Generator poupouri. Ratings. kW vs KVA Standby vs prime

This Generator Tutorial/discussion is most suited to three phase sets above about 15 KVA.

  • What is the difference between KW and KVA?
  • How are they determined?
  • What can you expect to find?

First, there are exceptions.
(And below about 10 or 15 KVA, there are a lot of exceptions…)

KVA (or Kilo Volt Amps) - This is the product of the rated Amps and the rated Volts, divided by 1000.
Amps. This is determined by the temperature rise of the windings under load.
Usually the maximum rated Amps remains constant, but a prime power set will be rated at a lower temperature than a standby set. The voltage may change. A set may be rated for 120:208 Volts star, or 120:240 Volts four wire delta, or 480 Volts star.

Example: A standby set may have windings rated for 100 Amps maximum.
If this set is to be used at 120:208V the maximum KVA will be 120V x 100A x 3 / 1000 = 36 KVA.
Generator sets rated at or above 15 KVA are typically 12 lead machines, suitable for use at 480 Volts.
If this set is to be used at 277:480V, the windings will be in series rather than in parallel.
The voltage across each winding will be 277V / 2 = 139 Volts.
277V x 50 Amps x 3 = 41.6 KVA.
However the set will still be rated at 36 KVA due to prime mover concerns. The same set may be rated at 50 Hz and used at common 50 Hz voltages. The KVA rating will change as the utilization voltage is changed.

The bottom line:
Do not exceed the rated current per winding.

:kW, KW or Kilo Watts.
The maximum kW is dependent on the power of the prime mover.
The prime mover or engine must supply enough mechanical kW to support the electrical kW output of the set.Typically, and more typically as the size of the set and the load increase, loads tend to be somewhat inductive.
With this in mind, it is common practice to rate sets at 80% Power Factor (PF). When sizing a set, you must know the nature of the expected load. If the load power factor will be below 80% you may size the set by the KVA rating. If the PF of the load will be above 80% PF you must size the set by the kW rating.
While three phase sets are rated at 80% PF, single phase conversions often result in the kW capability of the prime mover being greater than the KVA rating of the set.

Standby versus Prime ratings.
To understand the differences, it may help to look at the expected lifetime of typical sets.
A standby set may be exercised for two hours per month, and support 4, five hour power failures.
A total of 44 hours per year.
A prime set will run 24/7. There are typically at least two prime sets, alternating the load.
There are about 8700 hours in a year, so our prime set may be expected to run about 4400 hours in a year. A prime power set may run as much in a year as a standby set runs in 100 years. The build and ratings reflect this disparity in expected run times.

Example: Starting with a basic set, the standby set is rated at the maximum capability.
The standby ratings allows occasional overloads of 10%. The rating of the basic set is reduced to allow occasional loading up to the standby rating. The 10% buffer also allows the set to lose some power with age and still produce rated power after 4,000 or 8,000 hours of run time.

A prime set will often have added features that may or may not be present on the basic standby rated sets.
These may be:

  • A larger oil pan to allow longer running between oil changes.
  • An oil cooler.
  • Provisions for a remote radiator.
  • Any other accessories that the manufacturer feels will enhance the lifetime of the set.

From a recent Caterpillar publication, and more in line with what I have seen over the years for ratings.

  • Standby Rating: Output available with varying load for the duration of the interruption of the normal power source. The generator with the Standby Rating is typically used for the building standby power services. (Typical Load Factor = 60% or less; Typical Hours per Year = 500 h; Typical Peak Demand = 80% of standby rated ekW with 100% of rating available for the duration of an emergency outage.

  • Prime Rating: Output available with varying load for an unlimited time. The generator with the Prime Rating is typically used for heavy industrial loads, pumping station applications, peak shaving applications or cogeneration plant applications. (Typical Load Factor = 60% to 70%; Typical Hours per Year = no limit; Typical Peak Demand = 100% of prime rating used occasionally.

  • Continuous Rating: Output available without varying load for an unlimited time. The generator with the Continuous Rating is typically used for utility parallel operation, on-site generation or base load cogeneration plant applications. (Typical Load Factor = 70% to 100%; Typical Hours per Year = no limit; Typical Peak Demand = 100% of continuous rating used 100% of the time.


Hi Mike;
It’s good to have you here.
A question.
The background:
When I first got dumped into the deep end of the diesel generator pool, I didn’t have many resources.
I spent a lot of hours going over F. G. Wilson’s online specs.
I was comparing standby ratings with prime ratings, generator ends and diesel engines with generator ends and diesel engines.
At that time, almost 30 years ago, The sets were identical save for the rating:
a 50 KVA prime set would be rated as a 55 KVA standby set etc.
(In those days, if you looked very closely at the nameplate on a Caterpillar Olympian set, you would see in very small lettering:
“Made for Caterpillar by the F.G. Wilson company”.)
Now Cat owns F.G. Wilson.
The prime sets may have added accessories, larger sumps, oil coolers if not already spec’d and possible other additions to the same base engine.
The question; Has there been any change or is my knowledge still valid.
Thanks Mike.

1 Like


Mostly nowadays, at least in the units I see pretty regular, Standby and Prime units are usually similarly equipped as far as sump size, accessories like fuel and oil filtration, air filtration and rating capabilities. Continuos rated units typically have the larger sumps, larger and dual fuel and oil filtration systems, larger air cleaners with accomodation for precleaners, and usually better local monitoring packages for engine and generator parameters. Of course these are pretty broad statements, sometimes I run into units that have the same iron configuration and just have different flash files in their ECM’s depending on how much power you want to pull out of them. Newer engine controls have histograms for load factor, fuel consumption and other parameters that help the manufacturer and dealer figure out if you have exceeded the allowed limits and use that data in warranty and policy determinations.
So in general I’d say a lot of what you experienced in the past is the same, but some change with unit size and power rating, plus the fact that with emissions regulations in many countries, a lot of engine manufacturers have tried to reduce the number of engine configurations they have to certify. In smaller units like you appear to experience with, the difference in accessories is usually limited, but in a 2MW unit while the base unit is very similar, between a standby, prime and continuous rating there will likely be turbo and fuel system component changes, along with sump size and filter systems. Also depends on what the end user is willing to pay for.

Last night I was at a site that is having to run on their standby generator for the next couple of weeks due to a utility problem, when they bought the unit it had both a standby and prime rating, so overall they initially didn’t seem too concerned, what their facilities manager missed was that the unit has 150 hour service interval, so about once a week they will have to stop the unit, losing power onsite to do a service, not what they wanted to hear. Now they can try to run it longer, but they risk having an unplanned shutdown, mainly due a fuel filter plugging (their air pollution permit limited them to 22 hours a year for maintenance running and their fuel storage and system maintenance hasn’t been very good), and because the unit has an extended warranty, if they don’t do their maintenance as recommended, the may impact their warranty coverage. But they saved money up front! If the utility issue only actually lasts a couple weeks they will probably be ok, but with the ways things out here right now, who knows, especially since we are not out of an active fire season yet this year.

A little history about CAT and FG Wilson, even before CAT bought them (actually was part of a trading of multiple companies that also involved Kato) FG Wilson packaged a pretty large number of smaller (less than 300ekW) for CAT in North America and for CAT dealers in Europe the Middle East and Africa. The North American product was the early Olympian product line and most of the overseas units carried the FG Wilson nameplates. When CAT got Perkins, it got a bit more complicated, especially since FG Wilson was one of Perkins biggest customers. So you will still see FG Wilson packaged CAT sets and FG Wilson branded sets in a lot of places in the world, depending on how they were sourced and usually where they were intended to end up. As a dealer in USA, we sold a number of FG Wilson sets into Latin America with CAT engines but warranty and service support by FG Wilson dealers, but at that time we were also a Generac dealer as well. Early on CAT just wasn’t as big a player in the small end market for the entire set, mostly due to costs, both initial on ongoing maintenance and service.

Hope that helps, MikeL

Thanks Mike