How Do You Size a Backup Generator for a Data Center?

A backup generator for a data center should be sized around the electrical load it is expected to support, how that load will be applied, the required voltage and electrical configuration, redundancy requirements and anticipated future growth. The generator’s kW rating is important, but it is only one part of selecting the right equipment.

For larger data centers, the required capacity may involve multiple generator sets or multi-megawatt equipment. Critical Infrastructure Group supplies commercial and industrial generators across a wide range of capacities, including 2001+kW generator systems for large data center and mission-critical applications.

Start With the Load the Generator Must Support

Generator sizing begins by defining which facility loads will be supported when normal utility power is unavailable.

Depending on the data center design, this can include:

• UPS systems and protected IT loads
• Cooling and mechanical equipment
• Pumps and motors
• Networking and communications equipment
• Controls and building-management systems
• Security and life-safety systems where applicable
• Other equipment designated for backup power

The project team can then establish the total required load in kW and kVA and identify how those loads will be transferred to the generator system.

For UPS-specific capacity planning, see What Size UPS Does a Data Center Need?

Generator Sizing Should Account for How Loads Are Applied

The total connected load is not the only consideration. The generator also needs to accommodate the way equipment is added to the system during transfer and startup.

FEMA’s Emergency Power Systems guidance explains that generator sizing should consider both the maximum load and how loads are applied. Equipment such as motors, pumps and cooling systems can create larger electrical demands during startup, particularly when several loads are applied at approximately the same time.

Review FEMA P-1019 Emergency Power Systems for Critical Facilities.

This is why generator selection is generally based on a detailed facility load profile rather than simply matching the generator’s nameplate kW rating to the sum of equipment ratings.

Voltage and Electrical Configuration Matter

The generator needs to match the electrical requirements of the data center and the equipment it will supply.

Important specifications can include:

• Required generator kW and kVA capacity
• System voltage
• Three-phase configuration
• Frequency
• Power factor requirements
• Available fault current
• Connection and distribution requirements
• Automatic transfer switch configuration

Providing these details when sourcing equipment helps CI Group identify generator options that are better matched to the existing or planned electrical system.

The Automatic Transfer Switch Is Part of the Generator System

Backup generation works together with the facility’s transfer equipment. When normal power is interrupted, an automatic transfer switch can transfer designated loads to the alternate power source once the generator is ready to accept them.

The ATS should therefore be coordinated with generator voltage, amperage, system configuration and the loads being transferred.

CI Group supplies Automatic Transfer Switch equipment for commercial, industrial and critical-power applications.

For facilities maintaining existing transfer equipment, CI Group also provides Automatic Transfer Switch Service & Maintenance.

Redundancy Can Change the Generator Configuration

A data center may use more than one generator to achieve the required capacity and redundancy objectives.

For example, a facility may distribute the required load across multiple generator sets and include additional capacity as part of an N+1 configuration. Larger data centers may also use paralleled generator systems to provide the required total capacity.

The appropriate arrangement depends on the facility’s electrical design, required load and operating objectives.

See What Is N+1 Redundancy in a Data Center Power System? for a closer look at redundant critical-power capacity.

Generator Rating Should Match the Application

Generator specifications may include standby, prime or continuous ratings. These ratings describe how the equipment is intended to operate under defined conditions, so the appropriate rating should be matched to the data center’s intended use and operating requirements.

Uptime Institute provides data center-specific guidance on engine-generator ratings and how generator capacity and operating ratings relate to data center infrastructure design.

Review Uptime Institute’s Engine-Generator Ratings technical paper.

When sourcing equipment, CI Group can help identify available generator sets that match the capacity, voltage, fuel type and rating requirements established for the project.

Plan for Future Data Center Capacity

As with UPS sizing, generator capacity should account for planned facility growth rather than only the current load.

A data center expansion may add additional UPS capacity, cooling equipment, high-density racks or AI/GPU systems. These additions can change the amount of facility load that needs to be supported by backup generation.

For facilities planning AI or high-density computing, see Power Infrastructure Requirements for AI Data Centers.

Including realistic future requirements during generator selection can make it easier to source equipment that supports both the current project and the facility’s next stage of growth.

Large Data Centers May Require Multi-Megawatt Generators

Large data center projects can require substantial generator capacity. Depending on the design, that capacity may be provided by individual large generator sets or several units working together.

CI Group’s inventory includes generators rated at 2001kW and above, providing equipment options for data center construction, expansion and generator replacement projects.

For more information about sourcing large equipment, see How to Source Large-Scale Generators for Data Center Build-Outs.

What Information Is Needed to Source a Data Center Generator?

Having the project specifications available makes it much easier to identify suitable generator equipment. Useful information includes:

• Required generator capacity in kW and kVA
• Required voltage and phase
• Critical facility load
• Major motor and mechanical loads
• Expected load-transfer sequence
• Required generator redundancy
• Fuel preference or existing fuel infrastructure
• Required generator rating
• Indoor, outdoor or enclosure requirements
• Sound-attenuation requirements
• Available installation space
• Project location
• Project schedule
• Planned future capacity

Once these requirements are defined by the facility’s engineering team, CI Group can help match the project with available generator equipment.

Source the Generator Around the Complete Data Center Power System

The generator is one part of the complete critical-power path. UPS systems provide immediate conditioned power, batteries support the required transition period, transfer switches connect the alternate source and generators provide longer-duration backup power.

Planning those systems together helps ensure the generator equipment is selected around the actual data center application.

Critical Infrastructure Group supports data center projects with generators, UPS systems, automatic transfer switches, UPS rentals and other Data Center Power Infrastructure solutions.