Battery Energy Storage for Emergency Power Systems

Introduction

The Regulatory Landscape: NFPA 111 and NFPA 855

For any system designated to support emergency loads, compliance with NFPA 111, Standard on Stored Electrical Energy Emergency and Standby Power Systems, is the primary benchmark. NFPA 111 defines performance requirements for Stored Energy Power Supply Systems (SEPSS) based on the type of load they serve—ranging from life-safety (Type A or B) to optional standby (Type M).

While NFPA 111 governs the performance of the system as an emergency source, NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, governs the physical installation and safety of the battery itself. Key updates in the 2026 Edition of NFPA 855 have removed prior 600 kWh capacity limits for rooftops and open parking garages, facilitating larger deployments in dense urban environments. Furthermore, the code now mandates both UL 9540A thermal runaway testing and Large Scale Fire Testing (LSFT) to determine safe separation distances from buildings and between adjacent units.

Technical Considerations: Power vs. Energy

A critical distinction in BESS design is the difference between power (kilowatts) and energy (kilowatt-hours).

• Power (kW) is the rating of the bi-directional inverter that determines the system’s ability to handle instantaneous loads.
• Energy (kWh) is the energy storage capacity of the battery, effectively the “size of the fuel tank” that determines runtime.

Sizing for dual-use applications requires an additive approach to energy capacity. Per NFPA 111 Section 7.2.2.1, where an SEPSS is used for purposes other than emergency or standby power (such as demand charge management), the energy storage system must maintain sufficient reserved capacity to provide the required emergency power after those other uses have been completed. This means the system must be sized to meet the energy management objective while always maintaining a sufficiently high state of charge (SoC) to meet the runtime requirements for the emergency role. While this increases the total energy storage requirement and initial capital expenditure, it offers a significant technical advantage: most of the time, the Depth of Discharge (DoD) will be less than 50%. This shallow cycling reduces stress on the cells, leading to an exceptionally long cycle life and protecting the long-term health of the asset.

Managing Motor Starts

Runtime Limitations

Relative to traditional diesel generators, a BESS has a much lower energy density. For context, one megawatt-hour (1 MWh) of battery storage is approximately equivalent to 70 gallons of diesel fuel consumed by a diesel generator. While a generator can run indefinitely with a steady fuel supply, a BESS will have a more limited runtime during a utility outage until it can be recharged via the grid or on-site renewables. Consequently, a BESS is most effective for emergency systems where required runtimes are modest—typically 4 hours or less—and loads are predictable.

The Economic “Value Stack”

Energy storage is widely considered an economic problem before it is an engineering problem. A BESS dedicated solely to emergency standby will never achieve a positive ROI if it sits idle most of the time. To justify the higher capital expenditure compared to an emergency engine-generator, the system must be monetized daily through one or more of the following:

• Demand Charge Management: Using the battery to “clip” facility peaks, potentially saving thousands of dollars per month in utility demand fees.
• Time-of-Use (TOU) Arbitrage: Charging during off-peak hours (low cost) and discharging during peak evening hours (high cost).
• Grid Services: Enrolling in utility programs for frequency regulation or voltage support. These services require near-instantaneous response times that only inverter-based resources can provide.

Safety and Siting: Managing Risk

Fire safety remains the primary hurdle for BESS in urban commercial and industrial applications. Developers should specify UL 9540 Listed products, which certify the safety of the battery, inverter, supporting sub-systems, and the enclosure as a complete system.

Conclusion: Balancing the Scales

The decision to utilize BESS for emergency power is a balance of resilience and revenue.

Advantages of BESS for Emergency Power Needs:

• Monetization: Unlike a generator, a BESS can pay for itself through daily utility bill savings.
• Response Speed: BESS provides instantaneous “grid-forming” power, whereas engine-generators typically require 10 seconds or more to start and stabilize.
• Sustainability: Ideal for facilities looking to reduce their carbon footprint or utilize on-site renewable energy.

Considerations Against BESS for Emergency Power Needs:

• Duration: Limited by battery capacity; not suitable for multi-day outages without substantial solar or renewable energy.
• Environmental Needs: Requires climate-controlled space and specific fire-safety infrastructure.
• Limited Starting Current: High-inrush loads like elevators or large motors can require inverter over-sizing or aggressive use of motor starting aids such as soft-starters or VFDs.

For many C&I clients, a practical microgrid design compromise is a hybrid approach: a BESS for daily energy management and short-duration ride-through, paired with a traditional generator for long-term resiliency.