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Generac’s "Mission Critical Continuous" Rating  Meets Data Center Power Requirements

Generac’s "Mission Critical Continuous" Rating Meets Data Center Power Requirements


Data centers often seek third-party certification from the Uptime Institute for use in their marketing. The Uptime Institute is a data center research, education and consulting organization. Generac now offers a “Mission Critical Continuous” rating to clarify our compliance to the Uptime Institute’s requirements for data centers.

To meet the most rigorous levels of requirements - Tier III and Tier IV - diesel generators are considered the primary source of power, not the utility. This means the engine-generator system must be scaled to carry the entire load of the data center, providing constant power with no runtime limit. Generac Industrial generators are compliant with these requirements. 

We frequently hear that generators must have a continuous rating to qualify for Tier III and Tier IV data center projects. That requirement is typically driven by a miscommunication from the building owner or architect who references the Uptime Institute’s guideline for Tier compliance. This is where confusion about specifying generators for these applications arises.

Generator rating definitions are provided by the International Standards Organization (ISO) under category 8528. ISO defines three principal power ratings:

  • Continuous Power (COP) The rating definition for COP is “Constant load: Unlimited running hours.” This rating applies to a generator set paralleled with an infinite bus (e.g. a national electric supply network or grid) where the generator set must run at 100% load, 24 hours a day.
  • Prime Power (PRP) The ISO PRP definition is “Variable load: Unlimited running hours.” This rating pertains to applications where no supply network or grid is available and the generator set is used to supply power 24/7.
  • Emergency Standby Power (ESP) The ISO ESP definition is “Variable load: Limited to 500 hours per year.” This rating is suitable for classic standby power applications.

Generac Industrial generators satisfy all the ISO ratings, including ISO COP.

While the Uptime Institute references the ISO definitions, they do not require Tier III and Tier IV facilities to use ISO continuous-rated generators (100% load).  These generators are frequently specified for data centers because it’s a “no brainer”. However, this often results in units being oversized for the application. A generator de-rated to 80% at continuous load meets the Uptime Institute’s requirements.

Using generator-engine systems that do not have the ISO continuous rating does not mean the engine-generator system is less capable, as it must carry the entire data center load during an outage. Continuous power engines are considered utility-grade. In fact, commercial-grade engines are the same physical product as utility-grade engines. They are built by the same manufacturer, using the same parts and materials, so they also withstand the demands of applications requiring no limitations on runtime.

Confusion is also caused by the need to comply with EPA emission requirements. EPA standards for stationary non-emergency generators require extensive exhaust after-treatment systems such as SCR’s and DPF’s. EPA emission standards for emergency standby generators have a 200 hour runtime limitation--a direct conflict with the Uptime requirements. However, EPA does acknowledge that gensets for data centers would only be operated for extended runtimes during a true catastrophe and at the same time, operate at only 80% load, so the gensets can obtain a nameplate signifying compliance with both mission-critical continuous standards and EPA.

Note: Uptime does not specify emissions requirements. Warranty and commercial aspects also do not prevent Uptime compliance, since the limitation on runtime hours is only a financial consideration.

Some generator manufacturers have created special “data center” output rating categories, primarily for marketing purposes. Generac Industrial Power Generators with the Mission Critical Continuous rating meet the same requirements as competitors, so customers can obtain certification from the Uptime Institute.

If your customer would like a Mission Critical Continuous rating on their generator system, you need to submit the application for review. Generac will review the loads and other aspects of the application and determine whether it can run without a limitation on runtime hours. Once approved, a letter will be provided that is acceptable to the Uptime Institute for the certification process.

If you have questions or need assistance with a specific application, please contact your local Generac Industrial Power dealer.

*The Tier standards are defined in the organization’s publication Tier Standard: Topology, available at the website,

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