Updated
Duty ratings
Standby vs Prime Generator: What's the Difference?
A standby rating is for backup during outages and, under ISO 8528-1, typically allows up to 200 hours a year with the load averaging no more than 70%; a prime rating allows unlimited hours on a varying load averaging no more than 70% of the prime figure. Your data plate shows which ratings your set carries.
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- Standby: outages, limited hours
- Prime: main supply, unlimited hours
- Continuous: steady load around the clock
- Onsite across Gauteng
Backup versus a main source
What is the difference between standby and prime?
A standby set starts when the mains fails, carries your load through the outage and stops when the mains returns. Manufacturers rate it for that pattern. Cummins, summarising ISO 8528-1, describes the standby rating as limited to 200 hours a year, with the average load no higher than 70% over any 24 hours.
A prime-rated set is your main supply. It can run without an annual hour limit, as long as the average load over any 24 hours stays at or below 70% of the prime rating. A continuous rating, where the plate shows one, is for a constant load at up to 100% of that figure, with no hour limit.
The same machine carries a different figure for each duty. Cummins rates its C330D5 at 330 kVA standby, 300 kVA prime and 249 kVA continuous. If you plan to run a set as your main supply, size it on the prime figure, not the larger standby number.
Load, hours and sites
Why the duty rating changes the visit
Prime generator use
The set is your supply, not your backup. Hours build quickly, so the hour limit in the service schedule usually arrives before the calendar limit.
Load variation
Standby and prime ratings both assume a load that varies through the day, averaging no more than 70%. A continuous rating, where printed, is the one for a steady, flat load.
Running hours
Duty decides how fast hours build. At 4 hours a day of outages, a set adds about 120 hours a month, which uses up a 200-hour standby allowance in under two months.
Generator sizing
A standby kVA and a prime kVA on the same machine are different numbers. If you size from the standby figure and then run the set as prime, it works beyond its rating.
Maintenance differences
More hours mean oil and filters come due sooner. A standby set still needs its calendar service while it waits. We plan the visits around how you actually use the set.
Commercial and industrial use
Offices often need standby duty. A plant that runs production on the generator needs prime or continuous duty. We check that the rating on the plate matches how you use the set.
Standby, prime and continuous compared
The hour and load limits are the ISO 8528-1 definitions as Cummins summarises them. The example ratings are for one real 50 Hz generator set. Your own data plate and the manufacturer’s definitions give the figures for your set.
| Rating | Typical use | Hours allowed | Load allowed | Example: Cummins C330D5 |
|---|---|---|---|---|
| Standby (ESP) | Backup during mains outages | Up to 200 hours a year | Varying, averaging no more than 70% over any 24 hours | 330 kVA / 264 kW |
| Prime (PRP) | Main supply with a varying load | Unlimited | Varying, averaging no more than 70% of the prime rating over any 24 hours | 300 kVA / 240 kW |
| Continuous (COP) | Main supply with a steady load | Unlimited | Constant, up to 100% of the continuous rating | 249 kVA / 199 kW |
Worked example: run the C330D5 as a prime set and the load can vary up to its 240 kW prime rating, as long as the average over any 24 hours stays at or below 70% of 240 kW, which is 168 kW. For a flat load around the clock, use the continuous figure of 199 kW instead.
Sources: Cummins white paper: generator set ratings and ISO 8528-1; Cummins: C330D5 generator ratings.
Related Pages
Sizes, hours and site types
- Generator Sizes kVA bands and what the plate means
- kVA to kW Converting each rating at 0.8
- Running Hours How use shows up on the meter
- How Often to Service Standby and prime service timing
- Standby Generator Maintenance Keeping a standby-rated set ready to start
- Commercial Generator Repairs Offices, retail and complexes
- Industrial Generator Repairs Larger sets and heavier duty
Common Questions
Questions about standby and prime ratings
What is the difference between standby and prime?
Standby is a backup rating for outages and, under ISO 8528-1, typically allows up to 200 hours a year with the load averaging no more than 70%. Prime is for a main supply and allows unlimited hours on a varying load averaging no more than 70% of the prime rating. Your data plate shows which apply.
Can a standby generator run all day?
During an outage, yes; that is its job. Over a year, though, the ISO standby definition typically caps use at about 200 hours. If your set runs as your daily supply, check its prime rating and size the load on that figure.
Is prime the same as continuous?
No. Prime allows a varying load averaging up to 70% of the rating, with unlimited hours. Continuous is for a constant load at up to 100% of a lower figure. Cummins rates its C330D5 at 300 kVA prime and 249 kVA continuous.
Why do some plates show two kVA figures?
Each figure belongs to a duty. The Cummins C330D5 shows 330 kVA standby and 300 kVA prime. The higher number applies only to standby use.
Does the duty change servicing?
Yes. A prime set builds hours fast, so the hour limit decides: at 24 hours a day, a 240-hour oil interval arrives every 10 days. A standby set is usually serviced on the calendar limit.
We run the generator for a shop during outages. Is that standby?
Yes, that is the pattern standby ratings cover, within the hour and load limits for your model. If long daily outages add up to hundreds of hours a year, we check the prime figure with you.
Does a factory automatically need a prime set?
No. How the factory uses the generator decides the duty. A set kept for outages can be standby duty. If production runs on the generator, the plate has to carry a prime or continuous rating for that load.
Where do I read the duty?
On the data plate, next to each kVA figure, or on the manufacturer’s specification sheet for your model. Send us a photo and we read it for you.
Still unsure what the fault is? Call or WhatsApp 082 057 4605 for onsite diagnosis.
Service Areas
Onsite visits across Gauteng
We come to your generator anywhere in Gauteng, 24/7. Choose your region for local suburbs, call-out details and the faults we see most there.
What to send before we attend
These details help us plan the visit and bring the right parts. Send what you can on WhatsApp; if you can’t find the model number, we still book the visit.
- Location
- The suburb or site, so the visit can be booked in the right area.
- Make and model
- The generator brand and model, if they are known.
- kVA
- The size printed on the data plate.
- Fuel type
- Diesel, petrol, LPG, natural gas, or dual-fuel.
- Controller model
- The module name on the panel, when it is visible.
- Fault or alarm
- What the set does, and any text on the screen.
- Photos
- The generator, the data plate, and the controller or alarm. Send these on WhatsApp.
Book a Visit
Send the rating plate
A photo that shows the standby or prime kVA is the useful one. Tell us how you use the set and we check the rating matches.
- Call
- 082 057 4605
- Message us on WhatsApp
What happens next
- We reply with questions about the generator, or to book a visit.
- A technician comes to you. For a breakdown we arrive within 1 hour.
- The fault is diagnosed on site and you get a written quote. No work starts until you accept it.
Charlie, lead technician: Degree in Mechanical Engineering, Degree in Electrical Engineering, Diesel Plant Fitter. Generator specialists since 2000.