What is the difference between off-grid and mini grid?

We call a system ‘grid-connected’ if it connects to a regional distribution network of several thousands of megawatts capacity.  Such a network will typically connect thousands or millions of electricity consumers together, served by large and distant power plants.  An example of a grid-connected small power system might be a household PV system.  Such a system may supply a local load, but when the PV system produces insufficient power, the grid can supply the load instead.

An ‘off-grid’ power system, by contrast, is isolated from other power systems, so it must carefully match supply with demand.  Off-grid power systems typically require some form of dispatchable, or controllable, power supply, such as diesel generator or a battery bank, that can react immediately to changes in electrical demand.

We would consider any small isolated power system to be an ‘off-grid’ power system.  A community power system that supplies the electrical demands or 10, 20, or even 500 households through a small distribution network (or ‘mini-grid’) is certainly an off-grid system.