On-Site Power Generation: Duty Ratings and the Industry Reference Guide
On-site power generation means producing electricity at the point of use rather than drawing it from a utility grid. For a mine in Mali, a data centre in Texas, or a cement plant in Iraq, it is often the only way to secure firm capacity. The single most misunderstood decision in on-site power generation is the duty rating, because the same physical machine carries several different power ratings depending on how it will be run.
On-Site Power Generation Duty Ratings
| Rating | Load Profile | Typical Annual Hours | Overload Allowance | Application |
| Emergency Standby (ESP) | Variable | Around 200 | None | Backup on grid failure |
| Limited Time (LTP) | Constant | Up to 500 | None | Scheduled outages, peak shaving |
| Prime (PRP) | Variable | Unlimited | 10 percent, 1 hour in 12 | Off grid sites with varying load |
| Continuous (COP) | Constant | Unlimited | None | Base load generation |
Ratings follow ISO 8528-1. Confirm the exact hour limits and average load factor against the OEM rating sheet, because manufacturers apply the standard with some variation. A machine sold at 2,000 kVA standby may only carry 1,800 kVA prime and less again continuous, which is why a rating quoted without its duty class tells you nothing.
The rating also compounds with site conditions. Ambient temperature, altitude, and fuel quality each derate the machine further, and those reductions apply on top of the duty class rather than instead of it. A set specified at standby and then derated for a 45C ambient can deliver materially less than the nameplate figure a developer signed against.
The Industry Reference Standard
The Electrical Generating Systems Association publishes the reference work most widely used across the sector, Onsite Power Generation: A Comprehensive Guide to Onsite Power. First issued in 1990, it has been reorganised repeatedly as the industry has changed. The sixth edition, expected in late 2026, restructures the material to follow how a generating system is actually encountered in the field and adds chapters on hydrogen fuel systems, fuel cells, mobile power generation, troubleshooting, and maintenance.
Note that the title changed. Earlier editions were subtitled A Reference Book, so older citations and purchase links point to superseded material.
Frequently Asked Questions
What is on-site power generation?
Electricity produced at the point of consumption rather than imported from a utility grid, using generator sets, turbines, or hybrid systems. It provides firm capacity where the grid is absent, unreliable, or too slow to connect.
What is the difference between prime and standby rating?
Standby is for emergency backup at roughly 200 hours a year with no overload capability. Prime permits unlimited hours on a varying load with a 10 percent overload for one hour in twelve. Prime rating is always the lower number.
Is on-site power generation cheaper than grid supply?
It depends on fuel cost, utilisation, and what downtime costs the operation. Where the grid is unreliable, the comparison is not tariff against fuel cost, it is generation cost against the cost of lost production.
How do I choose a duty rating?
Start from the expected annual run hours and load profile, not the peak demand figure. Sites that will run the plant as their primary supply need prime or continuous, and specifying standby instead is a common and expensive error.
Choosing the rating correctly is where on-site power generation projects are won or lost. USP&E Global has delivered 150+ projects across 35+ countries, from diesel generating sets through full EPC construction. Distributed generation and reliability data is published by the U.S. Energy Information Administration, the International Energy Agency, and the World Bank.
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