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Size a data-center battery in two separate dimensions: power (the UPS or inverter’s kW/kVA output) and energy (usable kWh delivered for the required time). Set power from the highest coincident critical load, including step loads and motor starts; set energy from the load profile, runtime, efficiency, state-of-charge window, temperature, aging and reserve. A generator-ride-through UPS may need only seconds, while islanded operation or peak-demand shaving can require minutes or hours. There is no universal data-center battery duration.
Start with the service the battery must provide
Write the requirement in one sentence before choosing a battery or UPS. Examples include: support the critical bus at 2,000 kW for 30 minutes until generator transfer; maintain selected cooling and IT loads during a controlled shutdown; or reduce grid import to 4,000 kW throughout the utility demand window. The sentence should identify the bus, maximum power, duration, transfer behavior and whether recharge must be completed before another event.
Power and energy are different design decisions
Power is the instantaneous output capability of the UPS, power-conversion system (PCS) or inverter, expressed in kW or kVA. Energy is the usable battery capacity, expressed in kWh. A battery can have ample kWh but an undersized inverter, making it unable to serve a sudden IT ramp. Conversely, a very large inverter connected to too little kWh will meet the load briefly but not for the required runtime.
For backup, size discharge power for the maximum coincident critical demand and any specified step-load, motor-start or overload requirement. For peak shaving, size it for the largest amount by which the battery must reduce grid import to stay below the target limit. Check power factor, harmonics, overload duration, short-circuit contribution and bypass behavior against the selected UPS or PCS.
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Define what is on the critical boundary
List every load that must remain energized during the event. Depending on the design, this may include servers, storage, network equipment, security, controls, cooling towers, chillers, pumps, air handlers and lighting. Do not assume that the IT nameplate is the data-center load. Mechanical systems can be a substantial and highly variable part of the critical bus, and excluding them can invalidate the runtime calculation.
- Critical IT: servers, storage, network and security appliances that cannot be interrupted.
- Cooling and mechanical: chillers, pumps, fans, CRAH or air-handler units and their controls, if they must run through the event.
- Controls and auxiliaries: building-management systems, switchgear controls, monitoring, fire systems and communications.
- Discretionary loads: equipment that can be shed to extend runtime or protect the demand limit.
Collect measured load data instead of relying on nameplates
Nameplate ratings are useful for an early conservative estimate, but final sizing should use measured coincident demand. Gather at least 15-minute utility-demand data for tariff and demand-charge analysis. For UPS backup and transient smoothing, collect one-minute data or finer; AI workloads may require sub-minute telemetry to reveal fast ramps.
Build a time-series load profile
Record real power, apparent power, power factor and, where relevant, voltage, current and harmonic data at the critical bus. Align IT, cooling, pump, network and control measurements to the same time base. Mark maintenance, generator tests, seasonal conditions and unusual workloads so they are not mistaken for normal operation.
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Use the profile to find the maximum coincident load, not the sum of unrelated equipment nameplates. Also identify the minimum stable load, recurring peaks, ramp rates and the longest interval above a proposed grid-import limit. A demand window that is only briefly exceeded may need far less energy than one that remains above the limit for hours.
Account for power factor and transient behavior
UPS sizing is affected by both kW and kVA. APC’s sizing guidance emphasizes that power factor matters when selecting a UPS, because a low power factor can require more kVA for the same real power. Use the actual or guaranteed power factor of the load rather than assuming unity.
Capture step changes from server boot storms, workload scheduling, chiller staging and pump starts. The inverter must meet the transient without an unacceptable voltage or frequency excursion. A battery-energy calculation cannot compensate for an inverter that cannot supply the required instantaneous current.
Choose the operating objective and runtime
| Objective | What determines duration | Typical sizing question |
|---|---|---|
| Generator ride-through | Generator start, stabilization and transfer time, plus a site reserve | Can the UPS carry the critical bus until the generator accepts load? |
| Controlled shutdown | Time to notify systems, complete writes and turn off equipment safely | How long does the documented shutdown sequence take at the worst load? |
| Islanded backup | Required autonomy, fuel availability and load-shedding plan | How many hours must selected loads operate without utility or generator power? |
| Peak-demand shaving | Length and shape of the tariff interval, clipping target and recharge opportunity | How much energy is above the grid-import limit before the battery can recharge? |
| AI transient smoothing | Ramp magnitude and the interval over which the fluctuation must be filtered | What percentage of the ramp must the battery absorb, and for how long? |
Schneider Electric notes that, in a generator-backed design, the theoretical UPS runtime is only the time until generators start and the load transfers, often cited as 10–20 seconds. That figure describes the generator-transfer example in its article, not a universal autonomy target. Include generator crank retries, breaker sequencing, stabilization, failed-start contingencies and a documented reserve when setting the actual requirement.
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A facility without dependable generation may require a longer autonomy period or a controlled shutdown. Peak shaving and islanded operation should be modeled as energy-management problems, not treated as an ordinary short-runtime UPS.
Step-by-step battery sizing workflow
1. State the boundary and success condition
Specify the bus, loads, maximum import or discharge power, runtime, transfer requirement, minimum reserve and acceptable load shedding. State whether the number is a normal operating target, a worst-case design value or a contractual service level. Identify the design temperature and end-of-life condition at which the requirement must still be met.
2. Create the load and event scenarios
Use synchronized interval data to create scenarios for normal operation, maximum IT utilization, cooling at design ambient conditions, maintenance configurations and likely future expansion. For each scenario, include the loads that remain online and the loads that are intentionally shed. A single average kW value hides both transient power and long high-load intervals.
3. Size discharge power in kW and kVA
For backup, set P_required to the maximum coincident critical-load demand, then verify the inverter’s step-load and overload capability. Include motor-start current or use approved starting methods if mechanical equipment is on the bus. For peak shaving, calculate the maximum difference between measured site demand and the permitted grid-import limit. Check the result at the actual power factor, not only at kW.
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4. Calculate usable energy
For a constant critical load, the first-order calculation is:
E_usable (kWh) = P_critical (kW) × runtime (h)
For a changing load, integrate the load curve:
E_usable = Σ P_critical,t × Δt
For peak shaving, integrate only the battery output above the grid-import limit. If site demand is 5,000 kW and the limit is 4,000 kW, the battery supplies 1,000 kW only while demand exceeds 4,000 kW; it does not automatically discharge at 1,000 kW for the entire tariff window. A longer or repeated exceedance increases kWh even when the clipping target remains unchanged.
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5. Convert usable energy to nameplate energy
The battery’s catalog kWh is not all available at the design point. A practical derating expression is:
E_nameplate = E_usable ÷ (η_discharge × usable_SOC_fraction × aging_fraction × temperature_fraction) + reserve
Use manufacturer runtime tables at the design temperature and end-of-life condition. Include inverter and battery discharge losses, the minimum and maximum state-of-charge limits, capacity fade, low-temperature effects, auxiliary consumption and a documented reserve. Legrand warns that aging and ambient temperature can shorten real autonomy and recommends a design margin.
Keep the reserve definition explicit. A 10% reserve can mean 10% additional energy, a minimum state-of-charge floor, or both; those are not interchangeable. Record whether the reserve covers measurement uncertainty, generator-start failure, degradation beyond the forecast or a second event.
6. Model recharge and repeated events
Specify the maximum recharge power, charging efficiency, utility capacity and time available before the next event. A battery that can meet one peak but cannot recharge before the next demand interval is undersized operationally. For generator-backed UPS systems, verify that charging does not overload the generator or violate the site’s step-load plan.
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7. Add redundancy and growth
Redundancy is an architecture choice, not a percentage added blindly to kWh. Eaton defines N+1 as the number of units or modules required to support the load plus one additional unit or module; after one failure, the remaining units must still support the load. In a 2N design, two independent paths each have full required capacity, allowing one complete path to be unavailable for maintenance or failure.
| Architecture | Capacity concept | Strength | Trade-off |
|---|---|---|---|
| N | Exactly the capacity required for the design load | Lowest initial equipment count and footprint | No unit failure margin; maintenance may require a bypass or load transfer |
| N+1 | Required capacity plus one module or unit | Continues serving the load after one unit failure | Requires verified sharing, maintenance procedures and enough physical separation |
| 2N | Two independent full-capacity paths | Strong fault and maintenance tolerance when loads are dual-corded | Higher capital, space, losses and coordination complexity |
NREL cautions that a large identical redundant unit can operate at a low load factor. Compare modular systems and fixed blocks at both today’s load and the forecast load; modular UPS systems can improve scalability and partial-load operation. Eaton gives 15% growth over five years as an example planning assumption, not a universal rule. Replace it with the facility’s signed capacity plan, rack deployment schedule and cooling expansion forecast.
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8. Size for AI workload transients
AI training and inference can create rapid, repeating ramps as accelerators move between compute phases. Vertiv states that a correct sizing approach starts with a detailed understanding of the AI load profile. Define two separate smoothing objectives: the percentage of fluctuation to filter and the duration for which the battery must sustain that filtering.
Size inverter power for the ramp magnitude and battery energy for the smoothing interval. For example, filtering a 20% fluctuation for 30 seconds is primarily a power and control problem; filtering the same fluctuation for 30 minutes becomes a materially larger energy requirement. Validate the control response, state-of-charge limits and interaction with generators before committing to a smoothing target.
9. Complete safety, interconnection and permitting checks
Stationary energy-storage systems must satisfy local electrical and fire requirements in addition to performance requirements. NFPA lists NFPA 855 as the active standard for minimum hazard-mitigation requirements for stationary energy-storage systems, with the 2026 edition shown in its current standards listing. Confirm the edition adopted by the authority having jurisdiction.
IEEE 2030.2.1-2019 addresses BESS design, operation, maintenance, integration and interoperability. IEEE 485-2020 describes methods for defining DC load and sizing lead-acid stationary batteries. Review ventilation, thermal management, separation, detection, suppression, emergency shutdown, seismic design, access, signage, utility interconnection and first-responder procedures with the authority having jurisdiction and the utility.
Worked example: a 2,000 kW generator-transfer load
Assume measured critical load is 2,000 kW and the generator-transfer requirement is 30 minutes. The design assumptions are 95% discharge efficiency, a 90% usable state-of-charge window, 80% end-of-life capacity and a 10% reserve.
- Ideal usable energy:
2,000 kW × 0.5 h = 1,000 kWh. - Apply discharge, state-of-charge and aging derating:
1,000 ÷ (0.95 × 0.90 × 0.80) = approximately 1,462 kWh. - Add the stated 10% reserve:
1,462 × 1.10 = approximately 1,608 kWhof nameplate energy. - Specify a UPS or PCS capable of at least 2,000 kW at the required power factor, plus any documented step-load or overload capability.
This is an illustrative method, not a site recommendation. The final selection must use the manufacturer’s certified runtime curve at the design temperature and end-of-life condition, include auxiliary loads and verify the actual transfer sequence. If cooling, pumps or controls are added to the critical bus, both the power and energy calculations change.
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Peak-demand shaving requires interval and recharge simulation
Demand charges are usually based on a utility-defined interval, commonly represented in 15-minute data, while the battery may respond every second. Simulate the actual tariff calendar, holidays, demand window, baseline demand, clipping limit, battery efficiency, minimum state of charge, maximum cycles per day and recharge power.
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Calculate the discharge profile
At each time step, set battery output to the amount needed to keep grid import at or below the target, subject to the PCS power limit and state-of-charge availability. Integrate that output over the interval to obtain required usable kWh. Include coincident cooling peaks and any second peak later in the day.
Test recharge feasibility
Model when charging is allowed, how much utility or generator capacity is available, and whether charging itself creates a new demand peak. A battery may meet the first event yet fail to provide the promised reduction after an incomplete recharge. Evaluate worst-case weather, operating schedules and back-to-back peaks rather than a single representative day.
Separate economic and resilience reserves
If the same BESS supports emergency backup and demand management, reserve a state-of-charge floor for resilience. Document who can override economic dispatch, what happens during a utility outage and how the control system prevents peak-shaving commands from consuming emergency energy.
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Do not choose VRLA, lithium-ion or another stationary technology from nominal kWh alone. IEEE 1679-2020 recommends evaluating stationary storage by performance, service life and safety attributes. Request comparable, end-of-life data from each supplier.
| Comparison item | What to request |
|---|---|
| Delivered power | Continuous and short-duration kW at end of life, design temperature and specified power factor |
| Usable energy | Temperature-corrected kWh within the permitted state-of-charge window, including auxiliary consumption |
| Runtime | Certified runtime at the actual load curve, not only a nominal constant-load point |
| Recharge | Maximum charging power, efficiency, time to target state of charge and generator compatibility |
| Life | Cycle life, calendar life, warranty limits and capacity-retention definition |
| Physical design | Footprint, weight, floor loading, clearance, acoustic output and thermal-management needs |
| Safety and code | Fire testing, detection and suppression interfaces, ventilation, separation and listed configurations |
| Controls | Battery-management-system data, alarms, cybersecurity interfaces and UPS or microgrid interoperability |
| Maintenance and replacement | Labor, monitoring, module replacement method, outage requirements and disposal obligations |
| Project cost | Total cost over the project life, including augmentation, replacements, inspections and electrical work |
Common sizing errors and how to correct them
- Converting a kW target directly into kWh: multiply the battery output by the modeled duration and integrate changing demand; a longer peak needs more energy even if the clipping kW is unchanged.
- Using IT nameplate power as total facility load: add measured cooling, pumps, networking, controls and other loads that remain online.
- Quoting a 10–20 second runtime as a rule: treat it only as Schneider’s generator-transfer example and include the site’s actual start, transfer and failure assumptions.
- Ignoring end-of-life and temperature: require certified runtime and capacity at the design temperature and end-of-life condition, then apply a documented reserve.
- Adding redundancy after the energy calculation: model N+1 or 2N paths explicitly, including the load each path carries after a failure or during maintenance.
- Overlooking recharge: simulate the next event, charging power and utility limit; a battery that cannot recover state of charge is not meeting the operating requirement.
- Missing AI ramps: use fine-grained telemetry and define both the fluctuation percentage and smoothing duration before selecting inverter power.
- Skipping code review: confirm the locally adopted fire, electrical, seismic, ventilation and interconnection requirements with the authority having jurisdiction.
Document the design so it can be verified
The sizing package should contain the one-sentence service requirement, one-line electrical boundary, interval-data sources, load scenarios, power-factor assumptions, kW and kVA calculations, energy integration method, derating factors, reserve definition, runtime and recharge simulations, redundancy diagram, growth forecast, control sequences and code basis.
During commissioning, test the measured load against the model, verify transfer and bypass behavior, demonstrate the specified step-load response, confirm alarms and state-of-charge limits, and run a controlled discharge or equivalent factory-accepted test. Record results at the design temperature where practical and update the operating model when IT or cooling capacity changes.
Quick Recap
Standards and guidance to include in the review
- IEEE 485-2020: methods for defining DC load and sizing lead-acid stationary batteries.
- IEEE 2030.2.1-2019: BESS design, operation, maintenance, integration and interoperability.
- IEEE 1679-2020: evaluation of stationary storage by performance, service life and safety attributes.
- NFPA 855: minimum hazard-mitigation requirements for stationary energy-storage systems; NFPA’s current listing shows the 2026 edition.
- Schneider Electric BESS white paper: version 1.1 released 13 November 2024, useful for design and integration context.
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