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There is no honest universal generator size for a liquid-cooled data center. Size the system from the loads that must operate during the defined utility outage, including the IT UPS, coolant-distribution units (CDUs), pumps, controls, heat-rejection equipment and any remaining air cooling. Then check both steady-state engine kW and alternator kVA, model motor and UPS transients, apply the required power and cooling redundancy case, derate for the site, and validate the complete transfer sequence with the generator and cooling vendors.
Liquid cooling changes the load schedule; it does not eliminate the need for one. A direct-to-chip installation can move much of the server heat into a liquid loop, but that loop still depends on powered pumps, CDUs, controls and facility heat rejection. The correct result is a project-specific rating and architecture, not a percentage added to IT nameplate power.
What the generator must keep running
Start with the outage operating scenario, not the servers’ nameplate watts. Write down what “online” means for this facility and for how long. The answer may differ for a short utility interruption, a prolonged outage with fuel replenishment, a maintenance condition, and the loss of one generator or one cooling component.
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Define the required operating case
- Utility-failure duration and the allowable UPS ride-through interval before generator power is accepted.
- Required IT capacity during the outage: the full installed load, a contracted capacity limit, or a deliberately shed portion.
- Cooling capacity and supply temperatures that keep every protected server within its thermal limits.
- Whether the design must survive a component failure, planned maintenance, or both while the utility is unavailable.
- Power and cooling topology: N, N+1, 2N, distributed redundant, or another arrangement.
- Fuel autonomy, refueling assumptions, emissions limits and the jurisdiction’s operating restrictions.
Power redundancy and cooling redundancy are separate decisions. An N+1 generator arrangement does not automatically provide N+1 CDUs, and a redundant liquid loop does not compensate for an undersized electrical path. State the normal, failure and maintenance cases explicitly before calculating capacity.
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Why liquid cooling changes generator sizing
Liquid-cooled servers add electrical equipment that an air-cooled load schedule may omit. A typical direct-to-chip path can include rack or row CDUs, primary and secondary pumps, valves, sensors, controls, heat exchangers, dry coolers or chillers, and building automation interfaces. Many facilities remain hybrid: air handlers, fans or room cooling continue to remove memory, storage, power-conversion and other residual heat. Do not assume that all server heat has moved to the liquid loop or that all facility cooling can be turned off during an outage.
Cold-plate systems capture “about 70-95%, varying by cold plate and server design” of IT heat output, according to an October 2026 Uptime Institute commissioning article. That is thermal context, not a generator-sizing multiplier. The unremoved heat and the equipment that transports and rejects liquid heat still belong in the backed-up load schedule.
Can the liquid loop bridge generator start?
Sometimes. ASHRAE’s Chapter 20: Data Centers and Telecommunication Facilities notes that residual liquid in header pipes can often maintain critical cooling until generators start, with full cooling restored by adding small supplemental pumps on UPS backup. This is a design strategy, not a permission to omit pumps or controls from the analysis. Determine the loop volume, heat capacity, flow decay, server protection limits and actual generator-start interval, then test the transition. If a small UPS-backed pump is required to preserve flow, its input, starting behavior and battery autonomy are part of the electrical design.
Build a complete backed-up load schedule
Create one schedule for every load that is energized by the emergency source in each operating case. Obtain measured or manufacturer data rather than relying on a single nameplate value.
| Load group | Items to include | Data to record |
|---|---|---|
| IT and UPS | UPS input, bypass equipment, distribution losses, battery chargers and the protected IT load | Running kW and kVA, power factor, harmonic current, transfer behavior, recharge or recovery current, and any walk-in function |
| Liquid loop | CDUs, primary and secondary pumps, VFDs, valves, sensors, controls and heat exchangers | Running and starting kW/kVA, motor starting current, VFD ramp, minimum flow, staging and failure response |
| Heat rejection | Chillers, dry coolers, cooling towers, condenser pumps, fans and remote radiators | Operating mode, ambient derating, starting sequence, minimum capacity and control interlocks |
| Residual air cooling | Computer-room air handlers, in-row units, fan walls and supplemental ventilation | Continuous and staged loads, fan starts, controls and the portion required for the hybrid design |
| Facility support | Lighting, life-safety systems, controls, monitoring, fuel systems and network equipment | Continuous load, emergency priority, inrush and whether it is on the generator, UPS or another source |
Record electrical characteristics, not just watts
For each item, capture voltage, frequency, phase, running kW, apparent power (kVA), power factor, nonlinear or harmonic behavior, inrush, motor-start current, VFD characteristics and the time at which the load is applied. Identify whether a load is always present, starts immediately, steps on after a permissive, or is shed when capacity is constrained.
A generator is limited by both engine power in kW and alternator capacity in kVA. Cummins states in its Generator Set Application Manual, Chapter 3: Sizing: “A generator set is a limited power source both in terms of engine power (kW) and generator volt-amperes (kVA), regardless of the type of excitation system.” A schedule that sums only kW can therefore pass an energy check while exceeding alternator current or producing unacceptable voltage distortion.
Calculate steady-state kW and kVA
For each operating case, sum the coincident running loads rather than every nameplate value. If a load’s kW and power factor are known, its apparent power is approximately:
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kVA = kW / power factor
For a three-phase feeder, apparent power can also be checked from line voltage and current:
kVA = √3 × line-to-line volts × amps / 1,000
Use the equipment manufacturer’s values for nonlinear UPS input, VFDs and corrected power-factor supplies. Do not assume that adding individual kVA values gives the final alternator requirement when phases, harmonics, diversity and operating modes differ; the electrical engineer should verify the one-line model and neutral and harmonic limits.
Account for UPS behavior
The generator may feed the UPS rectifier, the UPS may transfer through bypass, or both states may occur in different failures. Record the rectifier’s accepted voltage and frequency window, input-current distortion, battery-recharge current and any programmed ramp or “walk-in.” A UPS that gradually increases generator load can reduce the required transient capability compared with an instantaneous full-load step, but only if that sequence is documented, enabled and tested.
Do not add an arbitrary generic margin to the summed load to compensate for unknown behavior. Resolve the unknown behavior with UPS and generator data, a load-step study and an acceptance criterion.
Model starts, load steps and recovery
Steady-state capacity is only one part of the selection. During a utility failure, the generator starts unloaded or lightly loaded, the UPS transfers or ramps its rectifier, pumps and fans start, and chillers or other heat-rejection equipment may be enabled in stages. Each event changes engine torque, alternator voltage and system frequency.
Build the transition sequence
- Generator start: define crank time, minimum stable voltage and frequency, breaker-closure permissive and any required warm-up.
- Emergency bus energization: identify which controls, UPS inputs and small pumps are picked up first.
- UPS transfer or walk-in: specify transfer timing, rectifier ramp, battery contribution and recharge lockout.
- Cooling restoration: start supplemental pumps, CDUs, primary and secondary pumps, heat-rejection equipment and residual air cooling in the required order.
- IT restoration: connect loads immediately or in staged blocks, including any load shedding or priority logic.
- Steady operation: verify temperatures, flow, pressure, fuel consumption and generator loading after all permitted loads are online.
Cummins’ data-center guidance recommends defining transient and acceptance-test requirements and considering UPS walk-in rather than simply oversizing the generator to accept the entire load at once. The project specification should state the maximum allowed voltage dip, frequency dip, recovery time and equipment ride-through. Those limits come from the facility and connected equipment; they are not universal percentages.
Motor and VFD starting
Pumps, fans and compressors can draw several times their running current when started directly. A VFD may limit inrush but introduces harmonics and has its own ride-through and restart behavior. Obtain the actual start-current or VFD profile, the starting torque requirement and the sequence in which motors are permitted to run. If two pumps are intended to start together during a failure, model them together; do not use their separate running kW as a substitute for the combined transient.
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Coordinate power and cooling redundancy
Evaluate the generator and liquid system under the same normal, failure and maintenance scenarios.
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|---|---|---|
| Normal outage | Can the available generator set(s), switchgear and UPS path carry the required IT and facility loads? | Can the operating CDUs, pumps and heat rejection maintain server limits? |
| Single failure | Can the remaining generator capacity and distribution carry the required load after the specified generator, bus or feeder failure? | Can the remaining CDU, pump or loop carry required flow and heat rejection? |
| Maintenance | Can one set or power path be unavailable without violating the service objective? | Can the planned CDU or pump be isolated while preserving cooling? |
| Transition | Do transfer interlocks, breaker sequencing and UPS controls prevent an overload? | Do flow controls, thermal mass and supplemental pumps bridge the start interval? |
Uptime Institute describes N+1 CDU use as common where cooling redundancy is required. Vertiv explains that row-level N+1 requires an additional CDU. Those examples illustrate how the cooling topology changes the equipment count; they do not establish a universal topology for every facility. Shared and dedicated loops, 2N paths, isolation valves and control-system dependencies must be evaluated against the site’s availability objective.
Choose the generator rating and site configuration
After the load and transient cases are defined, select a generator rating appropriate to the duty and environment. Confirm all of the following:
- Duty class: standby, prime, continuous or a data-center-continuous rating. These categories have different permitted loading and operating assumptions.
- Site derating: altitude, maximum ambient temperature, humidity and radiator or remote-heat-rejection arrangement.
- Electrical compatibility: voltage, frequency, phase, neutral treatment, short-circuit contribution, harmonics and paralleling controls.
- Fuel system: usable tank volume, autonomy at the modeled load, replenishment time, fuel-quality management and jurisdictional limits.
- Emissions and permitting: engine certification, operating hours, exhaust treatment, acoustic limits and local approval.
- Physical integration: footprint, access for maintenance, ventilation, radiator location, cable routing and service clearances.
- Resilience: number of sets, bus arrangement, synchronizing controls, maintenance bypasses and the failure case that must remain online.
As a product example rather than a recommendation, Cummins lists QSK50 data-center-continuous configurations from 1,020 to 1,365 kW and 1,275 to 1,706 kVA, depending on configuration. Those manufacturer-specific figures cannot be generalized to a liquid-cooled facility, and the applicable output changes with rating, voltage and site conditions.
A project workflow from concept to acceptance
- Freeze the outage objective. Document the utility-failure scenario, IT capacity, ride-through time, cooling temperatures, fuel autonomy and N, N+1 or 2N requirements for each subsystem.
- Draw the one-line diagram. Show utility, UPS modules, emergency switchgear, generator sets, transfer switches, CDUs, pumps, heat rejection and all alternate paths.
- Issue load-data sheets. Request certified running and starting kW/kVA, power factor, harmonics, motor data, controls voltage and sequence requirements from every equipment vendor.
- Separate operating cases. At minimum, model normal outage, the specified generator failure, the specified cooling failure, maintenance operation and the worst permitted combination under the availability objective.
- Build the sequence of operations. Assign priorities and time delays for generator start, UPS transfer or walk-in, pumps, CDUs, chillers, fans, IT blocks and load shedding.
- Run a transient study. Use generator manufacturer software or an electrical-system study to test voltage and frequency response, alternator kVA, engine kW, motor starts, nonlinear UPS current and recovery time.
- Apply site and duty derates. Recalculate available output at the actual altitude, temperature, voltage, frequency, radiator arrangement and emissions configuration.
- Select the architecture. Compare one larger set, multiple paralleled sets, distributed emergency sources and the required maintenance and failure behavior.
- Review with vendors and the authority having jurisdiction. Resolve protection, coordination, emissions, fuel and controls issues before equipment is ordered.
- Commission the combined system. Test transfer, staged restoration, cooling flow, temperatures, alarms, failure modes, refueling assumptions and return-to-utility behavior with representative load.
How to compare generator architectures
There is no universal winner. Use the same project load cases and score each architecture against the criteria below.
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1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errors| Criterion | What to compare |
|---|---|
| Usable capacity | Site-derated kW and kVA in normal, maintenance and failure cases |
| Transient performance | Voltage and frequency dip, recovery time and motor-start capability for the specified sequence |
| UPS compatibility | Rectifier limits, walk-in behavior, harmonic performance and recharge control |
| Redundancy | Effect of a generator, bus, breaker, controller or maintenance outage |
| Fuel autonomy | Tank capacity, modeled consumption, replenishment logistics and fuel-system redundancy |
| Cooling integration | Radiator or remote heat rejection, ambient derating, acoustic limits and service access |
| Permitting | Emissions certification, operating-hour limits and local approvals |
| Commissioning evidence | Documented load-step results and integrated power-and-cooling tests |
Commissioning a liquid-cooled backup system
Factory data alone does not prove that the installed UPS, generator, switchgear, CDUs and cooling controls work together. Commission the sequence that the load schedule assumed.
Use representative thermal and electrical loads
Uptime Institute describes liquid-cooled load banks and rack emulators as a way to emulate liquid-cooled IT during integrated system tests. Treat this as an emerging commissioning tool, not a replacement for engineering review. The test should reproduce the expected electrical profile and liquid flow, pressure and heat-rejection behavior closely enough to exercise controls and alarms.
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Verify these acceptance points
- Generator start time and stable voltage and frequency before transfer.
- UPS transfer, rectifier walk-in, battery support and recharge lockout.
- CDU and pump start order, minimum flow, valve positions and loss-of-flow alarms.
- Chiller, dry-cooler, tower or radiator response at the design ambient condition or an appropriately controlled test condition.
- Voltage and frequency excursions at each load step and the measured recovery time.
- Server inlet, outlet and coolant temperatures throughout the transition.
- Operation after the specified generator, CDU, pump, feeder or control failure.
- Load shedding, restart and return-to-utility logic, including manual recovery procedures.
Common sizing mistakes and their fixes
Using IT nameplate watts alone
Why it fails: it omits UPS losses, CDUs, pumps, controls, heat rejection and residual air cooling.
Fix: build a coincident schedule for every backed-up electrical and mechanical load in each operating case.
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Adding a generic percentage “for liquid cooling”
Why it fails: liquid configurations differ in CDU count, pump power, heat-rejection method, IT density and redundancy. No broadly applicable published percentage determines generator size.
Fix: obtain equipment data and model the actual sequence and site derates.
Checking only steady kW
Why it fails: alternator kVA, motor inrush, UPS rectifier behavior and load steps can cause unacceptable voltage or frequency excursions.
Fix: run a transient study with certified starting and nonlinear-load data.
Assuming the UPS makes generator sizing irrelevant
Why it fails: the UPS battery bridges only its specified interval and may impose recharge or rectifier demands after transfer.
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Fix: include the UPS input profile, walk-in settings, battery autonomy and recovery current.
Assuming thermal mass guarantees ride-through
Why it fails: loop volume, flow, server limits and generator-start time vary, and a control or pump failure can remove the assumed bridge.
Fix: calculate the thermal interval, provide UPS-backed supplemental pumping if required, and test the actual transition.
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Why it fails: N+1 may apply to generators, CDUs or pumps independently, while a shared bus, control panel or heat-rejection path remains a single point of failure.
Fix: trace power and cooling paths end to end for normal, failure and maintenance cases.
A sizing worksheet for the design team
Use a separate worksheet for every required case rather than one blended total.
- List each coincident load and its running kW, kVA, power factor and harmonic characteristics.
- Mark each load as generator-backed, UPS-backed, shed, staged or unavailable in the case.
- Sum running engine kW and alternator kVA after applying the actual operating mode.
- List every start or step in time order, with starting current, duration and control ramp.
- Record the permitted voltage dip, frequency dip and recovery time for each connected system.
- Apply altitude, temperature, voltage, frequency, radiator and duty-rating derates.
- Repeat after each required generator, feeder, CDU, pump or cooling-path failure.
- Document the selected rating, the margin that remains after derating, and the evidence supporting it.
The output of this worksheet is not merely a generator model number. It is a coordinated set of ratings, controls, switchgear, fuel and cooling provisions that satisfies the stated outage and resilience case.
What information is still needed for an actual size
A numerical recommendation requires the facility load schedule, one-line diagram, UPS architecture and models, CDU and pump data, heat-rejection equipment, site altitude and temperature, voltage and frequency, emissions jurisdiction, fuel-autonomy target, transient limits and redundancy objective. Without those inputs, any single kW or kVA answer would be guesswork. Engage the facility electrical engineer and generator, UPS and liquid-cooling vendors to produce and validate the project-specific study.
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