You hold two generator quotes. The kVA figures are close, but one says "prime" and the other says "standby". This guide sets out the prime vs standby vs continuous generator rating side by side, gives you a rule for reading any quote, and lists what to ask the supplier to confirm in writing.
Why can the same generator carry two different kVA numbers?
A generator is limited by heat and wear, and both depend on how it is used. A set that runs for a few hours during a power cut is under much less strain than one that runs every day for months. So manufacturers publish a different rating for each pattern of use. Many base their definitions on the ISO 8528-1 standard, but the exact wording and limits differ by maker.
Often the engine and alternator are the same. What changes is the duty the maker allows. A higher number goes with a lighter duty, because the set will not run for long.
This is why "500 kVA" on one quote and "450 kVA" on another may describe the same machine. The numbers answer different questions. Before you compare the kVA, ask which rating each figure belongs to.
Note. kVA is apparent power. To see how it relates to kW, read how to size a generator.
What does a prime rating mean?
Prime power is for a variable load with unlimited running hours. Think of a site where the generator is the main source of power and the load rises and falls through the day as tools, cranes and cabins switch on and off.
There is a catch. A prime rating normally limits the average load over a long period to a stated share of the rating. Commonly this is around 70 percent, but the manufacturer sets it, and it differs between makers and models.
In practice, a prime set should not be held at its prime maximum all day. The maximum is for peaks. The average load is what the set can carry for long hours.
Some prime ratings also state a short overload allowance. Others do not. Some makers also publish a limited-time rating with its own hour limit, which is a fourth figure on the same set. The data sheet says which applies.
What does a standby rating mean?
Standby power is for emergency use when the mains supply fails. The set waits, starts when the power goes, and runs until the mains returns. It is not meant to be the daily source of power.
Because the running hours are limited, the manufacturer gives the set a higher kVA figure. That extra capacity is the reason a standby number must never be set beside a prime number. You are not looking at a bigger machine. You are looking at the same machine under a lighter duty.
A building with a mains connection, such as an office block or a clinic, is the usual user. The generator stays idle most of the year and runs for the outage. How the set takes over from the mains is a separate topic; see ATS and paralleling.
What does a continuous rating mean?
Continuous power is for a steady load held at a constant level, with unlimited hours. A plant that runs a fixed process around the clock, or a site with no mains at all and a flat demand, fits this pattern.
Compared with the other two, a continuous rating is usually the most conservative number for the same engine, though makers define it differently. It usually allows no overload. The set is rated for the long, flat load and has no extra headroom for peaks.
Continuous ratings are not often quoted for hire sets, because most real loads vary. If you see one on a quote, check that your load really is steady. If it swings, ask for the prime figure too.
Which rating fits a construction site, a building backup or a plant?
| Rating | Running pattern | Load profile | Running hours | Overload | Typical use |
|---|---|---|---|---|---|
| Prime | Main source of power | Variable, with an average limit set by the manufacturer | Unlimited | Some data sheets allow a short overload; check | Construction site, remote camp, event |
| Standby | Emergency backup only | Variable, during an outage | Limited emergency hours, set by the manufacturer | Normally none | Building backup, workshop, farm during mains failure |
| Continuous | Main source of power | Steady, constant level | Unlimited | Usually none | Plant with a fixed process, off-grid supply |
A set hired for a construction site that runs most of the day is normally a prime-duty application. Do not hire a standby-rated set for daily running just because its kVA number looks bigger.
Watch out. Running a standby-rated set as the daily source of power can shorten its life and may fall outside its warranty. If a quote says only "500 kVA" with no rating, you do not yet know what is being offered.
How do I compare two quotes with different ratings?
Use one rule: same rating first, then kVA.
- Find the rating on each quote: prime, standby or continuous. If it is missing, ask.
- Ask each supplier for the manufacturer's data sheet for the exact set offered.
- On the data sheet, find the figure for the rating you need. For a site that runs most of the day, that is the prime figure.
- Compare those figures, not the headline numbers on the quotes.
- Only then compare the other terms, as in how to compare equipment rental quotes.
For example, say Quote A offers "500 kVA standby" and Quote B offers "450 kVA prime". The first number looks larger. If both sets are the same model, A is not a bigger generator. For a site that runs all day, you compare the prime figure on A's data sheet with B's 450 kVA.
Do not convert one rating into another with a rule of thumb. The relation differs by manufacturer and model.
What should I ask the supplier to put in writing?
- Which rating is the quoted kVA: prime, standby or continuous?
- What is the kVA under the other ratings for the same set?
- Which manufacturer data sheet applies, and what is the exact set model?
- What average load limit does the prime rating carry, and what does it mean in practice for my load?
- Is any overload allowed, for how long, and how often?
- For standby: how many hours of use does the manufacturer allow, and is daily running covered by the warranty?
- Is the set suitable for my intended use, as I described it, under the rating quoted?
Describe your use plainly: running hours per day, whether the load is steady or variable, and whether mains is available. A supplier can only confirm the right rating if they know the real duty. For the full hire checklist, see how to rent a generator for a site.
Tip. Site heat, altitude and dust can also affect how much a set can deliver. Ask whether the rating applies at your site conditions, and what the supplier would change.
How to do this on Maedah
Open the Generators category to browse listed sets with their specs. Then post a request once. State the load, the running hours per day and your city, and say that you need the prime rating, or the one that fits your use.
Quotes from suppliers come back side by side. Because they are sealed, each supplier prices on its own terms. Check that every quote names its rating, then use chat to ask for the data sheet. Agree the final terms with the supplier directly.
