Like he said in his tweet, Musk and his companies have done this before. After Tesla acquired SolarCity last year, the company revealed that it had converted the island of Ta'u from diesel generator-based power to solar energy using thousands of solar panels and Tesla Powerpacks for energy storage. Tesla also brought its solar power and energy storage system to the Hawaiian island of Kauai as well as California earlier this year. And these systems don't take terribly long to get up and running. In the case of California, the Ontario station only took three months between groundbreaking and running.
The potential must have sounded appealing to Puerto Rico Governor Ricardo Rossello who tweeted at Musk that they should talk. Tesla has already shipped hundreds of its Powerall battery systems to the island and maybe that will be just the beginning. We could hear more very soon since it looks like Rossello and Musk will be talking today.