[Travis T. OJustice]

The percentage would be expected to be highest at night, when the wind speeds typically are highest and when demand is lowest.

Where in the world is the average wind speed higher at night than day?

[BMiles2112]

My understanding was that wind speeds were highest from early evening onward, decreasing as the night progressed, and lowest in the late morning. I could be off on that, I'm going off of things I've heard. I'd be happy to be corrected if you've got better information.

Taken together (demand and wind speed), it's still unsurprising that the highest percentage would have occurred overnight, when power is usually abundant.