Puerto Rico has begun using batteries connected to residents’ rooftop solar panels to provide backup power for its grid, helping prevent blackouts and offering an alternative to fossil fuel-burning peaker plants. It could be the first step toward building one of the largest virtual power plants of its kind. The yearlong pilot, launched late last year by Puerto Rico’s utility Luma Energy, will pull power from up to 6,500 households during energy shortages. It is part of a transformational effort ...