Off the grid is any IT environment or facility that is not dependent on public power companies for energy /electricity needs.
It is a facility that has on-premise infrastructure / equipment to generate and distribute electrical power and energy.
Techopedia explains Off the Grid
An off the grid facility is not connected to an external electricity provider or the public power grid. Rather, electricity generation unit such as power generators, solar panels, or wind turbines are installed within or close to the facility.
Such independent power generation equipment provides continuous power to the facility. Unlike public power /energy providers that are suspect to power failures and interruptions, the off the grid approach has minimal and/or foreseen downtime.
The off the grid approach is mainly implemented in data centers or high-end computing facilities that requires total availability of power resources in ensuring critical business /IT operations.