Microgrid

A microgrid is a group of interconnected loads and energy resources within a defined electrical boundary that acts as a single controllable entity and can connect to or disconnect from the grid.

That is the U.S. Department of Energy definition. In practice a microgrid pairs solar, batteries, and sometimes generators with a controller that balances generation and load. When the utility is present the controller optimizes cost. When the utility is lost the controller islands the site, and a grid forming inverter sets the voltage and frequency the rest of the equipment follows. The energy management system is usually that controller.