Backup generators and battery systems help critical facilities keep essential services running when utility power fails, but they do different jobs. Generators convert fuel into electricity; batteries supply stored energy. A microgrid can coordinate local resources and disconnect a facility or community from the main grid during an outage. The right design depends on which loads must stay on, how long they must operate, and how the system handles equipment failure and fuel or charging constraints.
What do generators and batteries each contribute?
A generator produces electricity while it has usable fuel and its equipment is ready to run. Its contribution therefore depends on the generator, transfer equipment and controls working, as well as fuel stored onsite or delivered during an outage. CISA’s Resilient Power Best Practices for Critical Facilities and Sites discusses generator and battery roles within resilient-power architectures, including fuel storage and failure tolerance.
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A battery energy storage system (BESS) supplies electricity that was stored earlier. Its outage contribution is limited by its stored energy, the load being served and opportunities to recharge it. Pairing storage with local renewable generation can help extend service or conserve generator fuel, but a battery alone does not provide indefinite power.
| Resource | What it contributes during an outage | Key planning dependency |
|---|---|---|
| Generator | Electricity produced from fuel while operating | Equipment readiness, fuel storage and resupply, and the systems that connect it to designated loads |
| Battery energy storage | Electricity from stored energy | Available stored energy, load profile and charging or recharging options |
| Microgrid | Coordinates local resources and can operate separated from the utility grid | Island-capable controls and resources able to serve the loads selected for outage operation |
| Uninterruptible power supply (UPS) | Provides power continuity for sensitive equipment where even a brief interruption is unacceptable | A UPS function must be specifically provided; a BESS does not automatically serve as one |
How does a microgrid keep local power operating?
The U.S. Department of Energy defines microgrids as: “Microgrids are localized electric grids that can disconnect from the main grid to operate autonomously.” In a critical facility, the local system may combine generators, batteries and, in some designs, renewable generation. When utility power fails, island-capable controls separate the local system from the grid, and the available resources serve designated loads.
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That sequence depends on the system being designed and configured for islanding. A BESS should not be assumed to protect sensitive equipment from every transfer interruption: CISA’s guide says it assumes a BESS does not provide uninterruptible power for sensitive equipment, such as computers, unless that role is explicitly stated. Equipment that cannot tolerate a brief interruption may need a separate UPS layer.
How should a facility decide what needs backup?
Resilience planning starts with essential operations, not with a generic generator or battery size. A facility identifies which functions must continue, the electrical loads needed to sustain them, and the outage coverage those functions require. The resulting design can prioritize selected critical loads rather than attempting to power every load in the facility.
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- Define the service objective: Identify the essential functions and the loads that support them, then determine the outage coverage those functions require.
- Check continuity needs: Identify equipment that cannot tolerate a transfer interruption and determine whether it needs a UPS in addition to the facility’s BESS or generator.
- Plan fuel and charging: Match onsite fuel storage and resupply access to the resilience objective. For batteries, account for stored energy, the load being served and how charging could continue.
- Plan for failures: Consider what happens if a generator, inverter, control or other component fails. CISA describes resilient designs that retain power for at least critical equipment if a backup component fails.
- Reduce demand where practical: DOE’s 2019 report, Energy Efficiency and Distributed Generation for Resilience: Withstanding Grid Outages for Less, explains that lowering facility demand reduces the energy and backup generation required to maintain essential functions during outages.
These considerations do not produce one universal architecture. Requirements depend on the facility’s essential loads, resilience objective, site conditions and available resources.
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CISA lists portable generators among options that can help key facilities continue operating during power disruptions. That does not make a consumer portable generator a facility-scale solution: it should not be presented as suitable for a hospital, water utility or large communications facility without specific authoritative support.
Critical infrastructure relies on engineered arrangements matched to its loads and operating requirements. A generator or BESS is only one part of that arrangement; the transfer equipment, controls, fuel or charging plan, and ability to tolerate component failures also matter. There is no facility-wide runtime, size or cost figure established here that applies across critical sites.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do real resilience projects compare options?
A rural community microgrid assessment
The DOE case-study report Voices of Experience: Microgrids for Resiliency describes Georgia Transmission Corporation evaluating options for a rural community with a history of outages on an exposed radial transmission line. The approaches considered were continuing with traditional wires, developing solar with battery storage, replacing existing diesel generators with a similar source, or installing two microturbines. The example illustrates that resilience planning compares alternatives against local reliability, technical and economic needs; it does not establish one universally best technology.
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Puerto Rico grid resilience work
A U.S. Department of Energy fact sheet on investments in Puerto Rico’s energy grid reports deployment of 430 megawatts of four-hour battery storage to support grid stability and reduce reliance on more expensive backup power. It also reports the acquisition of 14 temporary generators with about 340 megawatts of total capacity to address urgent generation shortfalls while repairs were completed. These are project-specific figures, not sizing targets or estimates for other facilities. The fact sheet’s publication year is not stated in the indexed record.
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