*2.2. Daily Power Profile of PV*

Figure 4a shows the power generation and the load demand of California Independent System Operator (ISO) on 31 March 2019. As the sun rises, the PV generation increases and the net load declines. Into the evening, the PV generation declines while the load demand increases. This creates a duck-shaped net load profile with steep ramp rates. The net profile of Figure 4a has a ramp rate as steep as 5300 MW/h. Such power ramps impose challenges in commitment of the slow conventional generators [32]. The problem becomes severe with the increased penetration level of the PVs as shown in Figure 4b.

**Figure 4.** Load profile of California ISO; (**a**) Load demand and generation on 31 March 2019, and (**b**) net load profile on March 31 for last 5 years (2015-19) [33].
