As this recent LA Times article explains that the people running these pot businesses either just open at a different location or actually buy an external, we assume gas powered, generator.
Black market cannabis shops thrive in L.A. even as city cracks down
It's not till the very end of the article, but:
"Although the utility shutdowns have disrupted some operations, many businesses have also simply reopened after obtaining an external generator. The detective who guessed the White Castle dispensary near Wilmington would be back in business in a week was almost right."