In short. A prime power generator is rated to supply a varying load for unlimited hours a year, at an average load of about 70 percent, with 10 percent overload allowed for one hour in twelve. It is the right rating when the generator is the main source of electricity rather than a backup: quarries, construction sites and factories where DISCOM supply is absent for many hours a day. Fuel logistics, service intervals and maintenance continuity decide the design.
How the prime rating differs from standby
The prime power rating (PRP under ISO 8528-1) sits one step below the emergency standby rating on the same engine. A set sold as 500 kVA standby is typically offered at about 450 kVA prime, because the engine must sustain the duty over thousands of hours rather than a few hundred. The rating carries three conditions that must be read together: the load varies, the average over any 24 hour period does not exceed about 70 percent of the prime rating, and a 10 percent overload is permitted for no more than one hour in every twelve hours of operation. Each manufacturer publishes its own version of these limits and the model datasheet is the final word.
Where the load is steady at or near full output for very long periods, prime is still not the right class. That duty belongs to the continuous power rating, which is lower again and permits no overload. The three classes form a ladder, and the page on standby power generators covers the top rung.
| Rating class | Annual hours | Load profile | Overload allowance | Typical use |
|---|---|---|---|---|
| Emergency standby (ESP) | Limited, often about 200 | Variable, average about 70 percent | None | Backup to a reliable DISCOM supply |
| Prime (PRP) | Unlimited | Variable, average about 70 percent | 10 percent for 1 hour in 12 | Main source where mains is absent or poor |
| Continuous (COP) | Unlimited | Constant at up to 100 percent | None | Base load, captive plant |
Reading the load profile across the day
A standby set is sized for a peak that lasts seconds. A prime set is sized for a profile that repeats every day. We ask for five figures before recommending a prime machine: the minimum load (usually the night shift or the weekend), the normal running load, the peak load, the largest single step the load can take, and the number of hours a day the set will run. The minimum load decides whether the engine will spend long periods below the loading at which it burns cleanly. The peak and largest step decide the transient capability. The daily hours decide how quickly service intervals arrive.
Load factor = energy generated per day (kWh) ÷ (prime rating in kW × running hours per day)
Here energy generated is read from the controller’s kWh register or estimated from the load profile, the prime rating is the nameplate kW at the prime class, and running hours is the time the set actually carries load. As an illustrative example, a granite processing unit near Hyderabad that generates 4,200 kWh in a 14 hour working day on a 400 kW prime set has a load factor of 0.75 (illustrative). That is slightly above the 70 percent average the rating assumes, so either the set should be the next size up or the peak hours should be shifted. A unit generating 2,000 kWh on the same set over the same hours has a load factor of 0.36, which signals a different problem: light loading and wet stacking.
Light load, wet stacking and the lower limit
Diesel engines are designed to run hot. Held for hours at light load, cylinder temperatures fall, combustion is incomplete, unburnt fuel and carbon collect in the exhaust, injectors coke and oil consumption rises. The condition is called wet stacking and it is the most common way a prime set is damaged on a varying-load site. No universal minimum loading applies to every engine; many manufacturers advise keeping sustained load above about 30 percent and loading the set harder periodically to clear deposits, and the installed engine’s guidance governs. Where the day’s load swings between 100 kW and 400 kW, two 200 kW sets with automatic start and stop on load, as described under paralleling, keep each running engine in its healthy range and provide a spare when one is being serviced.
Fuel as an operating system
On a standby site fuel is an asset that waits. On a prime site it is a daily consumable, and the fuel system becomes part of the plant’s availability. A 400 kW set at 75 percent load burns in the region of 70 to 80 litres of diesel an hour, so a 14 hour day consumes about 1,000 litres (illustrative). The site needs a bulk tank sized for the delivery interval plus a margin, a day tank with level alarms and automatic transfer, a water separator and filtration sized for the volume, and a procedure for checking each delivery. During the monsoon, water in bulk tanks is the usual cause of injector and pump failures on prime sets. The fuel consumption guide gives the estimation method and the remote and off-grid rental page covers temporary plants where fuel logistics dominate.
Maintenance arrives in weeks, not years
A prime set running 14 hours a day accumulates about 5,000 hours a year. With oil and filter changes typically due every 250 to 500 hours, that means a service visit every few weeks, a top overhaul within a few years and a major overhaul inside a period that a standby set would take two decades to reach. Plan the schedule from the start, stock consumables on site and build a maintenance window into the production calendar. The maintenance planning page sets out intervals by hours and by calendar time.
Continuity during service
If the generator is the only source, every service visit is a plant shutdown. The usual answers are a second set that carries essential load during the window, a rental set brought in for the overhaul, or an N+1 arrangement where the spare is permanent. For a factory the decision rests on what an hour of lost production costs compared with the capital for a second machine.
Common mistakes
- Buying on the standby kVA figure and running the set as the main supply; the engine operates above its prime rating basis and overhaul intervals shorten sharply.
- Sizing one large set for the peak and running it at 25 percent through the night; wet stacking follows within months.
- Installing a bulk tank without a day tank or water separator; a single contaminated delivery stops the plant.
- Planning maintenance on calendar months rather than running hours; oil changes fall overdue within weeks on a high-hours site.
- Having no plan for the service window; the first overhaul becomes an unplanned week of lost production.
What to send us
Send the daily load profile if you have logged it, or the connected load list with the shifts and hours each section runs, together with the largest motor and its starting method. Tell us how many hours a day and how many days a year the set will run, the fuel delivery arrangement available at the site and whether a DISCOM connection exists at all. Photographs of the site and any existing generator nameplate help. Use the generator sales enquiry and we will recommend a rating class, a set size or a multi-set arrangement with the fuel and maintenance plan to go with it.
Questions engineers ask
Can a standby generator be used for prime power?
Only at its prime rating, which is typically about 10 percent lower than the standby figure, and only within the prime load factor conditions. The hardware is often identical; what changes is the permissible loading and the hours. Running a set at its standby rating for long hours voids the rating basis and shortens engine life.
What is the 10 percent overload on a prime rating?
The prime class allows the set to deliver 110 percent of its prime rating for up to one hour in any twelve hours of operation, to cover an occasional peak. It is not a continuous margin and it is not available at all on the standby or continuous classes.
How much diesel does a prime generator use per unit generated?
At loads between 50 and 100 percent most modern diesel sets consume roughly 0.25 to 0.30 litres per kWh. The figure rises steeply below 30 percent load because the engine’s own losses are spread over fewer units, which is another reason to avoid long light-load running.
Is a prime power generator legal for daily use in Telangana?
Yes, subject to the state pollution control board consent, the applicable CPCB emission norms for the set’s class and the noise limits for the location. For new sets up to 800 kW, CPCB IV+ compliance is mandatory regardless of whether the duty is standby or prime.
Generators by rating
- Under 15 kVA generators
- 15–50 kVA generators
- 50–125 kVA generators
- 125–250 kVA generators
- 250–500 kVA generators
- 500–1000 kVA generators
- 1–2 MVA generators
- 2 MVA and above generators
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.