A data center generator must support the load after it settles and respond acceptably while loads connect, disconnect or change. Those are different questions. A running-load calculation can establish a preliminary kW and kVA envelope; it cannot predict the voltage dip, frequency dip or recovery associated with a particular load step.
This article builds a reproducible 500 kW example and identifies the extra information needed for a manufacturer-led sizing review. It does not select a generator model, set an allowable disturbance or certify a redundancy arrangement.
Start with a load boundary, not a GPU total
The generator-side running load is the electrical demand presented to the generator in the operating scenario being studied. A useful worksheet names each included circuit and excludes anything deliberately outside that scenario. IT power, facility utility demand and generator demand are not interchangeable labels.
For this hypothetical example, assume the following coincident generator-side electrical loads. The UPS row already includes its modeled losses and battery charging in this scenario; those must not be added again. None of the values is a recommended design allowance.
| Included load group | Running input | Boundary note |
|---|---|---|
| UPS-supported IT path | 280 kW | Generator-side input, not bare IT output |
| Cooling equipment | 140 kW | Only the equipment in the scenario |
| Other supported mechanical loads | 60 kW | Separate from the cooling row |
| Controls and other supported auxiliaries | 20 kW | No overlap with other groups |
| Total | 500 kW | Coincident steady-state assumption |
If the starting point is an IT estimate, use the AI data center power calculator to document that boundary first. Do not multiply this completed generator-side ledger by PUE: that would introduce facility overhead into rows that already include their assigned loads.
Calculate the steady-state envelope
Assume a user-selected planning reserve of 20% and an aggregate load power factor of 0.80. The reserve is a scenario input, not a code requirement or an allowance proven to cover transient effects.
- Planning running capacity = 500 × 1.20 = 600 kW.
- Planning apparent capacity = 600 ÷ 0.80 = 750 kVA.
- Without reserve, the same assumed aggregate power factor gives 500 ÷ 0.80 = 625 kVA.
The generator steady-state load calculator reproduces the 600 kW and 750 kVA result. Both outputs describe the selected planning assumptions. They do not mean that every generator advertised with either number will satisfy this application. The aggregate power factor must represent the combined scenario; it is not an arithmetic average of unrelated equipment nameplate values.
Equal final load does not mean equal load steps
Consider two hypothetical transitions that both finish at 500 kW. In case A, the generator is carrying 100 kW and the next event adds 400 kW. In case B, it is carrying 400 kW and the next event adds 100 kW. The arithmetic final load is identical, while the increments differ by a factor of four.
This comparison deliberately contains no motor-starting or UPS response model. It therefore cannot calculate either case’s voltage or frequency behavior. Its purpose is to show the missing dimension: a total does not encode the sequence that produced it. A later restarting load may also require review after other equipment is already operating.
Caterpillar’s generator sizing overview distinguishes running demand from starting demand and describes load-step modeling, including calculated voltage and frequency dips in SpecSizer. That is a substantially richer assessment than adding a reserve percentage.
Prepare a transient-review input package
Use the load ledger as the index for a second worksheet. For each event, record the load already connected, the equipment being added or removed, its input characteristics, and the intended timing. Mark unknown information explicitly rather than assigning an arbitrary universal starting multiplier.
- For motors and drives: request starting method and the relevant starting or ramp data.
- For UPS equipment: request input ramp settings, charging demand, input power factor and compatibility information for the proposed operating mode.
- For the site: record voltage, frequency, altitude, design ambient conditions and intended duty.
- For acceptance: obtain the permitted voltage and frequency excursions and recovery requirements from the responsible design team and equipment documentation.
Cummins’ generator-sizing paper explains why allowable voltage and frequency dips, site conditions and load sequencing affect sizing. Its T-030 application manual separately discusses UPS loads, charging and frequency sensitivity. These references identify questions to resolve; their examples are not universal settings for modern UPS products.
Keep ratings and redundancy separate
A duty rating is another dimension, not a synonym for spare capacity. Cummins’ Data Center Continuous ratings white paper discusses rating definitions and operating-duration conditions. Confirm the exact proposed rating and its documented conditions with the manufacturer; this article makes no Tier-compliance claim.
Likewise, an N+1 block count in the capacity planner is a conceptual capacity comparison, not proof of transient performance, protection coordination or transfer behavior. Hand over the running-load ledger, sequence worksheet and explicit unknowns together. The useful preliminary result is a traceable question for engineering review, not an equipment purchase guarantee.
Sources & further reading
- Caterpillar commercial generator sizing calculators ↗
- Cummins Power Topic 7007: How to size a genset ↗
- Cummins T-030 Liquid-Cooled Generator Set Application Manual, Issue 9 ↗
- Cummins Data Center Continuous Ratings white paper, February 2025 revision ↗