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Data center backup generators for mission-critical loads

To select the correct backup generator for your mission-critical data, learn the fuel, cost and efficiency considerations you should take into account.

Generators can act as your only lifeline if power fails, so consider your options to ensure your data center's generator meets both your initial and future needs.

The International Organization for Standardization (ISO) classifies generators in several ways depending on the amount of use: Emergency Standby, meaning these generators see 200 hours maximum yearly operation; Prime, which run for unlimited hours per year; and Continuous Rated. Emergency Standby generators are the norm, but ISO stipulates their use with a reliable utility.

Popular generator brands


Caterpillar manufactures their entire power systems, from engines and alternators to switchgear, fuel tanks and smaller genset enclosures. Their alternators only work if the engine starts and runs reliably, but they build the engines running their generators to the same standards as those that run their heavy equipment. Their standby-rated generators can run for the duration of an outage, exceeding ISO Standby and Uptime Institute Tier Certification requirements without upsizing or other special treatments.

Caterpillar offers more than 80 generator models that include diesel and natural gas options in both 50 and 60 Hz versions, and configurations specific to every country in the world. Mission-critical facilities tend to use 1.5 MW and larger Series 3500 and C175 diesel standby generators, with the most popular being the 2 MW to 2.5 MW units. Caterpillar also offers units up to 4 MW that meet all essential requirements. Pricing depends on voltage, fuel, whether the generator is enclosed and the specifics of the installation.


Cummins provides industrial diesel and natural gas standby generator and co-generation products. They design, manufacture and distribute their own engines, alternators, switchgear, fuel tanks, batteries, controls and enclosures. They also make hydrogen generation and fuel cell products. Diesel remains the prominent choice for mission-critical installations, with the most popular size range from 1000-3000 kW. Cummins recommends installing mission-critical generators either indoors or in effective outside enclosures, and incorporating dual starter batteries, block heaters and at least 12 hours of outside fuel capacity.

For a relatively basic mission-critical configuration in the 1000-3,000 kW range, Cummins suggests budgeting at least $350 per kW plus contractor markup and installation. This consists of a standby rated, indoor diesel generator; 480 volt, 105 degree rise alternator with main line circuit breaker; standard cooling system; dual filters for data center operation; redundant starter; batteries and chargers; critical grade exhaust silencer and flex piping; vibration isolators, freestanding day tank; block heaters; factory testing; on-site startup and testing; and a two-year warranty. Common additional cost data center options include enhanced high ambient cooling systems, 80-degree rise alternators, low nitrous oxide engine emissions calibrations, Tier 4 certified or compliant exhaust emissions systems and an Uptime compliant rating. Outdoor enclosures for 1000-2500 kW generators can run from $100,000-$500,000 depending on sound attenuation requirements and features. For 2750-3500 kW generators, enclosures can run $200,000-$600,000.

Cummins offers an onboard paralleling system, PowerCommand, which provides synchronizing and load control functions for parallel generators without the use of a master controller. It provides real-time status of all alarm and operating conditions with access from anywhere. Cummins systems also include AmpSentry, an Underwriters Laboratories-listed overcurrent protection system that enables operation without a circuit breaker, which simplifies selective coordination to minimize the potential for nuisance tripping.


Generac only produces generators, providing single-engine backup systems up to 3 MW capacity. Generac innovations include onboard paralleling, which simplifies generator paralleling by removing requirements for traditional switchgear and providing all components from the same manufacturer. This enables you to grow from a smaller kW unit that meets initial power demand to as high as 100 MW with add-on units. Generac specializes in alternators, sound attenuated enclosures, extreme performance enclosures, a variety of control systems and different size base tanks.

Diesel has been the traditional choice for generator fuel, but Generac focuses on smart grids, grid reliability and environmental regulations in their products. Natural gas and bi-fuel generators offer extended run times that protect facilities for longer periods of time during a crisis. Gas and bi-fuel generators also produce fewer emissions compared to diesel. Generac is the only vendor to offer bi-fuel generators that come fully integrated and EPA-compliant from the factory.


Kohler designs and manufactures its own alternators, fuel tanks and engines, but also makes its own switchgear, controllers and enclosures for their full generator size range. Kohler's 4 MW product is Tier 4 certified, and incorporates a selective catalytic reduction emissions control system to reduce exhaust after treatment components. This makes it less expensive and easier to maintain. Enclosures are all IBC seismic certified, 186-mph wind load-rated and UL 2200 Listed.

An applications engineering team provides total system integration from the factory. All products meet ISO G3 standards. Sales and service run through a global distributor network.

Other considerations for selecting a generator

Regulations. The U.S. National Electrical Code requires buildings to have a life-safety system to support safe evacuation. When a single generator can satisfy this requirement, life safety takes precedence over essential systems. You should keep life safety on a separate generator. This separate generator can also serve as the redundant generator for mission-critical systems as long as you electrically isolate the functions.

Generator rightsizing. Manufacturers offer software to help rightsize generators. Oversizing can present costly mistakes for organizations. Handle future growth expectations with multiple smaller gensets.

Reliability and availability. A reliable generator starts within 10 seconds and delivers power for the duration of an outage. If something trips a breaker, that power might become unavailable to critical loads. Isolate loads such as life safety, cooling and IT on different generators to preserve availability.

Redundancy configurations. In redundant systems, all generators should start simultaneously after a power failure and shut down once power becomes stabilized. Parallel generators that immediately synchronize and provide maximum initial capacity provide the best approach. You often must oversize single generators in order to handle inrush currents from chillers, air conditioners and UPS systems, which can cost more than paralleling smaller, redundant units -- especially if you can use onboard paralleling. Onboard paralleling is simpler than external paralleling gear, and it enables full factory testing of completed systems before shipment. It also makes adding generators easier and shortens commissioning time. Another approach is to prioritize loads with a stepped transfer system, separating them by a few seconds to keep initial load surges to a minimum.

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