What is the difference between kW and kVA?

The primary difference between kW (kilowatt) and kVA (kilovolt-ampere) is the power factor.  kW is the unit of real power and kVA is a unit of apparent power (or real power plus re-active power).  The power factor, unless it is defined and known, is therefore an approximate value (typically 0.8), and the kVA value will always be higher than the value for kW.  

In relation to industrial and commercial generators, kW is most commonly used when referring to generators in the United States, and a few other countries that use 60 Hz, while the majority of the rest of the world typically uses kVa as the primary value when referencing generator sets.

To expand on it a bit more, the kW rating is essentially the resulting power output a generator can supply based on the horsepower of an engine.  kW is figured by the horsepower rating of the engine times .746.  For example if you have a 500 horsepower engine it has a kW rating of 373. The kilovolt-amperes (kVa) are the generator end capacity.  Generator sets are usually shown with both ratings.  To determine the kW and kVa ratio the formula below is used.

.8 (pf) x 625 (kVa) = 500 kW


What is a power factor?

The power factor (pf) is typically defined as the ratio between kilowatts (kW) and kilovolt amps (kVa) that is drawn from an electrical load, as was discussed in the question above in more detail.  It is determined by the generators connected load.  The pf on the nameplate of a generator relates the kVa to the kW rating (see formula above). Generators with higher power factors more efficiently transfer energy to the connected load, while generators with a lower power factor are not as efficient and result in increased power costs.  The standard power factor for a three phase generator is .8. 

 


What is the difference between standby, continuous, and prime power ratings?

Standby power generators are most often used in emergency situations, such as during a power outage.  It is ideal for applications that have another reliable continuous power source like utility power.  It’s recommend usage is most often only for the duration of a power outage and regular testing and maintenance.

Prime power ratings can be defined as having an “unlimited run time”, or essentially a generator that will be used as a primary power source and not just for standby or backup power.  A prime power rated generator can supply power in a situation where there is no utility source, as is often the case in industrial applications like mining or oil & gas operations located in remote areas where the grid is not accessible.   

Continuous power is similar to prime power but has a base load rating.   It can supply power continuously to a constant load, but does not have the ability to handle overload conditions or work as well with variable loads.  The main difference between a prime and continuous rating is that prime power gensets are set to have maximum power available at a variable load for an unlimited number of hours, and they generally include a 10% or so overload capability for short durations.


What does an Automatic Transfer Switch do?

An automatic transfer switch (ATS) transfers power from a standard source, like utility, to emergency power, such as a generator, when the standard source fails. An ATS senses the power interruption on the line and in turn signals the engine panel to start.  When the standard source is restored to normal power the ATS transfers power back to the standard source and shuts the generator down.  Automatic Transfer Switches are often used in high availability environments such as data centers, manufacturing plans, telecommunication networks and so forth.