How Do We Size a Generator?

Prime power rating is for when the generator is used for continuous power whereas standby power is for when the generator is used only occasionally. It is at a lower engine rating. The prime power rating is usually around 75 or 80 % of the standby rating. So the gist is that if a generator is to be used as prime power it needs to be bigger than one supplying the same size load on a "standby" rating. This is just an outline, I'm sure others can fill out more.

It is necessary to determine the load that is suitable for the application. The size is not necessarily the total connected load but is the maximum operating load plus a maegine you feel comfortable with.

Once the load is determined, apply the through put effeciency of the type of alternator you like to use to determine the input shaft kW requirement. Typical industrial synchronous alternators have through efficiency of over 90%.

Now determine the prime mover kW rating. This is wherer prime duty and standby duty comes in. Industrial engine manufacturer publish the ratings for each application with the auxiliary equipment power requirement.

After determining the prime mover rating and the duty, you can define the cooling (and heat recovery, if applicable) system to meet the requirement. Engine/Generator controls and distribution swtchgear will have to be desinged as per the engine and generator (alternator) in use.

Note that this is applicable for a single engine, isolated power source. If more than one engine-gen set is to be operated or connected to national grid, then other system requirements arise.

Hope this is of some help

pulto45

Rule of thumb is: 125% of demand load + 100% largest motor start expected = tcl.

For critical applications, add a safety factor, but keep in mind that efficiency suffers
when the generator is too much oversized. - It's a judgement call.

The <...we...> simply picks up the telephone and discusses the requirement with <...generator...> manufacturers directly.