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Size a data-centre UPS from the measured critical load in both kW and kVA, add a documented growth allowance, and then select enough modular or parallel capacity for the required redundancy. Size the standby generator for the entire emergency facility load—not just IT—including cooling, pumps, life-safety systems, communications and other house loads. Set battery runtime from the actual outage sequence and battery discharge curves; a generator-backed UPS commonly bridges about 10–15 minutes, but the correct value is the time needed for detection, generator start and warm-up, transfer, UPS acceptance and any controlled shutdown. Final generator and UPS ratings require manufacturer sizing software, site conditions, local code review and a qualified engineer.
Start with a complete data-centre load schedule
The load schedule is the foundation for every later decision. A server nameplate or a utility-bill average does not show the maximum electrical demand, motor-start current, recharge behaviour or redundancy requirement. Record the operating state that the power system must support, including normal operation, peak IT demand, maintenance conditions and credible startup events.
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Separate critical IT and facility loads
| Load group | What to record |
|---|---|
| Critical IT | Measured operating kW, credible peak kW, kVA, power factor, rack or row growth and any high-inrush equipment |
| Cooling and mechanical plant | Chillers, computer-room air handlers, pumps, fans, cooling towers, controls, standby sequencing and motor-start characteristics |
| Life-safety and security | Fire detection and suppression controls, emergency lighting, access control, CCTV, alarms and legally required systems |
| Communications and controls | Network rooms, building-management systems, monitoring, controls and telecoms that must remain available during an outage |
| Other house loads | Lighting, receptacles, battery chargers, fuel systems, water systems and any other load connected to the emergency distribution |
Use measured operating kW wherever it is available, then add credible peak and startup values. Record kVA and power factor for every major load. A generator may have to start motors while simultaneously accepting a step from the UPS; an average kW figure cannot demonstrate that it will hold voltage and frequency during that event.
Define the operating cases
Create separate schedules for the highest expected IT load, normal facility peak, maintenance with one module or path unavailable, generator-only operation and any staged load-restoration sequence. Identify which loads are intentionally shed and which must remain connected. The generator schedule should show the order in which chillers, pumps, air handlers and UPS systems come online, because that order determines the largest electrical step.
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Size UPS capacity in both kW and kVA
UPS kW is real power—the work performed by servers and other equipment. UPS kVA is apparent power and reflects current in the conductors, inverter and upstream equipment. The two ratings are related by power factor but are not interchangeable. Convert the scheduled real power to apparent power using the selected UPS output power factor, then verify that the resulting load is below both the UPS kW and kVA limits.
Apply growth without confusing it with redundancy
Eaton’s UPS sizing guide recommends at least 15% growth over five years. Treat that as a documented capacity allowance on top of today’s required load. It does not create N+1 capacity: the extra unit or module needed to survive a failure or maintenance must be added separately.
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Select a topology that preserves the required capacity
Choose modular or parallel UPS equipment so the remaining modules can carry the required load when one module is failed, isolated for maintenance or unavailable after a fault. Check the capacity in the exact maintenance state, including static bypass, maintenance bypass, switchgear and distribution restrictions. A nameplate total that is available only when every module is healthy is not an N+1 design.
Online double-conversion UPS systems are normally used where the load must be isolated from utility disturbances and bridged to a generator. External battery modules can extend the bridge, but the runtime must be taken from the manufacturer’s tables at the actual load, battery temperature and end voltage rather than from amp-hour arithmetic alone.
Understand the available equipment scale
Eaton lists enterprise UPS families spanning these apparent-power ranges:
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|---|---|
| Small enterprise and rack/server-room systems | 8–40 kVA |
| Mid-range systems | 50–360 kVA |
| Large parallel systems | 250–1,200 kVA |
| Megawatt-class systems | 1–1.7 MW |
These are published family ranges, not a recommendation for a particular facility. Select a model only after the engineered load schedule, power factor, growth and redundancy state have been checked.
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Set battery runtime from the outage sequence
Battery runtime is a bridge to another power source, not a substitute for the generator’s fuel supply. Define every event between utility failure and stable generator power:
- Utility-loss detection and UPS transfer to battery.
- Generator crank, start confirmation and engine warm-up.
- Automatic-transfer-switch sensing and transfer delay.
- UPS verification that generator voltage and frequency are acceptable.
- UPS transfer from battery to generator input.
- Battery recharge and any orderly shutdown time if the generator fails to start or is rejected.
Eaton’s UPS fundamentals handbook describes 10–15 minutes with a generator as a typical configuration and says most generators start within one minute, with five minutes as the maximum in that guidance. Those figures are planning references, not a universal runtime requirement. A site with a longer transfer sequence, cold-weather starting risk, staged mechanical startup or a mandated shutdown procedure may need more.
Use the manufacturer’s discharge curves
Obtain the selected battery manufacturer’s discharge curves for the actual UPS load, battery temperature, aging assumptions, recharge state and specified end voltage. Confirm whether the curve is for the complete string, the expected number of strings and the intended autonomy at the minimum design voltage. Nameplate amp-hours by themselves do not establish minutes at the required load.
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Check the recharge burden
After the UPS returns to generator power, battery recharge can be a substantial additional load. Confirm the recharge current limit and duration with the UPS manufacturer and include it in the generator’s transient and steady-state assessment. If recharge is unrestricted, the generator may see a second step immediately after the initial transfer.
Choose the redundancy architecture
Redundancy is a capacity and path decision, not merely a count of UPS cabinets. Include bypasses, switchgear, distribution boards, cables, controls and the maintenance state in the analysis.
| Architecture | Meaning | When it fits |
|---|---|---|
| N | The installed capacity equals the minimum capacity required to support the load. | Sites whose availability target accepts loss of capacity during a single failure or maintenance event. |
| N+1 | One additional unit or module is available beyond the number required for the load. | The common architecture when one module or unit must fail or be removed for maintenance without dropping the protected load. |
| 2N | Two independent power trains, each capable of carrying the required load. | Facilities whose availability target requires full path independence and dual-corded or otherwise separately supplied equipment. |
Eaton describes N+1 as the most common redundancy architecture, with N referring to the number of units or modules required to support the load. A design that has N+1 UPS modules but a single bypass switch, common distribution board or shared cooling dependency does not provide complete path redundancy.
Test maintenance and failure states
For each architecture, model a module failure, a UPS isolation, a bypass operation, a downstream breaker trip, planned maintenance and the loss of one generator where applicable. Verify that the surviving equipment still meets the kW and kVA demand after the five-year growth allowance. For 2N, trace both complete paths from source through UPS, switchgear and distribution to the load; a shared component can defeat the intended independence.
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Size the standby generator for the whole emergency facility
The generator normally carries the UPS-backed IT load plus the cooling and mechanical plant needed to remove heat, as well as life-safety, communications, security and other house loads. Cooling can materially exceed the IT-only demand, so a generator sized from server kW alone is commonly undersized.
Evaluate steady-state capacity
Calculate the emergency schedule in steady kW and alternator kVA. Include the selected UPS input power factor, cooling operating points, pumps and fans, lighting, controls, battery chargers and any loads that remain connected during generator operation. Confirm the generator rating class and the permitted continuous or standby duty for the intended operating profile.
Evaluate motors and step loads
Check every motor-start event, including chillers, pumps, compressors and large fans. Review locked-rotor current, starting method, sequential-start timers and the largest simultaneous step. Then assess the UPS rectifier’s input characteristics, harmonic current and battery-recharge step. A generator can meet the average kW while still failing the first motor start or the UPS transfer because voltage and frequency dip outside the equipment limits.
Apply environmental derating
Generator output changes with altitude and ambient temperature. Apply the manufacturer’s derating for the actual installation elevation, maximum temperature, enclosure and ventilation arrangement. Confirm fuel quality, fuel-system capacity, required fuel autonomy, tank placement, refuelling access and the generator’s emissions configuration for the jurisdiction.
Use manufacturer sizing software
Voltfield states that final sizing should be confirmed by a qualified engineer, usually with the manufacturer’s sizing software. Anvilfield likewise identifies cooling demand, transient response and generator rating as key inputs. Use those tools with the actual load schedule and start sequence rather than selecting the next nominal kW size from a catalogue.
Make the UPS and generator operate as one system
UPS-generator compatibility is a functional requirement. Eaton warns that if a generator’s frequency range is too wide for the UPS to accept, the UPS may interpret it as an unstable source and remain on battery.
Verify frequency and voltage windows
Compare the generator’s steady-state and transient voltage and frequency performance with the UPS input and bypass windows. Confirm acceptance both when the UPS transfers from battery and when the generator is carrying the load. A generator that is electrically healthy at no load may still fall outside the UPS window during motor starting or recharge.
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Coordinate controls and protection
Check harmonic compatibility between the UPS rectifier and alternator, short-circuit contribution, protective-device coordination, grounding and neutral switching. Confirm automatic-transfer-switch logic, generator start contacts, inhibit signals, load-shed commands, bypass operation and the UPS’s recharge-current controls. Define what happens if the generator fails to start, is rejected by the UPS or trips after transfer.
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Specify acceptance tests
Write the commissioning test around real events: utility loss, generator start, transfer, staged cooling startup, UPS battery return, recharge and failure recovery. Include load-bank or facility-load tests that demonstrate voltage and frequency transient limits, breaker coordination and the required runtime. Record the measured start time and transfer delays instead of relying only on settings shown in drawings.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Use reference figures carefully
Schneider Electric’s EcoStruxure Reference Design 99 illustrates the scale difference between IT and total facility demand: it lists 3,672–3,818 kW of IT capacity, a 6,250 kW total facility peak and five minutes of UPS runtime at rated load. Those values describe that reference design, not a generic data-centre ratio. They show why the generator schedule must include the mechanical plant and why runtime must be stated at a defined load.
When comparing your own design with a published reference, match the boundaries: IT-only versus total facility power, rated versus measured load, normal versus peak cooling, and runtime at rated load versus runtime at the actual operating point.
Compare candidate designs on the decisions that affect resilience
| Decision axis | Question the design must answer |
|---|---|
| Real and apparent power | Does the UPS and generator remain within both kW and kVA limits at the selected power factor and worst operating case? |
| Growth headroom | Is the documented five-year allowance of at least 15% included before redundancy is added? |
| Redundancy and maintenance | Can the required load remain online with a module, unit, bypass, switchboard or complete path unavailable? |
| Battery bridge runtime | Does the battery curve cover detection, start, warm-up, transfer, UPS acceptance and controlled shutdown at the design temperature and end voltage? |
| Generator step-load and motor starts | Can the alternator and engine hold voltage and frequency through the largest motor start, UPS transfer and recharge step? |
| Cooling and non-IT loads | Are chillers, pumps, fans, life-safety, communications, security and house loads included in the emergency schedule? |
| Fuel autonomy | Does the tank and fuel system support the required operating duration, testing, refuelling plan and environmental conditions? |
| Space and weight | Can the building support UPS cabinets, battery strings, generator, tanks, switchgear, ventilation and maintenance clearances? |
| Efficiency | Is the UPS and generator operating efficiently at the expected load rather than only at rated capacity? |
| Code, emissions and permits | Do local electrical, fire, emissions, fuel-storage, environmental and authority-having-jurisdiction requirements permit the proposed arrangement? |
A practical engineering workflow
- Inventory the facility. List every critical, mechanical, life-safety, communications, security and house load, with measured kW, peak kW, kVA, power factor and starting data.
- Freeze the operating cases. Document normal, peak, maintenance, generator-only and staged-start conditions, including loads that will be shed.
- Calculate UPS demand. Convert kW to kVA with the selected UPS output power factor and check both ratings.
- Add growth. Apply at least 15% over five years as a separate planning allowance, then add the explicit module or unit required for N+1 or 2N.
- Select battery autonomy. Map the outage sequence and select strings from manufacturer discharge curves at actual load, temperature and end voltage.
- Build the generator schedule. Include UPS input, cooling, motors, pumps, life-safety, communications, security, lighting and recharge demand.
- Run transient and derating studies. Use manufacturer software to test motor starts, step-load acceptance, UPS rectifier behaviour, altitude, ambient temperature, voltage, frequency and fuel runtime.
- Validate and commission. Obtain qualified-engineer approval, confirm code and permitting requirements, then perform integrated utility-loss, transfer, staged-start, recharge and failure tests.
Common sizing failures and their corrections
- Sizing from IT kW alone: rebuild the generator schedule with cooling, mechanical, life-safety and house loads.
- Using kW as if it were kVA: apply the actual UPS power factor and verify both equipment limits.
- Counting growth as redundancy: keep the 15% planning margin separate from the additional N+1 or 2N capacity.
- Choosing batteries by amp-hours: use discharge curves for the actual load, temperature and end voltage.
- Assuming a one-minute start is guaranteed: model the complete detection, crank, warm-up, transfer and UPS-acceptance sequence; Eaton’s guidance allows up to five minutes.
- Ignoring recharge: include the post-transfer recharge current as a generator load and transient.
- Checking only steady-state generator kW: test motor starts, UPS transfer, harmonic current and step-load voltage and frequency response.
- Declaring N+1 from the cabinet count: trace bypasses, switchgear, distribution and maintenance states for shared points of failure.
- Ignoring site conditions: apply altitude, ambient-temperature, ventilation, fuel, emissions and permitting constraints before ordering equipment.
- Skipping integrated testing: prove the sequence with the actual controls and connected loads, not just a document review.
Code, permitting and final sign-off
Local electrical and fire codes, emissions limits, fuel-storage rules, environmental conditions, utility-interconnection requirements and the authority having jurisdiction can change the feasible design. Confirm generator rating class, transfer equipment, grounding, fire protection, exhaust, acoustic treatment, fuel autonomy and maintenance access during design review.
The final single-line diagram should show normal and emergency sources, UPS modules and bypasses, battery strings, transfer equipment, generator distribution, load-shed logic and every shared component. A qualified engineer should reconcile that drawing with the load schedule, manufacturer sizing software, battery curves, transient study and commissioning test plan before procurement.
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