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Before discussing home gensets selection, let's go over some basics. Generator is a latin word that means originator or maker. In power industry, this term refers to a device that produces electricity. We know that although electricity does occur naturally, it does not exist in the forms that currently can be practically utilized. For practical use it is produced from other sources of energy. Technically speaking, in electric generators electricity is produced from mechanical energy. The mechanical energy in turn can be generated from chemical, nuclear or thermal energy contained in various types of fuel. It can also be obtained from renewable resources. A machine that transforms energy in fuel into rotational energy to drive generator's rotor is called prime mover. Steam turbines, internal-combustion engines, gas combustion turbines, water and wind turbines are the common types of prime movers.


The operation of electric generators is based on the phenomenon of electromagneticinduction: whenever a conductor moves relative to a magnetic field, voltage is induced in this conductor. Particularly, if a magnet is spinning inside a coil, a periodic AC voltage is induced between its terminals. For more information see our tutorial on how generators work with an animation that illustrates their basic operation.

When an external circuit connected to the coil terminals the induced voltage (called electromotive force or emf) will create an electric current resulting in energy being delivered to the load. Thus, the kinetic energy that spins the source of the magnetic field is converted into electricity. Note that the current flowing through an external load in turn creates a magnetic field that opposes the change in the flux of the coil, so the coil opposes the motion. The higher the current, the larger the force that must be applied to the magnet to keep it from slowing down.

In practice, the magnetic field is most often induced by an electromagnet rather then a permanent magnet. It consists of so-called field coils mounted on an iron core. A flow of electric current in the field coils is required to produce magnetic field. This current may be obtained either from an external source or from the system's own armature. The initial field is produced by residual magnetism in the electromagnet's cores. When the prime mover starts spinning, the armature at first operates in a very weak magnetic field and therefore produces small emf. This emf creates a current in field coils, which increases magnetic flux, which in turn increases emf in the armature. This process continues until the rated output voltage is reached.


In power plants the electricity generating devices are most often driven by steam or hydraulic turbines or by diesel engines. The same concept of producing electricity is widely used in small consumer-grade units. In commercially available generators for home use, an alternator is integrated with an internal-combustion engine into a single assembly. Such a device called genset is the most common type of emergency backup power sources for the home. A genset is often casually called just a generator even though it also includes an engine. There are two main types of such devices that differ by their connection and activation methods: fixed (standby) and portable. Fixed generators are permanently connected to the building wiring system and are also hooked up to a fuel line or a tank. They require professional installation of both the fuel line and a special redundancy system. The latter isolates the utility from your genset. Not surprisingly, permanent devices cost more than portables. However, they have a big advantage-- they can provide practically continuous power for as long as the fuel is supplied. Portable models are intended primarily for a temporary connection to several appliances via extension cords rather than to the whole house. They are normally powered from an on-board fuel tank and therefore need frequent refueling, although some more expensive models can also be connected to an external source for continuous operation. A portable unit is generally cheaper than a standby and can be used without any professional installation. However, if you want to connect it to the house wiring you still need to install a transfer switch. Choosing the best device for your application involves selecting the right type, choosing the fuel, and a proper sizing based on the amount of power you may need during an emergency.

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