What is a microgrid? From a hospital that keeps its lights on to a village with no grid

Island a campus to keep its hospital lit, then power a village that no grid reaches

F7 of Energy Systems, Step by Step: island a fictional campus in a storm, choose which loads its battery and generator can carry and for how long in July and January, then size panels and a battery for a village with no grid.
  • Foundations · L1
  • 20 min guided · about 10 min practice
  • After lesson 9 and F6

Prairie Campus in a storm, and a village with no grid (illustrative)

The campus's loads; its sources: 600 kW of solar, a 500 kW battery holding 2,000 kWh, a 400 kW generator with fuel for 24 hours at full power.
Load Power In an outage
County hospital 300 kW critical
Water and heating plant 100 kW critical
Dorms 250 kW can be shed
Classrooms and labs 550 kW can be shed
  • Ordinary grid-connected solar switches off when the grid fails. The battery's grid-forming inverter, or the generator, can form an island.
  • At night the island can give 900 kW (battery plus generator); the campus draws 1,200 kW, so some loads must be shed. The hospital and the water plant must stay on.
  • Keeping the hospital, the water plant and the dorms (650 kW): 20 hours from midnight on 28 July 2025, 8 hours from midnight on 20 January 2025.
  • Keeping the hospital and the water plant (400 kW): 51 hours from midnight on 28 July 2025, 40 hours from midnight on 20 January 2025.
The village's smallest designs on the sliders, for an average day. Toy check: one average day; the battery can give any power the village needs, so only stored energy is tested, not cloudy days.
The village Panels Battery
60 households alone 15 kW 40 kWh
With the mill at 10:00–14:00 20 kW 40 kWh
With the mill at 18:00–22:00 20 kW 70 kWh

With JavaScript on, you work through this step by step: a campus one-line diagram you can island, a 72-hour outage in July and in January, and a village to power.

Fictional: Prairie Campus, its county hospital and the village; their loads, sizes and fuel are illustrative. The island's rule: the generator runs first, the battery gives the rest, spare sun recharges it.

Sunshine: SPP's real hourly solar for 28–30 July and 20–22 January 2025, and its average 2025 day (EIA-930), scaled to EIA's 2025 capacity factors for U.S. utility-scale solar (Table 6.07.B).

What you just did, as diagrams

A microgrid
Loads and sources inside one boundary, run as one unit by a controller, that can island: open its switch and run on its own.
Forming an island
A grid-forming source, such as a battery's inverter or a generator, sets voltage and frequency; ordinary solar inverters follow it.
Power, then energy
Power (kW) decides which loads the island can carry at once; stored energy and fuel (kWh) decide for how long.
Seasons
The same campus kept 650 kW for 20 hours in July and 8 in January: weaker sun leaves more of the load to the generator and battery, so their energy runs out sooner.
No grid at all
A village mini-grid runs as an island every day; running big loads while the sun shines keeps its battery small.

The parts of a microgrid

  1. Grid feeder. The utility's distribution line that brings grid power to the campus.
  2. Switch to the grid. The point of common coupling: the one place the campus meets the grid. Opening it islands the campus.
  3. Controller. Decides when to island, which sources run and which loads stay on.
  4. Solar panels, 600 kW. Their inverters are grid-following: they follow a voltage that something else sets, so they cannot start an island alone.
  5. Battery, 500 kW and 2,000 kWh. Its grid-forming inverter can set the island's voltage and frequency at once.
  6. Diesel generator, 400 kW. Fuel for 24 hours at full power. Once started, it can form an island too.
  7. County hospital, 300 kW. Critical load: it must stay on.
  8. Water and heating plant, 100 kW. Critical too: the hospital needs water and heat.
  9. Dorms, 250 kW. Can go dark in an emergency, if they must.
  10. Classrooms and labs, 550 kW. The largest load, and the first to shed.

Kinds of microgrid

The first four rows follow DOE's microgrid overview (2019); the village row follows the World Bank's 2025 energy-access report.
Kind What it is for Grid link In this lesson
Commercial or industrial Cut costs while connected; backup power in an outage Connected, can island –
Community, city or utility Reliability, emissions and energy goals Connected, can island –
Facility, such as a campus Link critical facilities and their equipment Connected, can island Prairie Campus
Rural community or island Burn less imported fuel; more resilience Sometimes connected –
Village mini-grid Electricity where no grid reaches None The village
Go deeper · AnalystDoes the microgrid pay off?
  • Keeping the same loads on for longer may need more storage or fuel: a bigger battery, a bigger tank.
  • Whether that pays depends on what an hour without power costs the hospital, a farm or a household. Analyst lesson A14 prices an outage and sizes the campus against it.
  • Who owns the microgrid decides who pays for it and who carries the risk: a utility, a co-op, the county or a private company.
Go deeper · ModelerLetting a solver size and run it
  • This lesson runs the island by one fixed rule: generator first, then the battery. It is a rule, not the best way.
  • A sizing model makes each size a decision variable and writes every hour’s balance, and each outage, as rules; lesson 10 writes a small model like this down.
  • Modeler lesson M5 sizes a microgrid that must survive an outage, with on/off decisions solved as in lesson 11.

Check yourself

Practice, about 10 minutes: three myths, two questions, then sort six systems.

Myth or fact? “My rooftop solar keeps my lights on in a blackout.”

  • Myth
  • Fact
Answer

Myth, for an ordinary grid-connected system: under IEEE 1547-2018 its inverter stops feeding an unintentional island, normally within 2 seconds. Backup needs a switch that isolates the home and equipment that can island; a battery keeps it going without sunshine.

Myth or fact? “A microgrid is just a big backup generator.”

  • Myth
  • Fact
Answer

Myth. A microgrid runs loads and sources inside one boundary as one unit, with a controller and a switch to the grid; it can serve one building or many. A generator alone, with no controller or local network, is backup.

Myth or fact? “Doubling the battery doubles how long the hospital stays lit.”

  • Myth
  • Fact
Answer

Myth. Keeping the hospital and the water plant from midnight on 28 July 2025, 2,000 kWh lasts 51 hours and 4,000 kWh lasts 67: the fuel and the sun set the rest.

A battery holds 2,000 kWh and can give 500 kW. The hospital and the water plant draw 400 kW, with no sun and no generator. How long does the battery carry them?

  • 4 hours
  • 5 hours
  • 8 hours
Answer

5 hours: 2,000 kWh ÷ 400 kW. The 400 kW fits under the battery's 500 kW, so its energy is the limit.

Same battery, still no sun or generator, but the labs stay on too: 950 kW. What stops the island?

  • Power: 950 kW is more than the battery's 500 kW
  • Energy: 2,000 kWh runs out
Answer

Power. The battery cannot give more than 500 kW at once, so the island falls short at once, long before its 2,000 kWh run out.

Microgrid or not? Sort six systems

  • A campus with solar, a battery, a generator, a controller and one switch to the grid
  • A village's panels, batteries and wires, with no grid connection at all
  • A house with rooftop solar and an ordinary grid-connected inverter
  • A standby generator on its own, with no controller, that starts after the grid fails
  • A wind farm connected to the transmission grid
  • An island's diesel generators, wind turbines and batteries, with no link to a mainland
Answers
  • A microgrid: a campus with solar, a battery, a generator, a controller and one switch to the grid. Loads and sources inside one boundary, run as one unit, and able to island.
  • A microgrid: a village's panels, batteries and wires, with no grid connection at all. An isolated microgrid: it runs as an island all the time.
  • Not a microgrid: a house with rooftop solar and an ordinary grid-connected inverter. It cannot run alone: its inverter stops when the grid does.
  • Not a microgrid: a standby generator on its own, with no controller, that starts after the grid fails. Backup, not a microgrid: no controller runs sources and loads as one unit. A microgrid can still serve a single building.
  • Not a microgrid: a wind farm connected to the transmission grid. A power plant that sells to the grid; it serves no local loads of its own.
  • A microgrid: an island's diesel generators, wind turbines and batteries, with no link to a mainland. A whole small grid run as one unit, isolated from any larger one.

Numbers, sources and what’s next

Numbers to remember

Number What it means Source
4.5 h · 11.0 h Average time a U.S. customer was without power, major events included, at utilities that report by the IEEE method: 269.2 minutes in 2025 and 662.6 minutes in 2024. EIA, Electric Power Annual, Table 11.1 (IEEE method, all events) · accessed 3 Nov 2026
461 · 3.1 GW Operating U.S. microgrids in DOE's Microgrid Installation Database when it launched, and their total capacity. U.S. DOE · 26 May 2021
over 666 million People without electricity in 2023. Mini-grids and off-grid solar are among the ways to reach them. World Bank, Tracking SDG 7 (2025) · 25 June 2025
2 s How fast an ordinary grid-connected inverter must normally detect an unintentional island and stop feeding it; up to 5 s by agreement with the utility. IEEE Std 1547-2018 · published April 2018

Next

A14 · Does a microgrid pay off, and for whom?

In the real world

In Puerto Rico, a small microgrid that serves a community organization kept the power on for nine days during Hurricane Fiona, while other parts of the island were dark (U.S. DOE, 8 November 2023).

In an industry internship, the course author worked on this question: how should solar PV, battery storage and backup generation be sized for an industrial campus to meet demand through grid outages?

The work used a techno-economic optimization model to size a resilient microgrid for the campus.

The company's data, models and results are confidential and are not shown here.

From the course author's research: in the modeled supply chains, each open factory or warehouse could run its own microgrid, with on-site wind, solar and storage, an energy balance every hour, and energy bought from and sold to the grid.

  • Holds when: stylized networks with synthetic distances and costs, candidate sites in California, Arizona and Nevada, and one year, planned week by week for products and hour by hour for energy.
  • Leaves out: grid outages, which the studies do not model.

The full case: case file CF1.

Sources and further reading

Spotted an error, or stuck? Write to hlli@ksu.edu. Corrections are made in place and dated here.

Text licensed CC BY 4.0, code MIT: teachers and students may reuse and adapt it, with credit to Honglin Li.