Describe the loads and the demand pattern
Start with a list of the equipment the generator is expected to supply. Include the maximum anticipated demand and an indication of average loading. A single total power figure leaves important questions unanswered: does demand remain relatively steady, or does it vary during the working day? Explain how the figures were obtained and identify any information that still needs measurement or assessment.
Record the relevant equipment characteristics for the design engineer, including motor starting requirements and nonlinear electronic loads. Caterpillar identifies load type as an important factor in alternator selection and explains that nonlinear loads can introduce additional thermal effects. The practical task for the buyer is to describe the application clearly; the engineer then evaluates how those characteristics affect equipment selection.
Caterpillar — Understanding generator set ratings · Caterpillar — A holistic view on generator set ratings
Define the operating duty
Explain whether the generator is intended for backup during utility interruptions or for regular power generation. Give the supplier the expected operating pattern and estimated annual running hours. Peak demand, average loading and running time should be considered together when selecting a manufacturer rating.
Standby, prime and continuous ratings describe different conditions of use. Manufacturer definitions can also differ from standard categories, so ask which definition a proposed rating follows. State whether operation will be isolated from the utility or connected in parallel, and flag any part of that arrangement that remains undecided. This gives the engineer a more useful starting point than an assumed spare-capacity percentage.
Caterpillar — Understanding generator set ratings · Caterpillar — A holistic view on generator set ratings
Provide the site conditions
Include the site's altitude, expected ambient temperatures and known airflow restrictions. Caterpillar identifies these environmental conditions as factors that can affect generator performance. The supplier needs this information before confirming that a particular package is suitable for the installation.
Describe the available installation area and identify anything still awaiting a site survey. Rather than prescribing ventilation dimensions or a universal derating factor, ask the supplier to explain how the proposed equipment has been assessed for the stated conditions. If site information is provisional, keep that uncertainty visible in the enquiry and in the comparison of proposals.
Ask suppliers to explain their assumptions
Use the same load and site information when requesting competing proposals. Ask each supplier to identify the rating definition used, the operating duty assumed and the environmental conditions considered. Where a recommendation depends on an estimate, request confirmation of what additional information is needed before the specification can be finalized.
Useful questions include: How were peak and average demand considered together? Which load characteristics influenced the alternator recommendation? How were temperature, altitude and airflow restrictions assessed? These questions help make the basis of a proposal understandable. They do not replace a project-specific engineering assessment or the selected manufacturer's equipment documentation.
Caterpillar — Understanding generator set ratings · Caterpillar — A holistic view on generator set ratings
Technical references
- Caterpillar — Understanding generator set ratings
- Caterpillar — A holistic view on generator set ratings
AI-assisted guide with automated source and language checks. It provides planning context; final engineering decisions require a project-specific assessment. Editorial approach ↗