Published: June 2026 Technical Level: Advanced Category: Power Systems Design
Battery energy storage systems are increasingly evaluated as alternatives or supplements to diesel engine-generator sets for backup power applications in commercial, healthcare, data center, and critical infrastructure facilities. The engineering case for BESS backup power rests on several technical advantages — instant response, no fuel storage or emissions, predictable maintenance requirements — but the sizing methodology is more exacting than for generator-based backup because the battery cannot replenish its stored energy during an outage and must have sufficient initial capacity to sustain all critical loads through the required autonomy period. This paper develops the systematic methodology for backup BESS sizing: critical load assessment and segregation, autonomy time selection based on NFPA 110 classification requirements, battery capacity and power sizing accounting for depth-of-discharge limits and efficiency losses, and the verification that the system meets the NFPA 110 performance requirements for its application class. The practical constraints that limit BESS backup power applicability — primarily the economics and physical size requirements at long autonomy times — are addressed, along with the hybrid BESS-generator architecture that captures the advantages of both technologies for facilities with extended backup requirements.
NFPA 110, Standard for Emergency and Standby Power Systems, governs the design, installation, testing, and maintenance of emergency power supply systems (EPSS) in facilities where a power outage creates a life-safety risk. The standard classifies EPSS by the maximum allowable transfer time (Type 10 for 10-second transfer, Type 60 for 60-second transfer, Type M for manual transfer) and the minimum required autonomy time (Class 2 for 2 hours, Class 6 for 6 hours, Class 48 for 48 hours). A hospital's essential electrical system life safety branch must meet Type 10, Class 2 requirements — transfer within 10 seconds and sustained operation for at least 2 hours. An emergency generator set has historically been the only practical technology for Type 10 compliance; the generator starts automatically, reaches operating speed and voltage within 10 seconds, and can sustain operation indefinitely as long as fuel is available.
A lithium-ion BESS exceeds the Type 10 transfer time requirement because its power electronics respond in milliseconds — far faster than the 10-second limit — making it technically superior to engine-generators on this metric. The constraint is not transfer time but energy capacity: the BESS must store sufficient energy before the outage begins to sustain the critical load for the full required autonomy period, and it cannot replenish that energy during the outage. For a 2-hour Class 2 application, this is straightforward — even a modest battery installation can sustain the typical life safety branch loads for 2 hours. For a 48-hour Class 48 application, the required battery capacity is massive and economically noncompetitive with diesel generation. The practical range for BESS backup power, without hybrid generator supplementation, is Class 2 to Class 6 (2 to 6 hours) for most commercial and critical infrastructure applications.
The foundation of accurate BESS backup sizing is an accurate accounting of the critical loads that the system must sustain during an outage, segregated from loads that can be shed. The load assessment process begins with a load schedule showing all loads that will be connected to the backup bus, their nameplate kVA or kW ratings, their power factors, and their duty cycle during backup operation. For most facilities, the backup load is substantially less than the total connected load: lighting may be reduced to emergency levels only, HVAC may be shut down to life safety supply air only, production equipment may be shut down entirely, and only the loads explicitly required for life safety or business continuity remain.
The critical load for backup BESS sizing is the maximum simultaneous demand of all loads that will be energized from the backup bus at any point during the backup period, not the sum of all connected load nameplate ratings. Two correction factors reduce the nameplate sum to the design demand. The demand factor accounts for the probability that each load is operating simultaneously at full nameplate rating — in practice, motor loads operate at a fraction of their nameplate rating under normal conditions. The diversity factor accounts for the fact that not all loads operate at their peak simultaneously. For a conservatively designed backup system, the demand factor is applied at the individual load level using measured operating data where available, and the total backup demand is verified by sub-metering or power quality monitoring before the BESS sizing is finalized.
The usable energy capacity required for backup operation is:
Where: is the required usable energy in kWh.
is the critical load demand in kW.
is the required autonomy time in hours.
is a safety factor, typically 1.10 to 1.25, that accounts for forecast uncertainty in load demand and battery capacity measurement tolerances.
The required installed (nameplate) battery capacity is larger than the usable capacity because: lithium-ion batteries must not be discharged below a minimum state of charge (typically 10 to 20 percent of nameplate for LFP chemistry, to protect cell life); the round-trip efficiency of the battery system (AC-coupled: typically 90 to 92 percent one-way) reduces the energy available at the critical load bus relative to the stored energy; and the battery's capacity decreases over time as it ages. For a backup system designed to meet Class 2 requirements at end of the battery's warranted life (typically 10 years or 3,000 to 5,000 cycles), the installed capacity must be sized for the end-of-life condition, not the beginning-of-life condition.
The installed nameplate capacity, accounting for these factors, is:
Where: is the maximum depth of discharge (0.80 to 0.90 for LFP, 0.85 for NMC).
is the one-way discharge efficiency (0.92 to 0.95 for modern systems).
is the state of health at end of warranted life (typically 0.80, meaning 20 percent capacity loss).
For a hospital data-closet and life-safety load of kW requiring hours of backup at a safety factor of , the usable energy requirement is:
Sizing the installed capacity for an LFP system with , discharge efficiency , and an end-of-life state of health :
The installed nameplate capacity of 816 kWh is 48 percent larger than the 552 kWh usable requirement — the difference is the energy that the depth-of-discharge limit, the discharge inefficiency, and the end-of-life capacity fade make unavailable to the critical load. Sizing to the beginning-of-life capacity instead would leave the system unable to meet its NFPA 110 autonomy requirement in its final warranted years, which is the most common and consequential undersizing error in backup BESS design.
The power rating of the BESS must be sufficient to supply the critical load's maximum demand — including motor starting inrush currents if motor loads are present — without exceeding the inverter's overload rating. Motor starting is the most demanding power requirement: a 20 hp motor starting across-the-line draws approximately 650 percent of its full-load current for 2 to 5 seconds, requiring the inverter to supply a peak current of roughly six times the motor's running current for the starting interval. Not all inverters support motor starting without derating; the system designer must verify with the manufacturer that the inverter's short-time overload capability accommodates the worst-case starting condition.
NFPA 110 Section 5.3 requires that a BESS used as an EPSS be listed for emergency use (UL 9540 listing for the complete system) and that the system be capable of operating the connected emergency load for the minimum time specified for its class. The battery management system must provide an alarm when the system's autonomy time falls below the minimum required for its class — indicating that the battery capacity has degraded to the point where the autonomy requirement can no longer be met — and must be tested at intervals per NFPA 110 Section 8.4 by discharging the system against its connected load for the full rated autonomy period and verifying that the load is sustained. This test is more rigorous than the engine-generator 30-minute monthly test and must be planned to ensure that the building's normal power can sustain critical operations for the test duration, or that the test is conducted during a period when the utility power is available as backup.
The analysis in this paper connects to several companion studies in this library. Readers concerned with the upstream and downstream engineering will find Battery Energy Storage for Peak Demand Reduction develops a closely related aspect of the same problem, while Battery Energy Storage System Safety extends the treatment into an adjacent domain. For the broader methodological context, Emergency and Standby Power System Design provides complementary depth.
Battery energy storage for backup power demands a more exacting sizing methodology than generator-based backup because the battery cannot replenish its stored energy during an outage and must therefore contain, at the outset, the full energy required to sustain all critical loads through the mandated autonomy period. The methodology developed in this paper proceeds from a rigorous critical-load assessment through the energy and power sizing that NFPA 110 reliability requirements impose, and the central engineering conclusion is that the energy rating, not the power rating, is usually the binding constraint for backup duty, because autonomy periods of several hours drive the kilowatt-hour requirement well beyond what the instantaneous load alone would suggest. For the engineer specifying a BESS as a generator alternative or supplement, the operative discipline is to size for the worst-case coincident critical load over the full required duration, apply the round-trip efficiency and usable-capacity derates honestly, and reconcile the result against the NFPA 110 class designation that the occupancy requires, because an undersized backup battery fails precisely when it is needed most.
[1] NFPA 110, Standard for Emergency and Standby Power Systems, 2022 edition, NFPA, 2022.
[2] NFPA 70, National Electrical Code, Article 700, 701, 706, 2023 edition, NFPA, 2023.
[3] NFPA 855, Standard for the Installation of Stationary Energy Storage Systems, 2023 edition, NFPA, 2023.
[4] UL 9540, Standard for Energy Storage Systems and Equipment, 4th edition, UL, 2023.
[5] IEEE Standard 1547-2018, Standard for Interconnection and Interoperability of Distributed Energy Resources, IEEE, 2018.
[6] EPRI, Energy Storage for Backup Power Applications, EPRI Technical Report 3002018004, 2021.
[7] Sandia National Laboratories, Energy Storage for the Electricity Grid: Benefits and Market Potential Assessment Guide, Sandia Report SAND2010-0815, 2010.
[8] IEEE Standard 485-2010, Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications, IEEE, 2010. (Methodology basis extended to lithium-ion.)