How Generators Work- Part One

Generators Part One is a must-read for anyone interested in understanding the basics of generators. In this guide, Mark Delle Bovi outlines everything from fuel considerations to load shedding, setting the stage for further exploration in 'Generators Part Two.'

The topic of generators stirs up heated debates within the industry. In this article we will discuss the basics of generators, and some of the best practices to use when sizing generators. The task of properly sizing the power generation system for a building falls to the designer and/or manufacturer, not the NEC. To understand generator sizing we will need to look at some basic electrical theory concepts that come into play. 

Generator outside home

What type of generator do I need?

Some questions that are important to ask are below:

  1. What fuel will the generator use? Natural Gas, Propane or Diesel. 
  2. What voltage does the generator need to supply? 120, 240, 208, 480V?
  3. Is the generator supplying single phase or 3 phase?
  4. Duty rating: is the generator standby rated or prime rated? Prime means the generator can overload ten percent of its capacity for a certain amount of time.

Answering these questions is the first step in the process of sizing a generator. A diesel generator will function completely differently than a natural gas generator, often requiring some limitations in its capabilities. 

Power Factor

Power factor comes into play in most instances when sizing a generator. The power factor calculates the ratio between the real power absorbed by the load (watts) and the apparent power flowing in the circuit (VA). Power factor possesses no units and presents a value between 0 and 1, with 1 representing a purely resistive circuit that doesn’t draw extra current. The important thing to remember is that the lower a load’s power factor, the more current it will draw. Let’s look at an example. 

EXAMPLE: Calculate the current for a 480V 3 phase 45kW load at 0.8 PF and at 0.5 PF. 

We can observe that the lower power factor requires more current to deliver the same amount of power at a given voltage. This is an important concept to understand when sizing generators. It is also important to understand the difference between apparent power and real power, as we want to make sure we are using the correct units for sizing. As an example, calculate all loads in apparent power first, and then convert them to real power using the correct power factor. We rate generators based on their ability to produce power in watts or kilowatts at a defined power factor. Three-phase generators will have a power factor of 0.8, while single-phase generators will have a power factor of 1.   

Example of a 0.8 Power Factor Genset
Example of a 0.8 Power Factor Genset – Source:

What type of loads will the generator serve?

Motor loads are hardest on the generator alternator, due to the large inrush current from the startup. Soft-start motor starters and frequency drives help prevent this to an extent. The use of across the line motor starters allows the extraction of up to six times the motor’s running current from the generator itself.

Other loads to consider are nonlinear loads such as variable frequency drives (VFD), large uninterruptible power supplies (UPS) or even battery charging systems.  Nonlinear loads can induce unwanted harmonic currents into the system, which can lead to voltage distortion and instability.

In situations where a generator is serving large amounts of motor or nonlinear loads, the generator should be oversized to prevent voltage and frequency distortion. There are many free generator sizing tools out there that can be used for this, but a good rule of thumb is to provide 50 percent spare capacity. 

Generator Fuel type

Generators can be either diesel fuel, or natural gas/propane fired. Diesel generators are much more robust and provide quick start up, providing smaller engine sizes compared to natural gas or propane generator sets. Natural gas and/or propane generators offer an alternative to diesel – onsite fuel storage is not required, they generally run quieter, and burn cleaner than diesel. Oftentimes generators are supplying life safety systems in buildings, which require a maximum of 10 seconds to startup. Natural gas generators can have difficulty achieving this in some of the large-size generators. For this reason, diesel generators are generally recommended for sizes 500kW and larger. 

Emergency Diesel Generator
Emergency Diesel Generator- Source:

Load Shedding/Load Stepping

Load “stepping” allows the generator to progressively add loads over a certain amount of time. It’s always best to progressively load the generator up, rather than connect all loads instantly at the same time. Adding more load at once leads to larger dips in voltage and frequency. The lower the inrush current, the more stable the system will be. In the inverse process, called “load shedding”, we systematically disconnect loads to reduce the total load on the system. Load shedding is accomplished the same way as load stepping – via transfer switches. The generator controller is programmed to switch over certain loads at given time intervals. 

Portable Generator with Load Shedding

Portable Generator with Load Shedding – Source:

As an example, say we have three transfer switches in a building. One transfer switch is for life safety loads (such as emergency lighting), one is for optional standby loads (servers in a data center), and one is for a fire pump. Under normal operation if the power goes out, the generator will energize both the life safety and optional standby loads and not the fire pump, as there is no fire condition. However, during a fire condition, the generator will provide power to the life safety loads and the fire pump, but not the optional standby loads. This allows the sizing of the generator to be based only on the largest two loads, rather than all three. 


We’ve laid a solid foundation on generator basics, highlighting the critical role of power factors, real and apparent power, generator parameters, and load shedding. As we’ve seen, understanding the type of loads a generator will serve is key to an optimized system. Next, in “How Generators Work- Part two,” we will delve deeper into advanced techniques. Stay tuned as we continue our exploration into the complex and essential world of generators.

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