Generator Ratings (back to basics)

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    Steven Mill

    Here is another article written by one of the member of the Electrical Engineering community. His last article was about Electric Generator Terminologies.

    Hello friends, today’s topic, as the name suggests, is about generator ratings. I chose to share about this topic because when an Electrical Engineer is in field dealing with generators, then it’s very important for him to know the rating of under process generator otherwise he may cause a great loss.

    Generator rating is a set of specified values which are specified for that particular generator model by its manufacturers. It must be capable of supplying that much power output in an accurate way that is anticipated by the manufacturer.

    Need of Ratings

    These ratings prove helpful while buying the product, to help in correctly selecting the machine that is needed by the user, depending upon his usage and requirement, he can choose the appropriate rating for his work. It also differentiates between the manufacturing companies and the efficiency of generators that the company produces and how reliably those ratings are fulfilled by those generators.

    Units of Standard ratings

    The thing that should be remembered is that the ratings of the generators are usually expressed in Kilo Volt Amperes i.e. KVA and this is conventional for all energy supplying devices as well, such as transformers and UPS’s as well. A motor is similar in construction and working principles to the generator, but it is rated in Kilo Watts (KW). This is due to the fact that KW represents real power whereas KVA represents the apparent power output of the device.

    And as far as the energy supplying devices are concerned, their power factors depend on the load that is being attached to them, and hence the real power can never be predicted accurately. So in the apparent power, the power factor is approximately taken as one.

    The conversion from kVA to KW can be represented as follows:

    (P.F) * (kVA) = (kW)

    To calculate kV, the calculations can be performed as follow:

    (I x V)/1000 = kVA

    If simple Amperes are to be calculated, then we can follow the conversion like this:

    (KVA x 1000)/V = A

    To make the understanding better, here is a table which shows the common kV ratings of a generator, and the calculated loads that can be afforded by them:

    Generator Ratings

    Standard Definitions of Ratings

    Standard rating definitions are set by the International Standards Organizations (ISO), so that the comparison between different manufacturing companies and their equipment become easy, when all have the same standard parameters. For the same rating, a generator can deliver more power at the output if used for a lesser time duration, than the one used for more time duration.

    The standard definitions of some commonly used Ratings are given below:

    • Standby Power Rating

    The Generator set is capable of providing emergency power at times when no other source is available. Its power providing duration can be up to 500 hours for an entire year. The power factor for a stand by power rating should not be more than 0.7.

    • Prime Power rating

    Prime rated power is capable of providing the power for an unlimited time period to a varying load. Overload is also allowed but only ten percent of the rated value and that is also permitted to only once in twelve hours.

    This rating offers the option of a more varying load, but only with an average load factor.

    • Continuous Power rating

    In this rating, the generator can again provide a power supply for an unlimited period but only to a non-varying load. But the average output power comes out to be between 70 to 100 percent of the rated power output. The load should be “relatively steady”, which means that there should be no significant variations in it, otherwise the prime power rating could be a better option to consider. Also any overload capability is not available.

    • Limited Time Power rating

    If a prime rated generator is applied to limited power ratings, then it can be used in a parallel with the utility, and can run for duration of five hundred hours in a year. It also does not allow the over load capability of ten percent of the rated power.

    In the next post of this series, we will put some light on another type of generator, which is called Direct Current Generator. Direct Current generators are very rare to found but have unique applications. They are quite similar to synchronous generators in operation with slightest difference which we will see in detail in the next post.

    Till then, take care and if you want to write an article for the ECC blog just like me, simply send a mail to the team.

    Spir Georges GHALI

    Dear Mr. Mill ;


    Refer to this topic, I would mention to the following points :

    – In the table and all equations you mention only to the “ small Single Phase Generators ”, where are the 3 Phases Generators ?

    – The table mentioned to the small powers of Single Phase Generators and the relevant Loads in Watts, but those loads’ values are theoretical, because :

    • Most of actual loads aren’t 100% resistive, that means the PF is smaller than 1 ( exp. if we have a load “ P : 800W, PF : 0.70 ”, that means the minimum apparent power that we need is “ Smin : 800/0.70 = 1140VA ”, so, 1kVA generator can’t afford this load of 800W ).
    • Normally, the name plate of any generator mention to a “ PF value ”, and that value guides us to know to the maximum active power “ P : W  or  kW ” can be consumed of it. Noting that you mentioned to this point in “ Standby Power Rating ” paragraph.

    – I think we should also mention that the maximum loads of any Standby Generator should not exceed “ 80  to  85% ” of its Rated Apparent Power “ S : kVA ”.

    – It’s mentioned in “ Prim Power Rating ” paragraph that for this type a 10% of the rated apparent power is allowed as an Overload only one time each 12 hours, but it’s not mentioned the duration of this allowed overload ???


    Dear publisher,
    The first part of the current article is not to the point and misleading.

    To mention a clue, if a generator is rated as “X” KVA it as potentially “X” KVA at 0.8 PF
    That is it can give as maximum 0.8*X KWATTS, Which is the maximum real power (KW) it can give at a time.
    So if your overall P.F. of your installation is 0.8 you can reach the KVA rating of your generator plate as maximum load (taking into acount to stay a 70% of generators rating in average during lifetime of generator), however if your installation is of unity Power Factor for example you can only load 0.8*X KVA or KWATTS (X being the KVA rated power of the generator).

    To sum up for 1000KVA rated generator, your maximum load must never be above 800KWATTS. And if you have unity power factor load your maximum load is 800KVA (which is 800KW in this case) if your installation load PF is 0.9 your maximum load can be 800/0.9=889KVA (which is again 800KW but at 0.9 PF)

    Spir Georges GHALI

    Dear Mr. Dikran


    Most of Generators’ assemblers define the Apparent Power ” kVA ” with ” P.F. = 0.80 “, but we should always read the  Generator’s Name Plate to know the exact PF value be defined by the assembler.


    Dear sir,

    very exellent notes,

    thanks and regards,



    very good,


    thank you about this information but we need more of 3 phase gen.



    Nice share about Continuous Power rating and other topic to actually I search Information about this topic and find the different about it.



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