Inverter vs. Generator? You have questions and we have the answers. We tell you everything you want to know about inverter generators and regular generators. There are some key differences that you really need to be aware of before you decide which portable generator to buy.
Inverter vs Generator?
What’s the difference between a conventional generator and an inverter generator? For instance, say you place a conventional open frame generator, and an inverter generator from the same manufacturer side by side. Both must deliver the same 2800-watt running, 3000-watt startup power. Add to the picture, two identical air compressors and connect one to each generator. Fire up the generators and allow them some time to settle. Let both generators run at normal speed, the economy switch on the inverter generator must be off. Now switch on the power to the air compressors. Immediately both generators will feel the strain. You will hear both engines losing some speed. The beat changes, as the engine speed drops, you can hear it working harder. Within a second or two, the speed increases and settles at the correct RPM.
Apart from the vast difference in noise generated, there is no observable difference between the two setups. Yet we know there’s a cost difference between these generators and the much-touted clean power from an inverter. This begs the question, is an inverter generator worth it?
How much do prices differ?
To make a fair cost comparison between an inverter generator and a conventional one, I set out to find equally powered models from one manufacturer. Then I did the same for a second manufacturer to make sure that my cost comparison is fair.
I compared the complete range of generators from manufacturer A to see how their model prices compare. Then I selected two models, an inverter generator, and a cyclo-converter model. Both generators deliver the same wattage and I calculated the comparative dollar cost per kW. It turned out to be $776/kW for the Inverter generator and $509/kW for the cyclo-converter model. Then I did the same for manufacturer B and found that, for both, the dollar per kW cost is about 50% more.
Throughout the range, the $/kW prices changed considerably. Next, I examined all the inverter generators from the two manufacturers to find their most cost-effective models. I found that for manufacturer A their most cost-effective inverter delivers 2200 running watts. For manufacturer B, it is a 2800 running watts generator. For both, the 1000-watt inverter generator is the most expensive per kW at close to $1000/kW. This means that if you choose wisely, you can buy a lot more power for your money. Generally, expect the inverter generator to be about 50% to 90% more expensive than a conventional generator.
Price Difference : The Generators below reveal the real-time prices of inverter generators vs conventional generators. The columns show comparable generators for each type. For example, column 1 shows the Honda EU2200i inverter generator and directly below you will find a comparably sized conventional generator to give you an idea of price difference. Row2 shows a larger generator, etc — up to row 4. This is intended to give you real numbers so you can see for yourself how prices compare between inverter generators and regular generators.
Cost difference | Are inverter generators worth the extra cost?
In the above comparison, the inverter technology is about 50% more expensive. However, that comparison is incomplete. When we compare features, you’ll see that your extra money buys a lot more than just clean power.
- Some models have dual coil or multiple pole alternator stators that reduce noise and maintenance.
- Reduced engine wear by running at lower engine speed when using the economy switch.
- Reduced fuel consumption is possible with some models that have an economy switch.
- It delivers clean power that is safe for electronic devices.
- It is remarkably quieter.
- Some models offer a parallel mode to run two of the same generators in parallel, doubling the power output
- It has an appealing design.
Is the cost difference worth the advantages? It depends on your own set of requirements. What do you intend using the generator for? However, the cost difference still begs the question, is clean power that important? To answer this question, we need to look at the differences between the two technologies, and the reasons why we need clean power.
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Conventional, open frame generator technology
The conventional or synchronous generator, that’s been with us for a very long time, is versatile and reliable. They are fully portable and are used wherever we need power. Most modern synchronous generators use revolving field alternators. Meaning that the armature windings are stationary, and the electrically generated magnetic field rotates. Therefore, the slip rings and brushes deliver the DC excitation power to the revolving field windings. The power output is provided directly from the stationary armature windings.
The rotation speed controls the frequency of the AC voltage. Hence the term synchronous generator. Controlling the voltage and amperage is achieved by varying the current in the DC field windings. The engine delivers the power we use, and the generator converts it. The synchronous generator has precise control over the power it generates but not the frequency.
The frequency of the AC current depends on two factors: the number of poles in the alternator and the speed of rotation (rpm). A two-pole generator must run at a constant speed of 3600 rpm to provide 60 Hz. A four-pole generator must rotate at 1800 rpm to produce 60 Hz. The Engine can only deliver the necessary power at close to 4000 rpm. For this reason, a two-pole generator is used. The frequency and therefore, the generator’s engine speed must be precisely controlled, remaining at 3600 rpm. A microcontroller and stepper motor control the speed of the engine.
To control the current in the DC field windings, the generator’s voltage regulator measures the output voltage and compares it with a conventional reference voltage. The circuit adjusts the excitation current of the magnetic field windings up or down to maintain the output voltage at its rated value.
Drawbacks of Conventional Generators
The synchronous generator produces a lot of heat and cooling it is essential. Portable generators use air circulation to cool the engine and the alternator. A fan driven by the engine takes in cooler air from the atmosphere and blows it internally across the generator set. The heated air disperses back into the atmosphere. The open frame design of the generator helps to dissipate the heat. The design has a distinct disadvantage. You can hear the engine, fan, and generator noises and have no means of blanking them off. Because this noise can be annoying, you need to move the generator a distance away to make it bearable.
Another problem is that the signal derived from the alternator is not a pure sine wave, it is somewhat distorted but effectively simulates a sine wave. The following drawing shows what the wave from a typical open frame generator looks like.
The distortion of the signal is due to design flaws in the alternator but, for most applications, it does not really matter. Nevertheless, some sensitive electronics like WiFi units, cellphones connected to a charger, radios, computers, etc. can overheat and may malfunction.
Frequency shifts
Another problem is that the synchronous operation of an engine generator is not entirely controllable. It must deliver a constant frequency sine wave of 60 Hz with less than 1% deviation. However, the speed of the engine determines its frequency, which is difficult to control precisely. The engine must maintain a constant 3600 rpm to achieve this, but changing loads cause it to fluctuate. When the load suddenly changes, the engine cannot react fast enough, and speed changes lag behind. The variations in engine revolutions cause a frequency shift away from 60 Hz.
In this second image, the blue wave represents a pure sine wave with 0% THD. This is the ideal sine wave. When you compare the black line to the blue line, you can see the signal distortions drastically alter the shape of the wave. The frequency shift caused by engine speed changes is also visible as the two frequencies shift apart. A frequency shift will be noticeable in the speed changes of induction motors.
Total harmonic distortion
The complex sine wave (black line) is just a composite of multiple waveforms called harmonics. THD or total harmonic distortion is the sum of all harmonic waves of the voltage or current waveform compared to a pure sine wave. The harmonic distortions of the sine wave have been with us since the very first utility power was delivered. Nevertheless, they were negligible at first due to the lack of non-linear loads before the 1960s. Since then non-linear loads became more prevalent. These loads include electronic ballasts, computer power supplies, Industrial welders, industrial arc furnaces, and variable frequency drives.
Unwanted Harmonic distortion can increase the current in power systems, which result in higher temperatures. Higher frequency harmonics increases core loss in motors, resulting in excessive heating of the rotor core. Communication transmission can also be affected by these higher order harmonics and, if left unchecked increases temperature of electronic components. The harmonic interference can significantly shorten the life of electronic equipment and cause damage to power systems.
No national standard dictates THD limits, but IEEE Std. 519 recommends values for acceptable harmonic distortion. From this recommendation, voluntary restrictions on voltage harmonics are set at 5% THD and 3% for any single harmonic.
How does the inverter generator differ?
The inverter generator delivers a pure sine wave at the correct frequency and amplitude. This is independent of the engine speed. In the above drawing, the blue wave represents the output from an inverter.
An inverter generator uses an engine to drive a multipole brushless AC alternator that utilizes rare earth magnets to produce multiple high voltage AC signals. It has a high-frequency AC output, with fluctuations in both the frequency and voltage. The AC output from each pole is rectified to a DC current, which is combined into one DC output. This DC voltage (180-200 VDC) supplies power to the inverter module. The inverter controls power devices like the engine speed with a microcomputer.
The inverter has a designated circuit, generating its own 60 Hz sine wave. The sine wave is amplified and uses the high voltage DC current to produce the 120/240 Volt AC output. This output is independent of the signal values and distortions in the alternator. The microcontroller and a stepper motor vary the speed of the engine according to the power demand. Its purpose is to minimize fuel consumption and noise.
An economy, or “Speed control switch,” on the control panel can lock the engine speed to maximum power. It can have two or three modes of operation. On most inverter generators, it provides an economy setting. When selected, the inverter module controls the engine speed. This results in the quietest and most fuel-efficient mode of operation. It works best when powering resistive or constant loads like lighting, TVs, computers, electric heating, video games, etc.
When set to the constant speed mode, the engine speed does not vary, it remains at the most efficient speed. This is for inductive loads like air conditioners, drills, blenders, refrigeration, etc.
All manufacturers recommend switching this to a constant speed when experiencing high startup loads. The engine revs cannot adjust in time when the load suddenly increases, like a startup current. While the engine is picking up speed, the inverter is deprived of enough current. Therefore, it delivers less current to the load, extending the time of the higher load demand.
The advanced alternator used in inverter generators runs cooler than the synchronous generator of conventional generators. It isn’t totally enclosed and can be force cooled. Consequently, the generator can be covered with noise reducing panels. This not only makes the generator quieter, but has an appealing design too.
Noise levels
All generators produce mechanical noises from the engine and the alternator, as well as exhaust and air intake noise. Modern generators use improved designs and materials to reduce this as much as possible. As engine speed increases, noise levels increase. One way of reducing the noise level of a generator could be by running the unit at lower speeds. Unfortunately, the conventional generator must run at 3600 rpm to maintain the 60 Hz frequency.
Internal combustion engines perform best at higher engine speeds. Though diesel engines deliver maximum torque at slower speeds than gas engines. At low engine speeds, the engine is inefficient, improving as the speed increases. The best engine power is achieved close to 4000 rpm, so the requisite 3600 rpm is already a compromise. We could replace the alternator with a four-pole alternator, reducing the engine speed by half and still maintain the 60Hz frequency. However, at 1800 rpm the engine is underpowered and will not be able to respond to increased power loads.
Comparing inverter and conventional generator noise levels
To maintain some consistency, I only used noise level ratings from one manufacturer as tested by an independent laboratory. In comparison, I found that the conventional models were 8 dB louder than the inverter generator. To the human ear, it sounds twice as loud as the inverter generator. Notably, both generators used the same engine, and the tests were conducted over a range of loads. I also compared many conventional generators to inverter generators from different manufacturers. All inverter generators have lower noise levels when compared to open frame generators.
I am overly sensitive to noise, which is why I live in a quiet, rural area. To me, the term noise pollution is important. So you can imagine my reactions when I first experienced a camper using a generator. I think that the effort put into “super quiet” inverter generators is worth every cent. In my opinion, low noise levels give inverter generators the greatest advantage over conventional open frame models.
Fuel consumption
Many manufacturers claim that inverter generators are more fuel efficient than conventional generators. Logically, it can only save fuel if the power losses from an inverter generator are a remarkably lower than a conventional generator. A generator can only save fuel by being more efficient. To substantiate their claims, I compared the fuel efficiency of generators from different manufacturers. I also compared the fuel efficiency of the conventional and inverter generator of manufacturer A because I noticed that both use the same engine. It is, therefore, an ideal opportunity to determine if an inverter generator is more fuel-efficient than a conventional generator.
At its rated load, the inverter generator from manufacturer A has a fuel efficiency of 5.85 kilowatt-hours per gallon. The conventional model, with the same engine, has a fuel efficiency of 6.1 kilowatt-hours per gallon. I then looked at all the generators I reviewed and found many conventional generators that are more fuel efficient than inverter generators.
Based on what I found, used on non-linear loads, I cannot support the inverter generator fuel economy claims. Neither can I refute it because there may be some inverter generators that are more fuel-efficient. However, this won’t be as result of the inverter technology. I often find that for the same rated output, some engine and generator combinations are more efficient than others are.
There is, however, one notable exception. The economy switch on inverter generators can have an effect on its fuel efficiency if used with constant loads. If used for lighting, heating, and cooking and at less than 25% of the generator’s rated output, the inverter module can lower the engine speed. Then it will save fuel and will outperform a conventional generator.
Parallel mode | Inverter Generators
Running conventional generators in parallel is widely used in industry to provide redundancy in power backup installations. It requires specialized (expensive) switchgear that only knowledgeable electricians can attempt to install. Therefore, the parallel mode offered on some portable inverter generators is another unique advantage over conventional portable generators. This unique feature is possible because the inverter module supports sine wave synchronization.
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Paralleling two inverter generators typically uses parallel mode connectors on the generator with a connection cord. This cord connects the two generators in parallel. The parallel cord has its own interface with outlets that can deliver double the power available from the individual generators. Parallel cords connectors differ amongst generator manufacturers. So only two compatible inverter generators from the same brand can be connected together.
There is no risk to the generators when paralleling two units with only one generator running. The inverter modules can support this situation because you cannot start or stop both generators simultaneously. It also does not matter if one unit runs out of gas. Be aware that you can only connect or disconnect the parallel cord while the generators are not running.
How to decide which one is right for you?
Should you consider an inverter or conventional generator? I think it really boils down to a few advantages that you should consider and weigh up between the two technologies. Only you know your budget, what you plan to use the generator for, and which features are important to you.
When you use your generator, does clean power matter? Owners of a food truck, a small business that needs backup power, those who entertainment when tailgating, will appreciate clean power. On the other hand, if your main concern is to support the RV’s air conditioner, then raw power and a conventional generator could be desirable. That’s if the noise levels do not concern you.
If low noise levels are a major consideration, then the inverter generator is the only option open to you. The ability to use the generator in parallel mode could be a major advantage. Combined with the possible fuel savings, low noise, clean power output, and an appealing design. I think the cost difference is worth it.
