Describe the operating pattern first
A generator used during occasional utility failures has a different job from one supplying a site without a reliable grid connection. Before asking for a rating, describe expected operating hours, load variation, planned maintenance and the role of other power sources.
Do not choose a duty classification solely from a label in a sales listing.
Understand what the labels tell you
Standby ratings describe defined backup duties. Prime ratings describe defined duties in which the set serves as a primary power source, typically with varying load. The exact limits and conditions must be read in the manufacturer’s rating definition for the selected model and applicable standard.
| Procurement question | What to request |
|---|---|
| How often will the set operate? | The proposed rating and its operating limits |
| Is the load constant or varying? | Confirmation that the duty matches the load profile |
| Is overload capability needed? | The manufacturer’s written allowance, if any |
| Are site conditions demanding? | The applicable rating at the installation conditions |
Compare like with like
The same set may be advertised with more than one power figure. Comparing one supplier’s standby value with another supplier’s prime value can produce a misleading conclusion.
Ask for both kW and kVA, the duty designation, frequency and the reference conditions. Keep the original data sheet with your quotation comparison.
Avoid a universal rule
There is no single operating-hours or overload statement that should be applied to every product simply because it says prime or standby. Read the stated limits and have the supplier confirm suitability in writing.
Prepare a comparable enquiry and include your expected running pattern.
Sources & scope
- Cummins — T-030 Generator Set Application Manual · §2.4.1–2.4.3 Manufacturer-specific standby, prime and continuous rating conditions. Issue 9, February 2015; not represented as the latest edition.
General procurement guidance. Worked examples are illustrative; final selection requires the selected manufacturer's documentation and a project-specific engineering review.