Sizing a Power Generator in Plain English

Understanding the technical specs of power generators can give you a headache. Manufacturers overwhelm us with acronyms and huge numbers that are often misleading. It's completely natural that you might have gotten lost in all this!

Imagine your car. The speedometer says 250 km/h. Does that mean you can get in the car and drive at that speed 24 hours a day, 7 days a week? Of course not – the engine simply wouldn't survive it.

It’s exactly the same with generators. The biggest number on the sticker is usually the power the device can reach only for a short moment. To buy equipment that will serve you for years and won't break down after a week, we need to look under the hood of these magic acronyms.

So, if you stubbornly insist you know how to size a generator, please,  spare two more minutes and read this short article to the end with understanding, Mr. / Ms. Generator Sizing Expert.

Here is a simple guide on how to read generator power ratings based on the international standard (ISO8528-1), without the engineering gibberish, just the way you like it.


Deciphering the Labels: Basically 4 operating modes

The industry uses four main acronyms that define what kind of work the equipment was built for, how it should operate, and with what frequency.


1. ESP (Emergency Standby Power)

This is the "rescue" power. You buy a generator with this rating solely to save you in the event of a power outage.

  • Operating time: Max 200 hours per year.
  • How it operates: The load must fluctuate (variable), and on average it should not exceed 70% of the stated power.
  • Overload: None. You cannot squeeze a single watt more out of it.
  • For whom: A single-family home, a small shop saving fridges from defrosting when a storm takes down power lines.

2. LTP (Limited Time Prime)

The equipment is meant to work hard, but only for a predetermined, short period of time.

  • Operating time: Max 500 hours per year.
  • How it operates: It can run at 100% of this power the entire time (constant load).
  • Overload: None.
  • For whom: For example, to power pumps during periodic water pumping.

3. PRP (Prime Rated Power) – (The biggest trap lies here!)

This is the most frequently provided parameter. Equipment with this rating is meant to be your main source of power, not just a backup.
This is where most customers make a mistake, thinking that if they buy a generator with a 100 kW PRP, they can use it continuously (with continuous power) at 100 kW - No, that is not the case.

  • Operating time: Unlimited (can run all year round).
  • How it operates: The load should be variable, and on average cannot exceed 70% of the stated prime power.
  • Overload: Yes, it is the only one that allows a 10% overload, but only for 1 hour in every 12 hours of operation, without specifying the maximum continuous overload times.
  • For whom: Construction sites, festivals, production, continuous operation, continuous power supply, plants and factories cut off from the city grid.

4. COP (Continuous Operating Power)

A true workhorse. Built for heavy, monotonous work without a break.

  • Operating time: Unlimited.
  • How it operates: It can run non-stop at 100% load (constant load).
  • Overload: None.
  • For whom: Mines, massive server rooms operating under constant database load, ship power supplies.

The Translation Trap: Why "Prime Power" is not what you think

This is the most important thing you need to remember from this article. Often in stores, companies, by local and other manufacturers, the acronym PRP (Prime Rated Power) is mistakenly translated or explained as "continuous prime power". This is a mistake!! A mistake that has cost many people badly and unnecessarily spent money. Relying on such a description, they chose the wrong generator for their needs!!!

The word "continuous" in such misleading explanations refers to the fact that the generator itself can run continuously (24/7). HOWEVER, THIS DOES NOT MEAN that it can run at full power all the time !!!

If you buy a generator labeled PRP 100 kW and connect machines to it that draw exactly 100 kW all day and night, you will destroy it. According to the table (and the laws of physics), at PRP power, the average power consumption over the entire operating cycle cannot exceed 70%. The device must have moments of rest (variable load). If you need a constant 100 kW all the time, you must look for a generator with a Continuous Operating Power (COP) of 100 kW (such a generator will be physically much larger and more expensive), or one with a PRP about 30% higher than what you want to power.

How to size the power then? Rule of thumb!

If you are a regular user and don't want to get a PhD in electrical engineering, apply a simple rule:

  1. Add up the power of all the devices you want to turn on at once. (The total power you will be working with)
  2. To the obtained result, add a 30% to 35% margin.
  3. Compare the result you get with the Prime Rated Power (PRP) of the generator.

Thanks to this margin, the engine won't be "howling" at maximum RPMs, it will consume less fuel, and you will be sure that you have chosen your machine well.

So to sum it up, simply and to the point once again:

You plan to work with a load of 10 kW - you buy a generator with a prime power of PRP 16 kVA - which is 13 kW
You plan to work with a load of 100 kW - you buy a generator with a prime power of PRP 160 kVA - which is 130 kW
You plan to work with a load of 1000 kW - you buy a generator with a prime power of PRP 1600 kVA - which is 1300 kW


Proper generator sizing and service