Small hydroelectric power stations were applied to calculate electricity prices according to the avoided cost tariff, peak power generation hours are from 9:30 a.m. to 11:30 a.m. and from 5:00 p.m. to 8:00 p.m. excluding Sundays. On February 9, 2021, the Ministry of Industry and Trade issued Decision No. 478/QD-BCT allowing the shift of the peak power generation time of factories from the time frame of 9:30 am to 11:30 am to the time frame of 6 am to 8 am every weekday except for the owner. Japan. This change has a significant effect on the efficiency of these regulating centers that the reservoir has a limited useful capacity (Vhi/Vyc < 40%). When the reservoir capacity is large enough to meet the water demand for peak generation, the change in time frame has a subtle impact on energy efficiency (Vhi/Vyc>40%).