LoadCairn / Standby generator guide
How to size a data center standby generator.
Start with the running load, power factor, planning margin, and any motor-start allowance that matters to your scenario. This guide explains the transparent planning model behind the calculator and how to read its kW and kVA outputs.
Calculator model
Planning estimate only
Required kW = total load kW × (1 + margin % ÷ 100)
Required kVA = (total load kW ÷ load PF + motor-start allowance kVA) × (1 + margin % ÷ 100)
Reference basis = max(required kVA, required kW ÷ 0.8)
The formulas are a screening model for early planning. They do not select a generator or replace a site-specific electrical design.
01 / The planning question
What generator sizing is trying to answer
A standby generator has to support the real power the facility needs and the apparent power that its electrical equipment draws. Data center loads can include UPS systems, cooling equipment, pumps, fans, lighting, controls, and other auxiliary loads. Their combined demand and power factor influence the early capacity conversation.
The LoadCairn calculator keeps that conversation deliberately small: it uses one total running-load value, one power factor, one editable margin, and one optional motor-start allowance. That makes it useful for comparing scenarios before a complete load schedule or generator study is available.
Treat every result as a planning estimate. It is not a final equipment recommendation, a code-compliance determination, or a guarantee that a generator will start and carry the site load.
02 / Inputs and assumptions
Build a scenario from four inputs
Use the operating scenario you want to screen. Avoid mixing nameplate values, measured demand, and modeled peak demand without labeling the difference.
- Total running load
- Enter the expected real-power load in kW. It must be above zero.
- Load power factor
- Enter a value above 0 and no more than 1. Use measured or equipment-specified data when available.
- Growth / safety margin
- Enter a percentage from 0% through 100%. The calculator applies it to both kW and kVA requirements.
- Largest-motor starting allowance
- Optionally add a transient starting allowance in kVA. Leave it blank when no allowance is being modeled.
The calculator opens with an illustrative case of 100 kW, 0.80 PF, 20% margin, and no motor-start allowance. These defaults are editable examples, not facility data.
03 / Reading the result
kW and kVA answer different questions
Required real power · kW
This is the total running kW after the entered planning margin is applied. It is the real-power side of the estimate and helps describe how much useful electrical work the load requires.
Required apparent power · kVA
This starts with kW divided by the entered power factor, adds the optional motor-start allowance, and then applies the same margin. It describes the apparent-power burden in the simplified scenario.
Illustrative reference size
The calculator compares the larger of required kVA and required kW divided by 0.8 to an illustrative reference ladder, then rounds up to the next listed kVA value. That displayed size is a reference point for planning, not a recommendation for a particular manufacturer or installation.
04 / Boundaries
Know what the estimate leaves out
A usable early estimate is honest about its missing inputs. Review these assumptions before carrying a result into a procurement or design decision.
The model assumes
- The entered total load represents the running real-power demand in the scenario being screened.
- The entered power factor is used to convert running kW to running kVA.
- The optional motor-start value is additive on top of the full entered running load; the calculator does not subtract that motor’s running demand.
- The illustrative reference ladder uses 0.8 PF and rounds up to the first reference kVA value that meets the calculated basis.
It does not model
- Altitude, ambient temperature, site derating, and the generator’s actual rating conditions.
- Step loading, voltage and frequency dip, transient recovery, motor-start performance, and starter behavior.
- Harmonics, nonlinear loads, unbalanced phases, neutral current, short-circuit coordination, and protection settings.
- Fuel autonomy, emissions requirements, acoustics, enclosure, transfer equipment, paralleling, redundancy, and code compliance.
- Specific manufacturer ratings, availability, installation constraints, and the measured facility load profile.
Before selecting equipment, have a qualified electrical engineer or generator supplier review the actual load profile, starting sequence, site conditions, transfer equipment, applicable standards, and manufacturer data.
05 / Run a scenario
Compare your standby-power assumptions
Enter your planning scenario, review the live kW and kVA estimate, and inspect the calculation trace before taking the result into a more detailed design review.