Backup Power Planning for Utility Outages

Backup power planning starts with a simple question: which facility loads need to remain supported when normal utility power is unavailable?

Answering that question makes it possible to evaluate the UPS capacity, generator capacity, transfer equipment, fuel strategy and electrical distribution needed to support the facility during an outage.

For data centers, commercial facilities, industrial operations and other critical-power environments, the solution may include a combination of UPS systems, backup generators, automatic transfer switches and temporary UPS equipment.

CI Group helps organizations source and support this equipment across a wide range of capacities and manufacturers.

Start With the Loads That Need Backup Power

A backup-power system should be planned around the equipment the facility actually needs to support rather than around an assumed generator or UPS size.

Some facilities may need to maintain most normal operations. Others may identify a smaller group of critical loads and allow nonessential equipment to remain offline until utility power returns.

Depending on the application, protected or emergency loads may include:

• Data center and IT equipment
• Network and communications infrastructure
• Cooling systems required for critical equipment
• Building controls
• Security and access systems
• Industrial processes
• Pumps and motors
• Emergency and safety systems
• Other equipment required for continued facility operation

FEMA’s guidance for emergency power systems emphasizes evaluating facility loads and emergency-power requirements before an outage occurs.

Review FEMA’s Emergency Power Systems for Critical Facilities guidance.

Once the required loads are identified, the project team can establish the capacity, operating sequence and equipment configuration needed to support them.

Understand the Different Roles of the UPS and Generator

A UPS and a standby generator perform different functions within the backup-power system.

A properly configured UPS can provide immediate power to its protected load when the normal electrical source is interrupted. This helps bridge the period between loss of utility power and the point when an alternate source is ready to carry the load.

Depending on the UPS topology and application, the UPS can also provide additional power-conditioning functions during normal operation.

The standby generator provides a longer-duration source of electrical power once it has started, reached acceptable operating conditions and been connected to the designated facility loads.

For many critical facilities, these systems work together:

Equipment	Primary Role
UPS System	Provides immediate stored-energy support to protected loads during a power interruption and transition.
Battery System	Provides the stored energy used by the UPS for the available runtime.
Generator	Provides an alternate electrical source for longer-duration operation.
Automatic Transfer Switch	Coordinates transfer of designated loads between normal and alternate power sources.

Determine the Required UPS Capacity and Runtime

UPS capacity should be matched to the protected load while also considering redundancy, future growth and the operating requirements of the facility.

Battery runtime is a separate consideration. The appropriate runtime depends on factors such as:

• Protected UPS load
• Battery model and configuration
• Number of battery strings
• UPS efficiency
• Battery condition and age
• Operating temperature
• Generator-start and transfer strategy
• Facility operating requirements

CI Group’s UPS Battery Runtime Calculator can provide a planning estimate based on UPS load and battery configuration.

Calculated runtime should be treated as a planning value. Final battery and runtime requirements should be confirmed using the specifications and condition of the actual equipment.

Size the Generator for the Facility Load

Generator selection should also begin with the required load rather than simply choosing an available kW rating.

Important considerations include:

• Running load
• Starting and transient loads
• Generator kW and kVA capacity
• Voltage
• Phase
• Frequency
• Standby, prime or continuous rating
• Fuel type
• Redundancy requirements
• Future facility growth

Motors, pumps, compressors and cooling equipment can require substantially different power during startup than during normal operation. Nonlinear electronic loads can also affect generator selection.

The complete facility load profile should therefore be evaluated by the engineering team or other qualified professionals responsible for the electrical design.

For larger facilities, see How to Source Large-Scale Generators for Data Center Build-Outs.

Generator Ratings Are Not Interchangeable

The generator’s stated kW capacity is only part of its specification.

Manufacturers offer generator sets with ratings based on their intended operating profile, including standby, prime and continuous applications.

A generator intended for standby service should not automatically be treated as equivalent to equipment designed for a different operating profile simply because both display the same maximum kW rating.

The applicable generator rating should be confirmed when equipment is being selected for the facility.

Include Automatic Transfer Equipment in the Plan

The generator does not operate independently from the rest of the emergency-power system. Transfer equipment controls how designated loads move between the normal and alternate power sources.

Pacific Northwest National Laboratory describes the automatic transfer switch as the intermediary between the primary power source, the standby source and the connected load.

The ATS monitors source conditions and, depending on its configuration, can initiate the generator-start sequence and transfer the load after acceptable generator voltage and frequency have been established.

Review PNNL automatic transfer switch guidance.

Facilities should verify that their automatic transfer switches, switchgear and distribution equipment are appropriately matched to the sources and loads they are expected to support.

Understand the Power-Transfer Sequence

A backup-power plan should document what is expected to happen from the moment normal power is interrupted until the facility returns to its normal electrical source.

A typical sequence may include:

1. Normal utility power is interrupted.
2. The UPS immediately supports its protected load.
3. The transfer system detects that the normal source is outside acceptable parameters.
4. The standby generator receives a start signal.
5. The generator starts and reaches acceptable voltage and frequency.
6. The transfer equipment connects the designated load to the alternate source.
7. The facility operates on generator power as required.
8. When the normal source is restored and acceptable, the system transfers the load back according to its configured sequence.

The exact sequence depends on the electrical design, ATS type, controls and operating requirements of the facility.

Plan for Generator Runtime and Fuel Supply

Generator capacity answers the question of how much electrical load the generator can support. Runtime addresses how long the facility expects that source to remain available.

For a diesel generator, planning considerations can include:

• On-site fuel-storage capacity
• Expected generator load
• Fuel-consumption rate
• Usable fuel capacity
• Refueling arrangements
• Fuel quality and maintenance
• Site access for fuel delivery
• Applicable facility and code requirements

CI Group’s Diesel Generator Fuel Consumption Chart provides general fuel-use estimates across a range of generator capacities and load levels.

Actual consumption varies by engine, manufacturer, generator configuration, load and operating conditions. Manufacturer specifications should be used when exact fuel-consumption information is required.

Evaluate the Fuel Configuration

Diesel and natural gas generators are both used in commercial and industrial standby-power applications. The appropriate configuration depends on the facility and project requirements.

Factors to evaluate include:

• Site infrastructure
• Fuel availability
• On-site fuel-storage requirements
• Equipment availability
• Installation requirements
• Expected operating profile
• Emissions and permitting
• Maintenance requirements

For additional information, see Natural Gas vs. Diesel Generators.

Plan for Redundancy Where It Is Required

Facilities with higher availability requirements may need more than sufficient capacity under normal operating conditions.

Redundancy planning asks another question: what capacity and power paths remain available when equipment is being maintained or another component is unavailable?

Depending on the facility design, redundancy can involve:

• Multiple UPS modules or systems
• Additional UPS capacity
• Multiple battery strings
• Multiple generators
• Multiple transfer or distribution paths
• Redundant electrical distribution

For a deeper discussion of this approach, see Distributed Redundancy for Data Center Power.

Allow for Future Facility Growth

Backup power should not be evaluated only against today’s connected load if significant expansion is already planned.

Before adding new equipment, consider how planned growth will affect:

• UPS loading
• Battery runtime
• Generator capacity
• Transfer-switch capacity
• Electrical distribution
• Cooling infrastructure
• Available redundancy

This is particularly relevant for data centers adding higher-density AI or GPU equipment. See Power Infrastructure Requirements for AI Data Centers for additional planning considerations.

Temporary Power Can Support Planned Infrastructure Changes

Backup-power planning is not limited to utility outages. Facilities may also need temporary critical-power capacity while permanent infrastructure is being changed.

Temporary UPS equipment may be useful during:

• UPS replacement
• Planned maintenance
• Capacity upgrades
• Facility renovations
• Infrastructure expansion
• Equipment transitions
• Short-term critical-power requirements

CI Group offers UPS rental systems that can be incorporated into a planned maintenance, replacement or upgrade project.

Planning temporary power before work begins gives the project team time to coordinate capacity, electrical connections, equipment placement and scheduling.

Maintenance Is Part of Backup Power Readiness

Backup equipment should have an inspection, testing and maintenance program appropriate for the equipment and application.

PNNL recommends preventative and predictive maintenance for standby generators and notes that routine maintenance is important for preparing equipment for an outage. Its guidance addresses components including the engine, fuel system, lubrication system, cooling system, starting system, electrical system and transfer equipment.

Review PNNL standby generator maintenance guidance.

UPS equipment also requires planned service. Batteries, fans, capacitors and other components can have different maintenance and replacement requirements over the operating life of the system.

CI Group provides UPS preventative maintenance and UPS battery replacement for critical-power applications.

Test the Complete Backup-Power System

Testing individual components is useful, but facilities also need to understand how the overall power path operates as a system.

Depending on the equipment and application, testing may include:

• UPS operation
• Battery condition and runtime
• Generator starting and operation
• Automatic transfer-switch operation
• Controls and alarms
• Generator operation under load
• Return to the normal power source

The testing program should follow applicable manufacturer requirements, facility procedures and codes or standards that apply to the installation.

Account for Codes, Standards and Permitting

Emergency and standby power installations can be subject to electrical, building, fire, environmental and other requirements based on the facility, system classification and jurisdiction.

NFPA 110, Standard for Emergency and Standby Power Systems, addresses emergency and standby power sources along with transfer equipment, controls, supervisory equipment and related auxiliary equipment.

The current NFPA 110 edition is the 2025 edition.

Which standards and requirements apply to a specific project depends on the facility and installation. The project team should coordinate with the engineer, equipment manufacturers and authority having jurisdiction.

Backup Power Planning Checklist

Before finalizing a backup-power strategy, confirm:

• Which loads need to remain supported?
• What are the required kW and kVA capacities?
• Have starting and transient loads been evaluated?
• What UPS capacity is required?
• How much UPS battery runtime is appropriate?
• What generator rating is required for the application?
• What voltage, phase and frequency are required?
• Does the transfer equipment support the planned sources and loads?
• What fuel configuration is appropriate?
• How much generator runtime is needed?
• What redundancy is required?
• How will future growth affect available capacity?
• Is temporary power needed during maintenance or replacement?
• What inspection and maintenance program applies?
• How will the complete power-transfer sequence be tested?
• What permitting, codes and standards apply?

Build the Backup Power System Around the Facility

Effective backup power planning treats the UPS system, batteries, generator, transfer equipment, fuel supply and electrical distribution as parts of one coordinated power system.

The goal is to provide the required capacity for the facility’s current load while allowing appropriate room for redundancy, maintenance and planned growth.

CI Group helps organizations source new, surplus and pre-owned generators, UPS systems, automatic transfer switches and temporary UPS equipment for data centers, commercial facilities and industrial applications.