DISTRIBUTED energy resources (DER) systems are a solar energy-based concept that is gaining a foothold here in the Philippines, and as I have explained in previous columns, the idea is an effective solution in a couple of different ways. It does, however, have some limitations on its applicability, apart from those unavoidably associated with solar power, which is why I recently took a crash course to familiarize myself with a variation of the DER model that is rapidly expanding in the United States.