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Start with a measured or defensible forecast of IT load, define exactly which facility loads must remain energized, and then test normal, maintenance and failure conditions. A useful early estimate is total facility power = forecast IT power × assumed PUE. Generator capacity, however, must be calculated from the simultaneous load on the emergency bus, converted to kVA at the expected power factor and checked for starting transients, UPS behavior, derating and redundancy. These calculations establish a design basis; a licensed electrical engineer must produce the construction-ready design for the applicable utility and safety codes.
What you are actually sizing
A data center has several different power numbers, and confusing them is the fastest way to undersize equipment. The IT load is the electricity entering servers, storage, network equipment, telecom systems and other information-technology devices. Total facility power also includes cooling, UPS losses, battery charging, distribution losses, lighting, controls, security, fire systems and other infrastructure.
The utility service must support the facility in normal operation. The UPS must support its defined critical load during a utility interruption and while generators start and stabilize, or long enough to perform an orderly shutdown. The generator plant must support every load assigned to the emergency bus, not merely the racks. Each value should state its measurement boundary: utility service, UPS input, UPS output, PDU or rack branch circuit.
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Step 1: Define the design case and electrical boundary
Write the design basis before doing arithmetic. Record the following items for the specific building, location and operating scenario:
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- Utility voltage, phase, frequency and service configuration.
- Current operating mode and the forecast date for the design case.
- Availability objective, such as N, N+1 or 2N, including what “failure” and “maintenance” mean for the project.
- Present IT demand, expected peak demand and end-of-life growth forecast.
- Which loads are connected to the normal bus, UPS output and emergency bus.
- Whether each measurement is at the service entrance, UPS input or output, PDU, branch circuit or rack.
- Environmental conditions that affect equipment ratings, including altitude and ambient temperature.
DOE and Lawrence Berkeley National Laboratory define peak total electric demand at the data-center boundary and distinguish IT input from supporting infrastructure. State that boundary in every calculation. A utility-meter number cannot be compared directly with a rack-meter number unless the intervening losses and loads are identified.
Step 2: Build an IT-load model
Inventory every IT category
List servers, storage arrays, network switches, routers, telecom equipment, security appliances and any other equipment that will be powered as IT. Separate equipment that is genuinely critical from equipment that can be shed during an outage. Record quantity, expected operating range, power factor where available, and the intended retirement or expansion date.
Prefer measured demand to nameplates
Use rack-PDU or branch-circuit readings when they exist. Trend present load, 15-minute peaks and shorter transients where the metering system supports them. If measurements are unavailable, use vendor data with explicit assumptions for utilization, diversity and growth. Nameplate maximum watts are often a poor prediction of actual demand, but designing only from today’s average can leave no headroom for a high-utilization workload or future equipment.
Create at least three IT cases:
- Current operating case: the measured or best-estimate load today.
- Design peak: the coincident load expected during the selected planning horizon.
- End-of-life case: the load after planned growth, including rack additions and higher-density systems.
Do not add every device’s maximum rating as though all devices reached it simultaneously. Apply documented diversity and coincidence assumptions, and identify which assumptions are conservative.
Step 3: Add non-IT facility loads
Once the IT cases are established, add the infrastructure required to operate them. The list normally includes:
- UPS conversion losses, static-switch losses and battery-charging power.
- Transformers, switchgear, PDUs and other distribution losses.
- Chillers, cooling towers, computer-room air handlers, pumps, fans and controls.
- Humidification or dehumidification equipment.
- Lighting, receptacles, building-management systems and monitoring.
- Physical security, fire detection, suppression controls and communications.
- Any mechanical or life-safety equipment intentionally placed on generator power.
Cooling and power-conversion equipment can be a substantial fraction of facility demand, especially at peak outdoor conditions or during a restart. A design that multiplies rack watts by a convenient factor without identifying these subsystems cannot show what the utility or generator must actually carry.
Use PUE for an early total-power estimate
Power Usage Effectiveness (PUE) = total facility input power ÷ IT-equipment input power. A PUE of 1.0 would mean that every watt entering the facility reaches IT equipment; real facilities are higher because cooling, conversion and distribution consume power. ASHRAE emphasizes that the metric is meaningful only when the parties agree on the boundary and which consumption belongs to IT versus physical infrastructure.
For an early estimate:
Total facility power = forecast IT power × assumed PUE
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For example, 1,000 kW of forecast IT input with an assumed PUE of 1.35 produces an early estimate of 1,350 kW at the stated facility boundary. That is not a guarantee of operating demand: it is a planning value tied to the 1.35 assumption. As design progresses, replace the assumed PUE with measured or engineered loads for cooling, UPS, distribution and other subsystems.
Do not present a single “correct” industry PUE target. PUE varies with climate, cooling architecture, utilization and measurement boundary. A low annual PUE does not by itself prove that the emergency bus, UPS or generator is correctly sized for a peak event.
Choose a redundancy topology before sizing equipment
Redundancy changes both capacity and the cases that must be checked. The following descriptions apply to the complete power and cooling path, not just to generators.
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|---|---|---|
| N | Capacity exactly matches the defined load. | A single module or maintenance event may reduce service unless the load is shut down or transferred. |
| N+1 | One additional module can be lost while the design load remains served. | Check the remaining modules, distribution path and cooling capacity after the largest credible outage. |
| 2N | Two independent capacity systems can each support the defined load. | Independence must include switchgear, feeders, UPS, generators, fuel, controls and cooling paths; dual-corded servers alone do not create 2N. |
| Distributed redundant | Multiple smaller paths share load with spare capacity distributed among them. | Study load sharing, controls, fault isolation and the effect of losing a path or module. |
Evaluate at least the normal peak, a planned maintenance condition, a single-module or path failure, utility loss, generator-start conditions and any bypass configuration. The worst case may not be the highest normal load; a failure can force remaining equipment to operate at a higher percentage of rating while motors restart.
Size the utility service and distribution system
Convert the calculated real power to the service voltage and phase, then select transformer, switchgear, feeder and protective-device ratings. Continuous-load rules, design margins and local utility requirements apply. The engineering study must also address:
- Short-circuit current and equipment interrupting ratings.
- Protection coordination and selective tripping.
- Harmonic current from UPS rectifiers and other nonlinear loads.
- Expected power factor and its effect on kVA and conductor loading.
- Voltage drop, grounding and bonding.
- Future modules and physical space for expansion.
A kW figure alone is insufficient. Three-phase current, for example, depends on voltage, power factor and whether the stated voltage is line-to-line or line-to-neutral. The final one-line diagram and calculations belong to a qualified electrical engineer working under the jurisdiction’s electrical, fire and utility codes.
Size the UPS for the critical load and ride-through time
Define exactly what the UPS output must carry: usually the critical IT load and any directly connected controls or auxiliary systems. The UPS must bridge the interval from utility failure through transfer and generator stabilization, or provide the specified time for an orderly shutdown if generation is unavailable.
Specify both usable kW and kVA. A UPS can reach its kVA limit before its kW limit when the connected load has a low power factor. Include:
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- Battery autonomy at end of life, not only when new.
- Recharge time and the effect of recharge current on the generator.
- Static or maintenance-bypass behavior.
- Module configuration and the required N+1 or 2N condition.
- Future modules, cabinet space and upstream breaker coordination.
- Harmonic interaction and generator compatibility of the rectifier.
ASHRAE describes this ride-through function and notes that mission-critical UPS installations commonly use redundancy. A longer battery runtime may support a controlled shutdown, but it does not remove the need to analyze generator starting, fuel duration or cooling loads.
Calculate the emergency-bus load for generators
List simultaneous generator loads
Start with a load schedule for every device that starts or runs on generator power:
- UPS input, including battery-recharge demand.
- Cooling plant, pumps, fans, tower equipment and associated controls.
- Lighting, monitoring, security, fire systems and communications that must operate.
- Other legally required or operationally necessary building loads.
Then determine which loads can run simultaneously and which are sequenced. The basic running-load calculation is:
Generator running-load kW = simultaneous emergency-bus kW
Generator running-load kVA = emergency-bus kW ÷ expected power factor
Apply engineering and code margins after establishing the actual load schedule. Do not simply add every nameplate rating; that can grossly oversize the plant and still miss a transient problem.
Check starting and step-load behavior
Large motors and chillers can draw several times their running current during starting. Analyze starting sequence, inrush, voltage dip, acceleration time and the largest step load. Confirm that the UPS rectifier will accept generator voltage and frequency while other motors start. Verify minimum stable loading as well: a lightly loaded diesel generator can require a load bank or a different operating strategy, depending on the equipment.
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Generator output may be reduced by altitude, high ambient temperature, radiator configuration and other site conditions. Confirm the rating at the actual installation conditions, not just the headline rating on a data sheet. Size fuel storage for the required runtime and replenishment plan, and address emissions permits, sound limits, fire protection, seismic requirements and tank maintenance.
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Treat multiplier guidance as product-specific
A Schneider Electric USA FAQ for single-phase Symmetra UPS systems states: “A generator should be sized to 1.5 to 2 times the size of the total load to be attached to the generator.” The same guidance recommends sizing at 1.5 times frame capacity while not exceeding 75% of generator load capacity for that product family. The FAQ was published in 2012 and modified in 2024. These figures describe that UPS family’s guidance; they are not a universal multiplier for every generator, UPS or data center. The emergency-bus schedule, power factor, transients, derating and redundancy govern the actual selection.
Worked estimation example
Assume a planning case with 800 kW of forecast IT input and an early PUE assumption of 1.30. The early total-facility estimate is:
800 kW × 1.30 = 1,040 kW
That 1,040 kW is a facility estimate at the declared boundary; it is not automatically the generator load. Suppose the emergency schedule contains 800 kW of UPS input, 180 kW of cooling and pumps, 25 kW of lighting and controls, and 15 kW of security and fire systems. The simultaneous running load is 1,020 kW. At an expected 0.90 power factor:
1,020 kW ÷ 0.90 = 1,133 kVA
The engineer must then test motor starting, UPS recharge, step-load response, site derating, minimum loading, transfer sequencing and the selected N, N+1 or 2N case. A generator nameplate chosen solely from the 1,040 kW PUE result would not demonstrate that those conditions are met.
Compare generator, UPS and cooling choices
| Decision | Options | What to evaluate |
|---|---|---|
| Generator arrangement | One large set; multiple parallel sets; modular sets | Starting steps, maintenance, fault containment, minimum loading, controls, fuel logistics and failure of one set. |
| UPS technology | Static battery UPS; flywheel; diesel rotary UPS | Ride-through duration, maintenance regime, harmonics, recharge or refueling behavior and generator interaction. |
| Cooling system | Air-cooled; water-cooled; liquid-cooled | Peak electrical demand, pump and fan starting, restart sequence, water dependence and generator burden. |
| Measurement maturity | Utility or UPS aggregate meters; rack-level meters with continuous trending | Ability to identify coincident peaks, diversity, stranded capacity and subsystem losses. |
There is no topology that is automatically best. The correct choice follows from the availability objective, the load profile, maintenance strategy, climate, fuel plan and the consequences of losing a path.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Validate the model with metering and commissioning
Install or use metering at the rack PDU or branch circuit where possible. ASHRAE identifies the rack PDU as the final IT connection point and a source of fine-grained consumption data. Reconcile rack totals with PDU, UPS and utility readings so unexplained losses are visible. Trend 15-minute demand and shorter peaks when transient behavior matters.
Before acceptance, commission the complete sequence rather than only energizing equipment:
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- Perform transfer tests for normal-to-emergency and return-to-normal sequences.
- Use a load bank where required to prove generator capacity and stable operation.
- Test the largest motor or chiller starts and the planned UPS recharge sequence.
- Verify protection settings, selective coordination and emergency-stop behavior.
- Simulate utility loss, generator failure, UPS module loss, cooling failure and maintenance bypass conditions.
- Document measured voltage, frequency, kW, kVA, power factor, temperatures, transfer times and remaining fuel at each test point.
The PNNL, ASHRAE and NEMA performance-validation framework treats commissioning as part of establishing the design’s performance, not as an optional final demonstration. Update the load model with the measured results and preserve the assumptions for future capacity reviews.
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Common sizing mistakes and corrections
Using server nameplate watts as facility demand
Problem: Nameplates ignore utilization, diversity and planned growth. Correction: Meter racks or branches, create current, peak and end-of-life cases, and retain documented vendor assumptions where measurements are unavailable.
Applying PUE without defining its boundary
Problem: A PUE number can include or exclude loads differently across projects. Correction: State the numerator, denominator and measurement point, then replace the assumption with subsystem data as the design matures.
Sending only the IT load to the generator calculation
Problem: Cooling, pumps, UPS losses, controls and legally required systems may be unavailable when needed most. Correction: Build an emergency-bus schedule and identify every running and starting load.
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Using a universal 1.5× or 2× generator rule
Problem: A multiplier cannot capture power factor, motor starting, nonlinear UPS loads, derating or redundancy. Correction: Use product guidance only within its stated scope and validate the actual sequence with an engineer and commissioning tests.
Checking only normal peak operation
Problem: Maintenance bypass, a failed module or a restart can be the limiting case. Correction: Run a capacity matrix for normal, maintenance and credible failure states, including the largest step load.
Ignoring recharge and minimum-load behavior
Problem: A generator may be stable at steady load but struggle when a UPS recharges batteries or when the facility is lightly loaded. Correction: Include recharge current, minimum stable loading and any load-bank strategy in the sequence study.
Information to hand to the electrical engineer
A useful design package contains:
- Single-line boundary and voltage/phase information.
- IT inventory with measured or assumed kW, kVA, power factor, diversity and growth.
- PUE definition and subsystem load breakdown.
- Emergency-bus load schedule with starting and sequencing data.
- Redundancy topology and every maintenance and failure case.
- UPS usable kW/kVA, end-of-life autonomy, recharge and bypass requirements.
- Generator site conditions, derating, fuel runtime, emissions and sound constraints.
- Metering plan, commissioning scripts, load-bank requirements and acceptance criteria.
This package gives the engineer a traceable design basis for utility service, switchgear, UPS, generators, protection, fuel and controls. It does not replace jurisdictional calculations, permits or a construction-ready one-line diagram.
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