The same kW label can represent very different duties: continuous self-generation, scheduled daily operation or occasional outage backup. Unless rating basis and protected-load scope are explicit, similar quotations may describe configurations that are not comparable.
First, Confirm Whether This Article Matches Your Project
Owners, procurement teams, facility managers and electrical leads buying a first set, expanding, replacing an old unit or adapting to a changed load profile.
It separates symptoms, decision criteria, site inputs and engineering boundaries for generator rating definitions.
A sizing basis that records operating duty, critical loads, starting sequence, environmental derating and future-expansion limits.
Define the power-source duty before discussing generator size. State when the set will run, approximate annual hours, how load varies and which equipment must recover during an outage.
Check 01
Prime and Standby Describe Operating Duty
A prime-rated set supplies power continuously or for extended periods; a standby set runs mainly when the normal source fails. Their operating-hour, load-variation and maintenance requirements differ.
Confirm whether a reliable utility or other normal source exists, plus annual hours and expected run duration. Document whether load remains near a stable level and whether rotation or redundancy is required in the design, single-line, operating record or acceptance file. Without written evidence, do not assume the duty is defined.
- Confirm a Reliable Utility or Other Normal Source
- Estimate Annual Hours and Duration per Run
- Determine Whether the Load Is Relatively Stable
- Determine Whether Rotation or Redundancy Is Required
Check 02
Standby Power Does Not Always Need to Carry the Entire Facility
Including every load in the backup scope rapidly increases generator and distribution size. First identify what business continuity, safety and regulations actually require.
List loads that must recover immediately and equipment that may reconnect later. Document permissible load shedding during an outage and the UPS-to-generator handoff scope in the design, single-line, operating record or acceptance file. Without written evidence, do not assume the protected scope is complete.
- List Loads Requiring Immediate Recovery
- Identify Equipment That May Reconnect Later
- Define Loads Permitted for Outage Shedding
- Confirm the UPS-to-Generator Handoff Scope
Check 03
Why One Catalog Rating Is Not Enough
Brands and standards may publish standby, prime and other ratings together. Compare only the same rating basis, environmental conditions and power factor.
Confirm the kW-to-kVA relationship and whether the stated rating is prime or standby. Document reference conditions, derating and the limiting ratings of the engine, alternator and complete set. Without written evidence, do not assume ratings are comparable.
- Confirm the kW-to-kVA Relationship
- Verify the Prime or Standby Rating Basis
- Confirm Reference Conditions and Derating
- Compare Engine, Alternator and Complete-Set Limits
Check 04
How to Write a Clearer Request for Quotation
Putting operating duty and critical loads in the RFQ reduces quotation differences caused by each supplier making different assumptions.
Explain the relationship between normal and standby sources and attach the load list, starting methods and sequence. State target operating hours and site conditions, and require suppliers to justify capacity and reserve. Without written evidence, do not assume quotations share one basis.
- Explain the Normal-to-Standby Source Relationship
- Attach the Load List, Starting Methods and Sequence
- State Target Operating Hours and Site Conditions
- Require Capacity and Reserve Justification
Decision record
Put Each Decision in the Project Record, Not Just a Verbal Confirmation
Selection should not begin with brand and price. Turn loads, duty and site constraints into reviewable project conditions, then compare supplier proposals on the same basis.
| Decision Point | Prepare on Site | Confirm in Design or Acceptance |
|---|---|---|
| Prime and Standby Describe Operating Duty | Confirm a Reliable Utility or Other Normal SourceEstimate Annual Hours and Duration per Run | Determine Whether the Load Is Relatively StableDetermine Whether Rotation or Redundancy Is Required |
| Standby Power Does Not Always Need to Carry the Entire Facility | List Loads Requiring Immediate RecoveryIdentify Equipment That May Reconnect Later | Define Loads Permitted for Outage SheddingConfirm the UPS-to-Generator Handoff Scope |
| Why One Catalog Rating Is Not Enough | Confirm the kW-to-kVA RelationshipVerify the Prime or Standby Rating Basis | Confirm Reference Conditions and DeratingCompare Engine, Alternator and Complete-Set Limits |
| How to Write a Clearer Request for Quotation | Explain the Normal-to-Standby Source RelationshipAttach the Load List, Starting Methods and Sequence | State Target Operating Hours and Site ConditionsRequire Capacity and Reserve Justification |
Common Mistakes
These Shortcuts Leave the Risk Until After the Outage
Simply Adding Equipment Nameplate Ratings
Nameplate ratings do not show motor starting demand, coincident operation or recovery order. Capacity may look sufficient on paper yet fail to start the site loads.
Replacing Engineering Calculations with a Fixed Percentage
Reserve must separately address starting demand, environmental derating, future expansion and redundancy. One blanket percentage cannot define those boundaries.
Comparing Only the Generator-Set Price
A low purchase price is not a low total cost when fuel, distribution, installation, spares, maintenance and outage losses are excluded.
Handover Check
At Minimum, the Project Should Retain These Deliverables
End customers do not need to perform every calculation, but they should receive clear documents and know what data supports each conclusion.
Load Schedule and Starting Matrix
Record each load’s power, voltage, power factor, starting method, coincidence factor and recovery sequence.
Capacity and Rating-Basis Statement
State prime or standby duty, kW and kVA basis, environmental correction and capacity justification.
Expansion and Redundancy Limits
State whether future loads, paralleling conditions, maintenance windows and N+1 requirements have been allowed for.
Like-for-Like Proposal Comparison
Compare equipment, fuel, installation, maintenance, spares and service scope in one table.