a pumped-storage reservoir, a gas turbine and a lignite (brown coal) plant all count the same way here. The chart averages every day in the window onto one profile, so a single stormy week or a still December fortnight is smoothed into the average. Cold, dark, windless spells (Dunkelflaute) are shown one by one on Dunkelflaute.