A 90 kW AI rack does not map to one universal UPS size. Size the system in separate decisions: define the loads that must stay online, measure real power and apparent power, check both UPS kW and kVA limits, add planned growth, test the required failure case, set battery runtime from the operating plan, and validate the UPS against the rack’s transient workload. AI accelerators can change demand repeatedly, so average rack power alone is not enough.
The 90 kW figure is useful for scale, not as a sizing formula. A Vertiv reference design puts twelve 90 kW racks in a 1.1 MW facility and uses a 1,500 kVA UPS for IT, but that configuration also includes centralized distribution and direct-to-chip liquid and air cooling. A different voltage, topology, redundancy target, runtime or cooling architecture can require a different design.
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What UPS sizing actually has to solve
A UPS has two independent jobs. Its inverter must deliver the load’s instantaneous electrical demand, and its batteries must support that load for the required time. A unit can have enough output capacity but too little runtime, or long runtime at a load that exceeds its inverter or bypass limits.
For an AI data-center rack, the design therefore has four separate questions:
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- What equipment and auxiliaries are actually protected by the UPS?
- How many watts and volt-amperes do those loads draw, including peaks?
- What growth and failure condition must the installed system tolerate?
- How long must the batteries carry the load, and how must they respond to repeated AI transients?
Those answers produce a project-specific UPS architecture. They do not produce a reliable answer by multiplying rack kilowatts by a fixed ratio.
1. Define the protected load and the facility boundary
Start with a one-line view of the electrical system. List every load that must remain energized through a utility interruption, not every load in the building. Include the intended servers, networking, storage, rack power-distribution units and any controls or auxiliary devices assigned to the UPS.
Decide how cooling is powered
Cooling may be on the UPS, on generator-backed distribution, or on another independent path. Do not automatically add the entire cooling plant to a rack UPS. Trace the actual feeders and controls. Schneider Electric notes that cooling restart time after a transfer can matter at high rack densities; a design may need to keep selected pumps, controls or heat-rejection equipment energized even when the main cooling plant is not on the same UPS.
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Document whether the UPS feeds one rack, a row PDU, a room bus or a central IT distribution system. The boundary determines which transformers, switchgear, bypasses and downstream PDUs are part of the protected load. It also determines whether a rack-level UPS is electrically appropriate or whether the rack belongs on a facility-scale power train.
2. Measure watts, VA and power factor
Record the electrical behavior of each protected load over its intended operating range. For every device or aggregated PDU, capture voltage, current, real power in watts, power factor when available, and the minimum, typical and peak values. Reconcile meter readings with equipment nameplates and the planned operating envelope.
| Quantity | Meaning | How it affects selection |
|---|---|---|
| W or kW | Real power delivered to the equipment | Must be below the UPS continuous watt rating and within its overload curve. |
| VA or kVA | Apparent power, combining voltage and current | Must be below the UPS apparent-power rating, including expected peaks. |
| Power factor | The relationship between real and apparent power | VA equals watts divided by power factor when the factor is known. |
| Battery energy and runtime | How long the stored energy can support the load | A separate constraint from inverter output capacity. |
Eaton’s basic calculation is VA = voltage × amperage. If only watts and power factor are known, VA = watts ÷ power factor. Sum the VA of the connected equipment, then compare that total with both the UPS kVA and kW ratings.
Use measurements without losing the peaks
A nameplate maximum may be higher than normal consumption, while an observed average can hide short accelerator or power-supply peaks. Use time-series measurements or validated simulations where possible. Record the sampling interval and operating state so that a brief but consequential transient is not averaged away.
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Illustrative calculation, not a design recommendation
Suppose a hypothetical rack measures 90 kW at a power factor of 0.90. Its apparent load would be 90 ÷ 0.90 = 100 kVA. Applying Eaton’s illustrative 15% growth allowance would make the planning values 103.5 kW and 115 kVA before any redundancy or transient margin is considered. The actual UPS still has to be checked against its own kW/kVA ratings, overload behavior, battery requirement and failure case.
3. Check the UPS output limits in both dimensions
Read the UPS data sheet for continuous output watts and volt-amperes, not just the larger-looking nameplate number. A UPS with a high kVA rating can still run out of kW at the load’s power factor; a unit with enough kW can still exceed its kVA or current limit during a peak.
Review continuous and overload behavior
Check the duration and magnitude of the inverter’s overload rating, the static-bypass rating, crest-factor capability, and the allowable load power-factor range. Confirm whether the rating applies at the selected input and output voltage and at the site’s ambient temperature. Use the worst expected operating point rather than a nominal laboratory condition.
Keep capacity separate from runtime
Output capacity answers whether the UPS can carry the load at a given instant. Runtime answers how long its batteries can do so. Do not treat a larger battery cabinet as permission to exceed the inverter rating, and do not assume a high-output UPS provides the desired autonomy without a runtime test at the real load.
4. Add growth and design for the required failure
Growth and redundancy are different allowances. Growth covers additional equipment; redundancy covers equipment that is unavailable during a fault or maintenance.
Growth allowance
Eaton recommends at least 15% capacity above current equipment need and illustrates multiplying the load by 1.15 for a five-year growth allowance. That figure is vendor guidance, not a code requirement. Use the deployment schedule, accelerator refresh cycle, rack population plan and expected power-density changes to set the project allowance.
N+1 capacity
In an N+1 arrangement, one additional UPS unit or module is installed beyond the number required to carry the protected load. Calculate capacity after removing the largest intended unit or module; total installed nameplate capacity is not enough if the remaining equipment cannot support the load.
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2N and dual paths
A 2N design provides a second fully capable system. Schneider Electric describes the higher cost associated with this extra independence. Verify that each path has the required capacity, upstream feed, bypass, maintenance route and downstream PDU arrangement. A dual UPS nameplate without independent feeders or usable maintenance bypasses does not create true 2N availability.
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Check the failure you actually promise
- Single module or unit failure
- Maintenance bypass operation
- Loss of one input feeder
- Static-bypass transfer
- Battery string or battery cabinet isolation
- Unequal load sharing between parallel units
Specify the permitted interruption, transfer path and remaining capacity for each case before choosing the topology. N+1 and 2N are arrangements, not substitutes for a facility availability target.
5. Set battery runtime from the operating plan
Choose autonomy by deciding what happens during an outage. The objective may be to ride through utility disturbances until a generator starts, complete a controlled shutdown, or keep service available for a longer period while operators respond.
| Operating objective | Illustrative runtime cited by Schneider Electric | How to interpret it |
|---|---|---|
| Generator start and transfer | Typically 10–20 seconds | A representative transfer interval, not a guarantee for every generator or switchgear system. |
| Hyperscale examples | 1–2 minutes | An example of a risk and operating choice used in some facilities. |
| Cloud or colocation examples | About 5 minutes | An illustrative target, not a universal service requirement. |
| Finance examples | 10–15 minutes | A longer example reflecting a different risk tolerance. |
These values come from a 2020 Schneider Electric article and illustrate different decisions; they are not mandated targets. Specify the transfer time, shutdown sequence, generator start reliability, fuel plan and any contractual service objective for the actual site.
Size batteries at the real conditions
Select battery cabinets or strings only after the UPS family, load, voltage and autonomy are defined. Validate the manufacturer’s runtime at the actual load, the battery’s end-of-life capacity, ambient temperature, discharge rate and allowable depth of discharge. A new-battery runtime chart can overstate what an aging battery will deliver.
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6. Model AI transients, smoothing and recharge
AI training and inference can change rack demand repeatedly as accelerators synchronize, communicate and switch between workload phases. The UPS design must describe that time-varying profile rather than relying on a single average kW number.
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Measure the load shape
Vertiv’s 2025 paper recommends measuring or simulating three characteristics:
- Transient amplitude: how far the load moves above or below its baseline.
- Frequency or repetition: how often those changes occur.
- Duty cycle: how long the rack remains in each state.
Capture the profile at the electrical boundary that the UPS will see. A rack meter, PDU meter and facility meter can show different behavior because of aggregation and sampling.
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State how much fluctuation should be reduced and for what duration. Vertiv describes this as a fluctuation-reduction percentage and a smoothing window. The objective could be limiting the variation presented to a generator, reducing feeder swings, or preventing repeated inverter overloads. A percentage without a time window is not a complete requirement.
Validate battery response and recharge equilibrium
Battery capacity and chemistry must sustain the chosen smoothing objective without crossing operating state-of-charge limits. The recharge rate must also restore the battery between events. If recharge cannot keep pace with repeated transients, the UPS may pass fluctuations upstream and place additional stress on generators or the grid. Vertiv also warns that repeated UPS overloads can thermally stress power electronics.
Test the worst credible sequence, not only one isolated step. Include the initial state of charge, minimum and maximum load, recharge current, generator operating mode and the length of the smoothing window. The result should show that the battery neither runs down over repeated cycles nor violates the UPS manufacturer’s charge and discharge limits.
7. Understand what the 90 kW reference does and does not prove
Vertiv reference design number 0018 describes a prefabricated facility with 1.1 MW of total IT load across twelve racks. Each rack is specified at 90 kW, and the design lists a 1,500 kVA UPS for IT. The configuration combines centralized power distribution with direct-to-chip liquid cooling and air cooling.
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Those figures cannot be converted into a per-rack rule. The UPS serves the documented facility architecture, aggregation, cooling arrangement and operating assumptions. A twelve-rack site with different power factors, independent UPS paths, a different generator strategy or rack-level conversion equipment may need a different rating. Use the reference as a scale example and examine the complete one-line design, not just the 90 kW and 1,500 kVA numbers.
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8. Choose the topology for the critical-load architecture
Online double-conversion UPS technology is characterized by Schneider Electric as the choice for data-center and critical-infrastructure applications because the inverter continuously supplies the load and isolates it from many input disturbances. The topology decision still depends on the site’s voltage, fault-current, bypass and availability requirements.
Compare proposed configurations using the same protected load and the same failure scenario. The following checklist keeps unlike proposals from appearing equivalent:
| Comparison axis | Questions to answer |
|---|---|
| Usable output | What are the continuous kW and kVA ratings, power-factor range, crest-factor capability and overload duration? |
| Transient performance | Can the system follow the measured AI profile and meet the selected smoothing percentage and duration? |
| Runtime and battery | What runtime is delivered at the real load, at end of life and at site temperature? What are the chemistry, recharge rate and expansion limits? |
| Resilience | Is the architecture N, N+1, 2N or another arrangement? What happens after a unit, module, feed or battery failure? |
| Electrical fit | Do voltage, phase, feeder, connection, bypass, protection and PDU ratings match the site? |
| Thermal and space fit | Can the facility cool the UPS and batteries, and are footprint, clearances and maintenance access adequate? |
| Operations | Are monitoring, alarms, service access, replacement procedures and the growth path defined? |
No single option is established as a universal winner across these axes. The 2026 Schneider Electric AI sizing material is explicitly general guidance; an accurate application requires representative load and facility information.
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- Draw the boundary: mark the rack, PDU, control and cooling loads that the UPS must support.
- Collect time-series data: measure watts, voltage, current, power factor and peaks in each planned operating mode.
- Convert and aggregate: calculate VA for each load, sum the protected load and retain both kW and kVA totals.
- Apply the deployment plan: add a documented growth allowance and identify the largest unit or module that must be allowed to fail.
- Define autonomy: select the outage action, transfer interval and required runtime at end-of-life battery conditions.
- Specify AI behavior: provide transient amplitude, repetition, duty cycle, smoothing percentage and smoothing duration.
- Model recharge: prove that charging restores state of charge between repeated events without exceeding electrical or thermal limits.
- Review the one-line: check feeders, bypasses, protection, PDUs, generator interaction, cooling and maintenance paths.
- Commission under load: test normal operation, transfer, bypass, the planned failure case, transient sequences and battery recharge.
10. Validate a three-phase installation before purchase
High-density systems require a qualified project engineer to check actual input and output voltage, phase arrangement, current, available fault current, connection method, bypass and protective devices. The review should also cover rack-PDU ratings, harmonics and power quality where applicable, thermal conditions, battery limits, floor loading, clearances, maintenance access, controls and local electrical code.
Eaton notes that three-phase UPS sizing can be tricky and recommends specialist assistance. A manufacturer’s configuration tool or sales quotation cannot replace a coordinated electrical design, short-circuit study and commissioning plan.
Common sizing mistakes and the corrective action
- Using only kW: calculate VA and compare both output limits.
- Using only an average meter value: preserve the peak and transient profile in the sizing data.
- Applying 15% as if it were a code rule: tie growth to the actual five-year deployment plan.
- Calling total installed capacity N+1: recalculate after removing the specified failed unit or module.
- Buying batteries before defining runtime: select the UPS family first, then validate compatible packs at end of life and site temperature.
- Putting all cooling on the rack UPS by default: trace the facility architecture and support only the cooling loads assigned to that path.
- Ignoring recharge: test repeated AI events, not just one discharge.
- Assuming a small rack UPS fits a dense rack: Schneider Electric’s 2018 rack/tower catalog range of 1–20 kVA is aimed at smaller server or edge loads and does not cover a 90 kW rack.
- Scaling the 90 kW reference linearly: treat the Vertiv design as one integrated facility reference, not a per-rack formula.
Where rack-mount UPS equipment fits
Rack-mount UPS products are practical for smaller server, networking and edge-AI loads when their kW, kVA, runtime and bypass limits match the equipment. The cited Schneider Electric catalog covers a 1–20 kVA rack/tower range. A 90 kW rack is a facility-scale electrical design problem: it normally requires coordinated switchgear, distribution, cooling, batteries and often parallel UPS equipment rather than a single small rack chassis.
An external battery pack is an accessory decision only after the exact UPS model, load profile and autonomy target are known. Adding a battery cabinet to an incompatible model, or adding energy without checking charger and breaker limits, does not create a valid runtime design.
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What information an engineer needs for a final rating
A final UPS model and rating cannot be selected from the rack’s headline kW alone. Prepare these inputs for the design review:
- Measured minimum, typical and peak watts and VA
- Voltage, phase, frequency and power factor
- Rack and PDU one-line diagrams
- Cooling and controls assigned to the UPS
- Growth schedule and rack-population assumptions
- Required redundancy and maintenance-bypass strategy
- Outage objective and runtime at battery end of life
- AI transient amplitude, repetition, duty cycle and smoothing window
- Generator characteristics and transfer sequence
- Room temperature, space, floor loading and maintenance clearances
- Applicable local code and protection-study requirements
With those inputs, the engineer can test a candidate configuration against steady load, peaks, recharge equilibrium, bypass operation and the defined failure case. Without them, a quoted kVA number is only a preliminary capacity figure.
Bottom line
Size an AI-rack UPS as an engineered power system, not as a kW lookup. Measure the protected load in watts and VA, check both UPS output limits, add project-specific growth, prove the intended redundancy after a failure, and choose battery runtime from the outage plan. Then model the AI workload’s amplitude, repetition, duty cycle, smoothing objective and recharge equilibrium. The 90 kW rack reference demonstrates the scale of high-density infrastructure, but only the complete electrical, cooling and availability architecture can determine the correct UPS rating.
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