In the current situation of rapid development of renewable energy and slow growth of power demand

security and economic peak shaving of thermal power units with high parameter and large capacity is one of the important research directions for flexible load regulation. Taking the ultra-supercritical 660 MW unit as the object

through comparison of the difference of coal consumption rate and unit start-up and shutdown energy consumption between two units in low-load operation and one unit in high-load operation

this paper studies the economics of the units using two-shift system and low-load operation mode respectively. The results show that

the economy of the two methods is not only related to the total load

but also related to the peak shaving operation time. The energy saving advantage of the two-shift operation decreases with the increase of the total load. There exists a critical total load that makes both operating modes economically identical. When the total load is lower than the critical load

the economy of two-shift operation decreases with the peak shaving operation time

and there exists a critical downtime.