What Is a Microgrid?

A microgrid is a localized energy system that can operate connected to the main utility grid ("grid-tied mode") or independently as an island ("island mode"). It typically includes distributed energy resources (DERs) such as solar PV, battery energy storage, combined heat and power (CHP), diesel generators, or fuel cells; local loads (buildings, street lighting, EV chargers); and a microgrid controller that coordinates all these resources to maintain stable voltage and frequency within the microgrid boundary.

The defining feature of a microgrid — what distinguishes it from a simply connected set of distributed generators — is its ability to intentionally island: to disconnect from the utility grid and continue serving local loads autonomously, then reconnect seamlessly when the utility returns.

Microgrid Drivers

Resilience — the primary driver for most microgrid deployments. Military bases, hospitals, data centers, and critical infrastructure sites need continuous power that utility outages cannot interrupt. A microgrid with adequate DER and storage can maintain power indefinitely (with fuel cell or CHP) or for hours to days (with battery + solar).

Cost reduction — microgrids with on-site solar and storage can reduce demand charges, participate in grid services markets (frequency regulation, capacity), and optimize energy use across the local system.

Grid support — as DER penetration increases, microgrids that can respond to utility control signals provide valuable flexibility services: absorbing excess renewable generation, providing frequency response, and deferring distribution system upgrades.

Microgrid Components

Point of Common Coupling (PCC) — the electrical interface between the microgrid and the utility grid. A static transfer switch or mechanical disconnect at the PCC enables intentional islanding.

DERs — solar PV (intermittent), CHP or diesel generator (dispatchable and fuel-limited), fuel cells (dispatchable, low-emissions), battery storage (fast-responding, energy-limited).

Loads — all electrical loads within the microgrid boundary. In island mode, the microgrid controller may need to shed non-critical loads if DER capacity is insufficient.

Microgrid Controller (MGC) — the intelligence of the microgrid. The MGC optimizes DER dispatch in real time: maximizing renewable self-consumption, maintaining battery state of charge within limits, minimizing operating cost, and managing the grid-to-island and island-to-grid transitions.

Grid-Connected vs. Island Operation

In grid-connected mode, the utility grid maintains voltage and frequency. DERs operate in grid-following mode — inverters synchronize to the grid frequency and inject power as directed by the MGC. The grid absorbs or supplies the difference between local generation and local load.

In island mode, there is no utility reference. One resource (typically the battery inverter or CHP generator) must operate in grid-forming mode — establishing and maintaining stable voltage and frequency for all other devices to synchronize to. This is technically demanding: the grid-forming device must respond instantly to load fluctuations that the utility would normally absorb.

The transition from grid-connected to island (and back) requires careful synchronization to prevent voltage spikes, frequency steps, and equipment damage. IEEE 1547-2018 and IEEE 2030.7 (microgrid controller standard) define the requirements for safe and seamless transitions.

Microgrid Examples

University campuses — Princeton, Cornell, and UC San Diego operate campus microgrids with CHP, solar, and storage serving buildings across hundreds of acres. These microgrids provide resilience during utility outages and generate significant annual energy cost savings.

Military bases — US DoD has invested heavily in base microgrids under its Spiders and AFCEC programs, driven by the national security implications of base power reliability.

Remote and island communities — diesel-heavy remote communities in Alaska, Hawaii, and developing nations are replacing or supplementing diesel with solar + storage microgrids, dramatically reducing fuel costs and carbon emissions.