Compare written net kW and kVA for the same altitude, air temperatures, duty and set configuration. Keep each assumption and evidence source visible.
Do not size a generator for a hot or high site by subtracting one fixed percentage from a brochure rating. Give every supplier the same site data, duty, load profile and proposed set configuration, then compare written net kW and kVA at those conditions. If a supplier cannot tie the result to the exact set, measurement points and method, keep the quotation open.
This is a pre-RFQ and quotation-comparison method. It is not a site-derating calculator, a final model selection or an installation design.
Define the site conditions at controlled measurement points
Define the real installation and its measurement points before requesting a site rating. Record height above sea level and the design high, low and normal temperatures; a city name or weather-app screenshot is not a controlled site basis.
The word ambient is too vague on its own. The right test point can change by document and part. A supplier may use outdoor air, air entering a room or enclosure, air at the radiator, or air at the alternator inlet. These values may differ inside a room, canopy or container.
Return a site-condition sheet with these fields:
| Field. | Controlled input. |
|---|---|
| Elevation. | Height above mean sea level, unit, data source and any project design margin. |
| Outdoor temperature. | Design high and low, source, project basis and whether sun load is included. |
| Equipment inlet temperatures. | Name the engine or radiator cooling-air point and the alternator inlet when needed. Merge them only if the supplier uses one definition. |
| Installation arrangement. | Open set, weather or sound enclosure, container, generator room, rooftop or another stated layout. |
| Airflow limits. | Air inlet and outlet layout, hot-air return risk, nearby heat sources and any room-air study. |
| Other site inputs. | Humidity, dust, salt or corrosive air, fuel condition, and other data the maker asks for. |
Caterpillar's paper on enclosed generator sets explains that an enclosure can restrict and heat the cooling air. Its quoted enclosure limit uses outside air, not the hotter air at the radiator core. This is Caterpillar's test basis. It is not a rule for every supplier. Name each test point and ask each supplier which definition it used.
Lock the duty and electrical requirement before applying environmental limits
Fix the duty and electrical requirement before comparing any environmental correction. State whether the job needs standby, prime, continuous or another project rating, then give voltage, frequency, phase, power-factor basis, and the net kW and kVA required at the load point.
The ISO 8528-1:2018 catalogue entry covers generator-set application, ratings and performance. Use the edition named by the project, plus any local rules and supplier documents, to set the rating basis. The catalogue page gives no single correction for altitude or heat. It also does not prove that an offered set meets the standard.
Also send the load schedule, largest start or load step, start method and sequence. State the allowed voltage and frequency dip. A set may carry the steady load but still fail the required start.
Motor starts, block loads and non-linear loads need their own review. The industrial generator sizing guide shows how to prepare these inputs without hiding unknowns in one load total.
Separate engine, alternator and complete-package limits
Do not use one component's site table as the rating for the whole set. Require a whole-set rating basis, the limit controlling the result, and evidence for each relevant boundary:
| Boundary. | What the supplier should identify. | What not to assume. |
|---|---|---|
| Engine shaft power. | Exact engine setup, fuel, duty, base conditions, site method and power left at the site. | An engine percentage equals net electrical output. |
| Alternator heat and electrical limit. | Make and model, winding or heat-rise basis, voltage, frequency, inlet-air point and site factors. | One alternator table applies to every alternator or the whole set. |
| Cooling and enclosure limit. | Radiator and fan pack, room or enclosure layout, test basis and assumed airflow. | A larger nameplate fixes poor airflow or hot-air return. |
| Set loads and net output. | Fan and other set loads included in the net kW/kVA, plus the point where output is measured. | Gross part power is available to the buyer's load. |
| Duty and load steps. | Rating class, load profile, start order, block-load limits and proof for the offered set. | A steady site rating proves every start or load step. |
The STAMFORD/AvK AGN 012 site-rating note gives site factors for stated alternator types and conditions. This shows why an alternator needs model-, series- or rating-specific evidence. Do not apply those factors to another alternator or to a complete BEAR generator.
Caterpillar's paper on generator-set ratings discusses whole-set ratings and alternator heat limits. It supports a review of the full job, not just the name of the rating. It does not select a product for this project.
Reject universal percentages and unapproved multiplication
A percentage can be valid within the scope of the document that gives it. That does not make it a market-wide rule.
Do not write or accept a quotation calculation that silently assumes:
- one altitude loss per unit of elevation for every engine and fuel.
- one temperature loss above one universal reference temperature.
- an altitude factor and a temperature factor may always be multiplied.
- an engine correction, alternator correction and complete-set correction describe the same boundary.
- a factor is still required when the supplier's submitted net rating already includes it.
If a supplier multiplies factors, ask for the source document and revision. The answer must name the part or set, model, rating, voltage, frequency, duty and site range. It must state whether the factors may be combined and whether the net result already includes either factor.
Keep an unsupported result OPEN. A general online calculator cannot supply missing model-specific proof.
Request a written net site-rating evidence stack
For quote review, ask for a clear chain from the buyer's inputs to one offered set and its net output. Keep assumptions and exclusions in view.
When cooling, enclosure, controls, or other customized scope changes the site-rating basis, carry the affected component identities, assumptions, and review triggers into the BEAR OEM and private-label project review.
Require these fields in the supplier's site-rating return:
| Return field. | Required content. |
|---|---|
| Input identity. | Buyer site/load sheet revision, altitude, each temperature and measurement point, duty and electrical basis. |
| Offered configuration. | Set, engine, alternator, radiator and fan, enclosure or container, and any major set loads. |
| Base rating. | Catalogue or base rating and the conditions used for it. |
| Method used. | Model curve, table, software report or engineer review, with document title, revision and scope. |
| Part results. | Engine, alternator, cooling, enclosure and other limits. Name the controlling limit if one is found. |
| Net site output. | Available kW and kVA for the stated site and duty, output point and included losses. |
| Load-step result. | A separate answer for the stated start or load step and its pass basis. |
| Control. | Assumptions, exclusions, changes, reviewer, issue date and period for which the answer is valid. |
Assign one state to each returned answer:
CONFIRMED: the supplier gives a written answer for the exact set and stated inputs, with evidence, assumptions and exclusions.CONDITIONAL: a named assumption remains.NOT OFFERED: the supplier does not offer the set or says it does not meet the request.OPEN: proof is missing.
CONFIRMED records the supplier's answer. It is not independent proof that the set is right for the site.
Compare offers by net output at the same site and duty
Compare price or nameplate size only after supplier answers share the same input revision, duty, voltage, frequency and load cases. Map every site value to the supplier's named measurement point; values tied to different points remain outside the same comparison column.
Pause an apples-to-apples comparison when:
- one offer quotes catalogue kVA while another quotes net site kW.
- the offers use different temperature measurement points or altitude data.
- one includes an enclosure or radiator change and another leaves the package undefined.
- the engine and alternator evidence belongs to a different model, voltage, frequency or rating.
- the environmental method is not identified, or a calculator result is presented without configuration approval.
- the steady rating is returned but the required start or load step is unanswered.
The rated-versus-peak generator power guide helps reconcile rating labels. The voltage and frequency checklist controls the electrical configuration, while the generator document checklist keeps quotations, data sheets, drawings and test evidence on one revision.
Send one site-rating request to every supplier
Before asking for a revised quotation, confirm that the package states:
- exact site altitude and source.
- all design temperatures with units, measurement points and basis.
- installation and cooling-air arrangement.
- rating duty, voltage, frequency, phase, PF basis and required net kW/kVA.
- load schedule, start/load-step sequence and transient acceptance criteria.
- proposed configuration and component identities.
- applicable environmental documents and whether factors are already included.
- supplier-documented net site kW/kVA, identified governing limitation(s) if determined, assumptions and revision.
Use qualified application and site staff for the final rating, room airflow, electrical design and code review. This article does not do those tasks. For a first BEAR set review, send Miya the site sheet, load schedule, electrical basis, duty and estimated quantity. Do not assume that a model is available until BEAR confirms it.
