Generator Glossary: Understanding Backup Generator Terms

If you are in the market for a generator, it’s important to understand the lingo before you make a purchasing decision. Finding the right generator requires research and consultation, and unless you have a degree in mechanical engineering, you probably won’t understand all the terms you read about. That’s why we’ve compiled this glossary of generator terms. Use this guide to help you understand what you need in a generator and what you should be looking for when buying one.

Generator Basics


A generator is a machine that converts the kinetic or mechanical energy of a spinning turbine into electrical energy, which is used to power electrical devices. Some homeowners purchase a backup generator to supply the home with power during power outages or other emergencies. They can also be used by homeowners or companies operating in a place not connected to the electrical grid.


A genset refers to the entire generator unit. It consists of an engine, an alternator, a control panel, and a skid.

Backup Generator

A backup generator is a backup source of power that kicks in automatically when a home’s connection to the electrical grid is cut off. They can be powered by diesel, propane, natural gas, or gasoline.

Diesel Generator

A diesel generator is the most common type of backup generator. It runs on diesel, a highly efficient liquid.

Natural Gas Generator

A natural gas generator is connected to a home’s energy pipelines, meaning the gas is automatically pumped into the generator if the power goes out. The main risk of owning a natural gas generator is that during a natural disaster, the natural gas pipelines could be damaged along with the electrical lines, which would keep the generator from functioning.

Dual Fuel System

A dual fuel system, also known as a bi-fuel or hybrid-fueled generator, gives you the option to switch between sources of fuel. They are most common for portable generators. Though they tend to be more expensive, they provide the advantage of a wider range of fuel availability.

Stationary Generator

Unlike a portable generator, a stationary generator is connected to a house’s electrical circuit and is not able to be moved after it is installed. They are typically more expensive than portable generators.

Portable Generator

A portable generator is a less expensive alternative to a stationary generator that is not connected directly to the home’s electrical system. Instead, it can be transported to provide electricity when needed. They do not require installation and must be started manually. Many portable generators run on gasoline, but some use diesel, propane, or natural gas instead. For more information about portable generators, check out our blog on the subject.

Uninterrupted Power Supply (UPS)

Uninterrupted power supply uses a battery backup system in conjunction with a generator to provide uninterrupted power in the event of an outage. During an outage, UPS kicks in instantly and automatically. It is often used by hospitals, data centers, and other facilities where a continuous power supply is imperative.

Buying a Generator Second-Hand

Used Generator

A used generator is a generator that is bought second-hand from a prior owner or operator without having its internal components disassembled, inspected, or replaced. Buying a used generator entails a lower cost than a new generator but comes with the risk of a shorter lifespan.

Rebuilt Generator

A rebuilt generator has been taken apart and rebuilt following the original manufacturer’s specifications. It is less expensive than a new generator but more expensive than a used generator. The quality of rebuilt generators depends on the skill of the rebuilder, but they are typically thoroughly tested and come with a limited warranty.

Remanufactured Generator

A remanufactured generator is like a rebuilt generator but is made to be as close to the original engine as possible. It is more expensive than a rebuilt generator but less than a brand new generator.

Generator Rating Types

Generator Set Ratings

Generator set ratings are a way of understanding how a generator functions. They include ratings on the generator’s power output, reliability, and cost. Understanding generator set ratings can help you buy the generator best suited to your needs.

Emergency Standby Power (ESP) Rating

The emergency standby power rating refers to the maximum amount of power a generator can deliver during a power outage. An ESP rating means a standby generator can operate about 50 hours per year, with a maximum usage of 200 hours per year. It has no overload capacity and an average variable load of 70% of the ESP rating.

Mission Critical Standby Rating

The mission critical standby rating is a step up from an ESP rating. It applies to a standby generator with a typical usage of 200 hours per year and a maximum output of 500 hours per year. It has no overload capacity.

Standby Power Rating

Standby power rated generators are the most common. Their primary purpose is the provide power for a limited amount of time during a power outage. Like the mission critical standby rating, it applies to generators with a typical usage of 200 hours per year and a maximum output of 500 hours per year.

Prime Power Rating

A prime power rated generator is used by homeowners who do not purchase electricity from a utility company, but instead provide their own power. It can have either a limited or indefinite running time. The average variable load is 70% of the prime power rating, and there is 10% overload capacity.

Continuous Power Rating

Continuous power rating is used when the generator runs 100% of the time and the power load is thus constant throughout the year. It is used most often when the generator functions entirely off-grid.

Other Ratings

Emissions Rating

Emissions ratings track the amount of air pollutants such as carbon monoxide or nitrogen oxides released by the generator into the atmosphere. They confirm the generator meets legal regulations set by the Environmental Protection Agency.

kW Rating

A kW rating measures the power output a generator is capable of supplying. It is based on the horsepower of the engine.

Rating Voltage

A rated voltage is the voltage at which the generator is meant to operate based on its specifications.

Generator Cooling Systems

Air-Cooled System

n air-cool system cools the generator by taking air from outside the generator and funneling it into the generator. Cooling systems protect generators from damage from internal heat.

Liquid-Cooled System

A liquid-cooled system pumps water, oil, or coolant through the generator to keep it from overheating. This system is more effective for larger generators (typically 22 kilowatts or above) but more expensive than an air-cooled system.

Generator Parts


An alternator is the part of a genset that converts the mechanical energy produced by the fuel source into the electrical energy needed to power electrical devices in the home.

Battery-Charge Rectifier

A battery-charge rectifier is a device used to charge a home battery backup system. It works by converting alternating current (AC) voltage to direct current (DC) voltage.


A bearing is the part of a generator engine that restricts the range and rate of motion of different parts of the engine. Bearings reduce friction between parts and help the engine run more smoothly. There are different types of bearings, but they typically resemble a wheel.


A brush is a part of the engine that conducts electrical current between stationary wires and moving parts.


The core of a generator refers to the bundle of wires in the center of the engine.


The enclosure refers to the protective casing around the genset that is visible from the outside. Most come equipped with locks to prevent anyone not qualified to work with a generator engine from tampering with the genset.


A flywheel is a device in an engine designed to store the kinetic energy. When an engine is running, the flywheel rotates quickly within the engine, storing the energy in the form of movement before the alternator converts it into an electrical current.

Generator End

The generator end, or gen end, is another term for the alternator. It is the part of the genset that converts the mechanical energy produced by the fuel source into the electrical current used to power the home.

Load Bank

A load bank is a device that develops an electrical load and applies it to the generator engine. It is used to test the generator and determine its ability to handle an electrical load.


A lubricator is a device that supplies lubricant to the generator engine, usually in the form of oil. It supplies the lubricant in a metered amount to the pneumatic system of the generator, which is used to compress air and control energy flow.


A magneto is a small generator that uses permanent magnets to produce an alternating current. It is most commonly found in gasoline-powered portable generators, as well as some lawnmowers, chain saws, and other gasoline-powered machines.


A relay is an electrically operated switch that controls electrical circuits in the generator engine using an electrical signal.


The rotor is a rotating part of the generator engine caused by the interaction between the windings and the magnetic field. This helps produce mechanical energy, which is stored by the flywheel and converted into an electrical current by the alternator.


The skid is the base of the genset used to mount the other components.


The starter is the device that starts the engine. It rotates the engine so that it starts to operate under its own power. The starter can be electrical or manual.


The stator is the stationary part of the rotor system. It provides the rotating magnetic field that drives the motion of the rotor.

Transfer Switch

A transfer is either a manual or automatic switch that changes the electrical load from its usual source to an alternative source. When the power goes out, the transfer switch allows a home to switch from the power grid to the backup generator, restoring power to the home.

Water Jacket Heater

A water jacket heater is a patch of liquid used to keep the generator engine at the ideal starting temperature. To avoid a dangerous cold start, engine coolant must be kept at 100 to 120 degrees Fahrenheit.


A winding is a coil of wire used to conduct electricity within the engine. It is used in the rotor to generate electricity.

Electrical Terms

Electrical Grid

The electrical grid refers to the system of interconnected transmission lines that carry power from generating systems and electrical substations to consumers who use the electricity to power their homes and businesses. Disruptions to the grid can cause some customers to temporarily lose power.


Distribution refers to the delivery of electricity from electric power from where it is produced to its endpoint, where it ultimately provides power to an electrical device.


Peak refers to the maximum load which a generator can handle during a specific time.


Off-peak refers to a specific period of time when demand for energy is lower. According to the North American Electric Reliability Corporation (NERC), off-peak hours are 10 p.m. to 6 a.m. Monday through Saturday and all day Sunday.

Alternating Current (AC)

Alternating current refers to an electric current that repeatedly reverses its direction. It is the form of electricity used to power electrical devices in homes and businesses.

Direct Current (DC)

Direct current is an electric current that flows in only one direction.


The frequency is the number of electromagnetic waves that pass by a point in a second, measured in Hertz (Hz). Electrical frequency measures the number of times an alternating current changes direction per second. The standard electrical frequency in the United States is 60Hz.


A kilowatt (kW) is a way to measure power. One kilowatt is equal to 1,000 watts.


A kilowatt-hour (kWh) is a way to measure the flow of electricity from a generator or another device. One kilowatt-hour is equivalent to one hour of electricity flowing at a rate of one kilowatt.


A kilowatt-volt-ampere (kVA) is a unit of apparent power. It is an alternative way of measuring the power rating of a generator. In the United States, it is more common to measure the power of a generator using kilowatts.


The National Electrical Manufacturers Association (NEMA) is a large American trade association composed of about 350 electrical manufacturing companies. The organization publishes standards for electrical equipment to help ensure electrical equipment can be used interchangeably.

Full-Power Outlet

A full-power outlet lets you draw the full power of the generator out of a single outlet.

Mechanical Terms

Generator RPM

A generator’s revolutions per minute (RPM) refers to the speed at which the generator runs. A generator that runs at 3600 RPM will produce an electrical frequency of 60Hz, the standard electrical frequency in the U.S.


Gallons per hour (GPH) is a way to measure how much fuel is pumped into a generator or other device.


Gallons per minute (GPM) is an alternative way to measure fuel.

Generator Sizing

Generator sizing is the process of determining the size a generator needs to be to meet a home’s electrical needs. Before purchasing a generator, it is critical to work with a certified electrician to determine the ideal size and electrical capacity you need. A good place to start is our Power Calculator, which will help you determine your home’s energy consumption and the size of the generator you may need.

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