I'm guessing the title is a typo, and I'm guessing was meant to be "An UNusually...."

And the "a vs an" rule is more based on the sound that the next word starts with, rather than just the actual letter.

So for "usually", it's a "You-" sound

But for "unusually" it's an "Uh-" sound

Because the u in usually is pronounced like the consonant y, like “yew”. We use “a” before consonant sounds.

The u in unusually is pronounced like the vowel sound u, like “uh”. We use “an” before vowel sounds.

This is because two vowel sounds in a row is somewhat awkward and doesn’t flow as easily with how you move your mouth to pronounce things. Using “an” puts an extra consonant sound in between the two vowel sounds.

It’s odd because wide adoption of rooftop solar is still unusual, so I think the wrong word was used. It should read:

“An unusually wide adoption of rooftop solar…”

I think it's because "usually" starts with a Y consonant sound (yoo-sually), instead of a vowel sound

because while “usually” starts with a ‘u’, we typically pronounce it as if it started with a ‘y’