Transforming Data Center Energy: The Innovative FlexGen and Rosendin BESSUPS Collaboration

The June 2025 announcement that FlexGen and Rosendin have launched a “first-of-its-kind” utility-scale BESS (battery energy storage system) designed as a full UPS alternative for data centers could be considered a significant game changer in the data center power market.

The removal of the requirement for the traditional UPS infrastructure can significantly change how data center energy issues are addressed. According to  Pasi Taimela, Chief Innovation Officer of FlexGen:

Partnership Benefits

By the end of 2024, over 6400 MW of new data center capacity was under construction, just in the US, and with the growth of AI and HPC workloads, global demand could double by 2028, potentially reaching 325–580 TWh annually.  

Yet these facilities face increasing stress on grid capacity, long interconnection queues, and rising ESG expectations. And regardless of the stress level, reliability is non-negotiable. Even seconds of downtime cost $100k–500k/hour.

Tier 0 power security hinges on instantaneous, high‑quality backup—typically via UPS and diesel generators, which are increasingly seen as outdated, noisy, polluting, and too slow for evolving demands.

As local communities begin to legislate controls over data center development, the traditional backup power system, primarily diesel generation, is squarely in their sights as something that requires external controls.

One of the goals of this product partnership is to help define new architecture templates, procurement guidelines, and deployment strategies for BESS as UPS replacements, setting benchmarks for future deployments and potential long-term positive impact on data center targets for net-zero growth.

Why this BESS?

This partnership marks the first utility-scale BESS deployment explicitly designed to replace UPS for data centers. By placing the BESS outside the facility at medium voltage, it changes the perspective, circumventing the complexity and cost of indoor power system rewiring.

The system integrates Rosendin’s patented BESSUPS and FlexGen’s Soft-Grid Interconnect & Island-Grid Frequency Stabilization technologies. The goal is to enable deployment without costly electrical redesign inside the data facility.

The system delivers a utility-scale, medium-voltage system providing high-density, high-quality power sufficient for mission-critical data center loads. Overall, the solution is managed by FlexGen’s HybridOS energy management software, enabling real-time monitoring, intelligent dispatch, and precise transitions between operating modes.

BESS Is the Best (for AI)

This system, deployed outside the data center, on the medium-voltage side, can eliminate complex indoor UPS systems. This not only reduces capex, but also simplifies design, reduces fire and acoustic risks, and improves modularity.

It is also designed to accommodate the energy demands of fast-shifting workloads, such as AI training and HPC, that can quickly scale to tax the limits of existing power infrastructure already installed in existing data centers.

This BESS system is designed to support bi-directional dispatch, enabling data centers to act as virtual power plants by feeding power back during peak demand or grid instability. Selling power back to the grid, supporting utility services, and providing fast frequency response, even while continuing to act as a UPS, transforms the data center into a grid asset, not just a load.

With more locations requiring data center deployments to work with the existing utility grid, this capability is one that can help assure project approval and data center deployment.

Sustainability Cred

To further enhance sustainability and the drive toward net-zero, the BESS reduces dependency on backup diesel power generation and makes for a cleaner grid profile.

The BESS system itself delivers CEBMA-level power quality at scale, meeting Continuous, Evolvable, Black‑Start, Micrologic Aligned (CEBMA) standards, delivering clean, uninterrupted, high-performance power crucial for mission-critical data center, without onsite generators.

According to a FlexGen press release, real world, grid-connected validation is in process with the partners conducting live field tests to demonstrate:

• ·       Existing grid-forming PCS technologies supporting waveforms and response.
•    PCS firmware upgrades enabling full UPS replacement.
•    Seamless switching between grid-connected and island mode.

Bill Mazzetti, SVP of Rosendin, said of the project and collaboration:

Breaking the Backup Mold via Automation

Existing BESS deployments have been primarily used for grid support (e.g., peak shaving, frequency regulation) or to augment backup power systems alongside UPS and diesel generators. Whereas, the BESSUPS solution is designed from the ground up to replace the traditional UPS. It delivers instantaneous, power directly to critical load -- no diesel, no indoor UPS, no delay.

The partnership also believes that the use of FlexGen’s HybridOS, which was designed for data center resilience, is a significant step up from the way other deployments use BESS.

With State-of-Health (SOH) monitoring, fast transient response modeling, predictive dispatch & waveform compliance, and real-time EMS/SCADA coordination, the companies believe they have taken this a step beyond energy storage, moving into the realm of power engineering automation.

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