Alternators vs. Generators

Ian Mutuli
Updated on
Ian Mutuli

Ian Mutuli

Founder and Managing Editor of Archute. He is also a graduate architect from The University of Nairobi, Kenya.
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For most people who are looking to buy generators, knowing the differences between an alternator and a generator is usually not compulsory. You can buy just about any kind of generator you find on the market and it will still work as expected by providing you with the power you need. However, understanding the differences between a generator and an alternator can come with plenty of benefits. In this article, we will be looking at the main differences between the two and why you should be worried about the type of current that your generator produces.

Before we explain what an alternator and a generator is, it is worth noting that both can convert mechanical energy into electrical energy. The reverse is true too since if you supply either an alternator or a generator with electrical energy, you will get back mechanical energy. 

One thing to note about the two is that while they can produce electrical energy, the electric currents are not always of the same type. Alternators produce AC current whereas generators can produce either AC current or DC current, but we'll talk more about that later on.

An alternator is a special type of generator that converts mechanical energy into electrical energy which is generated as an alternating current (AC). Alternators are common in the automotive industry. As such, your car is probably one of the best places to find an alternator if you've never seen one.

Once you turn on your car's engine, the alternator will equally be powered up and will start converting some of the mechanical energy from the engine into electrical energy. Ideally, the mechanical energy from the engine simply rotates a magnet inside the alternator. 

When the magnet turns, it creates a magnetic field which changes the magnetic flux. This in turn produces an alternating current. The rotor in this case is the magnet which creates a rotating magnetic field while the stator outputs the alternating current.

Alternators are primarily known for their efficiency since they tend to generate electric current only when it is needed. This is why they are perfect for charging your car batteries. However, it is worth noting that an inverter and a voltage regulator stand in between your car's batteries and the alternator. This is because your car's batteries produce DC current. As such, you can only power it with DC, and that's why an inverter and voltage regulator are essential.

Typical portable generators that use propane, gasoline, diesel, or LPG also have alternators. These alternators are the ones that are responsible with providing you with AC power which is commonly used by many electrical appliances.

What is a Generator?

Generators, on the other hand, are devices that can convert mechanical energy into electrical energy and can produce either AC or DC. Finding a generator is far easier for most people since they are all around us and come in different types. The most common types are:

  • Standby Generators
  • Portable Generators
  • Inverter Generators

The reason why generators can produce either DC or AC power lies in the design. In a generator, the rotor consists of the coil of wires (armature) and not the magnet. As such, the magnetic field remains stationary, but the coiled wires move inside it. A current is thus induced in the wires. When this rotor is connected to a commutator, a DC current is produced. AC current can be produced when the rotor is connected to a slip ring. 

Generators produce voltage throughout and also consume the energy that they generate. Because of this, generators are ideal for use in large-scale electricity production and this is why they are popular.

One other thing to note about generators is that they can be used to charge fully drained batteries unlike alternators which should only be used to charge batteries that have some power. Interestingly, modern day generators have alternators in them. An inverter generator, for example, comes with a "digital alternator" rather than a mechanical one. A digital alternator has a set of rectifiers that dictate the type of current the generator produces. In most cases, an inverter generator will be sufficient for powering up small electrical devices.

The Difference Between AC and DC Power

So far, we've found out that alternators produce AC power while generators can produce either AC or DC power, but what do these two terms really mean? Alternating and direct current can best be explained by the graph below:

The Current War: Why Did Westinghouse (AC) Beat Edison (DC ...

As its name suggests, an alternating current consists of an alternating voltage that varies with time and at a constant frequency (usually either 50Hz or 60Hz). In AC, the flow of electrons reverses after every half cycle which results in the sine wave.

Though many electrical equipment usually use AC, there are a number of regulating devices that need to be put in place to control the amount of energy. Most of the time, these regulating devices convert AC power to DC. 

DC power, on the other hand, has a constant flow of electrons moving in one direction. This can be extremely useful in conserving power. Storing DC power is also easier and more direct than storing AC power. This is why you always need to keep your generator on to get power while a device like your smartphone can store DC power for using later.

Regardless of the advantages of DC, your typical generator will still provide AC power since AC can be easily stepped up and down.

Key Differences Between Alternators and Generators

Apart from providing either AC or DC power, there are several other things that make alternators different from generators. Below are some key highlights:

RPM Range

An alternator has a wider RPM range than a generator. This is because the magnetic field of the coil can be reduced or increased. Generators have a limited RPM range.

Power Output

The power output from an alternator will be significantly higher than that from a generator. A DC generator, for instance, will require rectifiers to smoothen the output. These same rectifiers require power and thus reduce the generator's efficiency.

Battery Charging

A generator can charge a completely dead car battery whereas an alternator may die in the process. The alternator will struggle to provide the power that is needed to run your car while charging your battery and may thus overheat and fail in the process.

Variable Voltage

The output voltage from a generator will always be constant while that of an alternator may vary. This is what makes a generator the ideal choice for powering electrical equipment for long durations.

Magnetic Field

With regards to the magnetic field, an alternator has a rotating magnetic field while a generator has a stationary magnetic field. This influences whether you get the energy from the stator or the rotor since an alternator supplies energy from the stator whereas the generator supplies the energy from the rotor.


Despite their differences, it wouldn't be worth settling on either an alternator or a generator as both have unique use cases. An alternator is more ideal when you don't need to worry about maintenance and when you don't want to be tied down by a specific RPM. In modern cars, this is crucial since it will prevent you from having to throttle up just so that your car battery can charge. 

On the other hand, a generator will still be a far better option when you need a constant supply of power and for a long duration. The power output from a generator is also ideal for charging a dead battery.

All in all, the key takeaway should be that generators and alternators simply convert mechanical energy to electrical energy. However, they do this in different ways which makes them ideal for specific situations. With regards to generators, the only time you'd really need to worry about an alternator is if your preferred generator comes with a digital alternator.

Ian Mutuli

About the author

Ian Mutuli

Founder and Managing Editor of Archute. He is also a graduate architect from The University of Nairobi, Kenya.
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