What is a pure sine wave inverter (All you need to know)?

If you are trying to understand what a pure sine wave inverter is and how it works, you are likely already looking into installing a solar system with batteries for your RV, or for your home.

If you have already done some research, you have likely already read about the two main inverter technologies: pure sine wave and modified sine wave.  

Pure sine wave inverters are up to 2 times more expensive than modified sine wave, therefore you might ask yourself are they any good? and what is a pure sine wave inverter?

A pure sine wave inverter will transform alternating current (AC) into direct current (DC) which can then be used to deliver high quality electrical current (similar to utility standards, voltage: 230V, frequency: 50/60hz) to all sorts of home appliances. In addition, pure sine wave inverters are also transformers. They rise the input DC voltage, for example 12V, to a much higher AC voltage, for example 230V.

In this article, you will learn all you need to know about pure sine wave inverters.

We will first explain in detail, what a pure sine wave inverter is and how it works. Then, we will quickly compare the two main inverter technologies. Finally, we will show why you should equip your electrical system with a pure sine wave inverter.

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What is a pure sine wave inverter?

Let’s start by understanding what a pure sine wave inverter is and why you would need one for your electrical system.

By now, you might already know that there are two types of current:

  • AC, for Alternating Current
  • DC, for Direct Current

What is the difference between AC vs DC?

a Brief history
When the world started its electric revolution (end of the 19th century, beginning of 20th), two famous scientists tried to establish their technology as a standard.

On the one hand, Nikola Tesla was pushing for the adoption of AC electricity and Thomas Edison strongly defended DC electricity. Obviously, Tesla won the battle as nowadays AC is the main electric current for all common household appliances and electric motors.

However, DC is still used and regained interest recently with the development of solar energy. Indeed, solar panels are producing DC power, and so are batteries.

Now that we explained a bit of the history to you, you may still be wondering what the main differences between AC and DC are?

First of all, electricity is a flow of electrons.

With DC, electrons flow in one way, from the negative electrode to the positive electrode. It could be represented by a straight and continuous current over time.

E3_tt_electricity

On the other hand, with AC, electrons flow alternatively in both ways in a periodic cycle. It could be pictured as a sine-wave current over time.

E3_tt_electricity

Now that you understand AC and DC are really different in their nature. Do not ever connect an AC appliance to a DC power source, it will be irreversibly damaged.

How do we convert DC into AC?

To overcome the compatibility problem between DC and AC, electrical engineers created a device that converts DC into AC, and alternatively AC into DC if needed: this is the inverter.  

A pure sine wave inverter, will therefore:

  • produce alternating current (AC) from a direct current (DC) source
  • deliver high quality electric current similar to utility standards (voltage: 230V, frequency: 50/60hz)

In addition, pure sine wave inverters are also transformers. They rise the input DC voltage, for example 12V, to a much higher AC voltage, for example 230V.

Now that you know what a pure sine wave inverter is, let’s see how it works.

How does a pure sine wave inverter work?

Previously, we saw that pure sine wave inverters first convert DC into AC, and then increase the input voltage (12V, 24V, 48V…) to reach an output voltage of 230V (transformer), usable by all our household appliances.

Let’s detail those two steps performed by a pure sine wave inverter.

From direct current (DC) to pure sine wave alternating current (AC)

In this section, we will provide a simple working principle that illustrates how pure sine wave inverters work:

The challenge is to turn a straight line (DC) into a wave (AC) in which the current flows periodically in both directions.

Let’s start with a simple switch. Switching the DC current periodically will give you a squared shape current alternating between zero (switch Off) and maximum value (switch On).

Graph of a current switch On and Off

Ok, now we have a periodical current but not flowing in both directions and still very far from a sine wave form.

To make the current flow in both directions, we would need a kind of automated switch that will alternatively flip the incoming direct current back and forth by reversing the contact. This could be illustrated as a rotating disk with criss-cross connections. The speed of the rotating disk will determine the frequency of the alternating current.  

Squared AC

Now, we have a periodical current flowing in both directions. However, it is still the shape of a square. It takes a bit more of electronic circuits to smoothen the square into a nicely defined sine wave.

what is a pure sine wave inverter
Pure sine wave AC

How do pure sine wave inverters increase voltage?

The second step is to increase the voltage of the pure sine wave current. Indeed, DC is usually 12V, 24V or 48V. Whereas AC is 230V.

The transformer will do the job. It is an electromagnetic device, made of an iron core wrapped with two coils of copper wire: the primary and secondary coil. The low voltage current enters through the primary coil and the high voltage current exits out of the secondary coil.

transformer
Schematic representation of a transformer from 12V to 230V

In the end, the two coils are not in contact and the current is transferred from one to another via electromagnetic induction.

The wiring density of the two coils controls the output voltage. In a step-up transformer (increasing the voltage) the secondary coil has a higher wiring density than the primary one.

Transformers are everywhere, for example your cell phone and laptop chargers are step down transformers.

What is the difference between a modified and pure sine wave inverters?

Looking into the inverters available on the market, you probably found out two models:

  • Pure sine wave (PSW)
  • Modified sine wave (MSW)

In the previous part of our article, we already outlined that a pure sine wave inverter will deliver the highest quality of electricity, similar to that provided by your utility company. As it is named, the current has a perfect sine wave shape with a stable frequency (50/60Hz) and output voltage (230V).

On the other hand, a modified sine wave inverter, uses rather cheap electronics to produce a “pixelized” wave. The wave is rather made of small steps than a continuous curve. It is like an approximation of a sine wave.

pure sine wave vs modified sine wave
Pure sine wave and modified sine wave graph

In the end we could summarized the main differences between a modified (MSW) and pure sine wave inverter (PSW) as follows:

Current QualityEfficiencyPriceFor all appliances
Pure Sine Wave++++++
Modified Sine Wave+++

What appliances need a pure sine wave inverter?

First of all, don’t throw away your modified sine wave inverter too quickly. Although, the output current is not of the highest quality, it can still be ok for some applications with no sensitive electronics. For example, electric heaters or water pumps. However, their efficiency will be reduced and they might overheat.

That said, almost all other appliances and electronic equipment will require a pure sine wave inverter. In addition, if your inverter is connected to the grid (like in many solar installations) you absolutely need a pure sine wave.

A pure sine wave inverter will run your appliances smoothly like if they were connected to the utility grid.

Below, we have listed some common appliances and electronic devices that would only run with a pure sine wave inverter:

  • LED TV
  • Microwaves
  • Refrigerators
  • Medical equipment
  • Variable speed electric motors

For all the reasons mentioned above, we would recommend you purchase a modified sine wave inverter only if you are on a budget and are not concerned about the efficiency of your appliances.

Why are pure sine wave inverters so expensive?

Pure sine wave inverters provide clean and reliable electric current, the same quality the utility grid delivers. To achieve this high quality, they use high-end electronic components, unlike modified sine wave inverters that contain only cheap electronics.

Therefore, for a similar wattage output, a pure sine wave could be 2 times more expensive than a modified sine wave.

However, do keep in mind that pure sine wave inverters will protect your equipment and therefore increase their life duration. In addition, you will save energy with a pure sine wave inverter because of its high efficiency (+90%).

What size pure sine wave inverter should I buy?

Pure sine wave inverters come in many sizes and power outputs to satisfy all needs.

Let’s first consider a car pure sine wave inverter:

Car Inverter – Max 1500 Watts

Many pure sine wave inverter models offer a cigarette socket to connect directly into your car. The source of energy will come from the car battery or from the car alternator.

In this configuration, your maximum inverter wattage should be 1500 Watts. There is no need for you to get a more powerful inverter, simply because your car alternator cannot produce enough power.

A typical car alternator produces between 40 Amps and 120 Amps at 12V. Therefore, a maximum power of 480 W to 1440 W.

Don’t worry, 1500 W of power is already huge and you could run many appliances and electronic devices at the samw time with this size inverter.

Solar inverter 1000 – 10kW

Pure sine wave inverters are widely used in solar off-grid and grid-connected systems. Usually, the inverter is connected to a solar charge controller (regulating the power of the solar panels) and to an energy storage system (Powerwall) or a lithium battery bank.  Some solar inverter models also have built-in solar charger.

There is virtually no power limit for a solar inverter. However most solar inverters range from 1000W to 10kW.

However, the rule of thumb, would be to get an inverter as powerful as your solar panel array. For example, if you have 3kW of solar panels then you could get a 3kW inverter.

As solar systems can be easily upgraded, we would recommend you get an inverter slightly more powerful than your solar panels.

Final thoughts

In this article, we showed that there are two types of inverters: modified sine wave and pure sine wave.

Pure sine wave inverters are made of high quality electronics and are highly efficient in providing electric current with the same characteristics as your utility company. They will not damage your appliances or overheat them.

On the other hand, modified sine wave inverters, are cheaper, but less efficient and cannot run all types of devices.

In the end, we highly recommend you get a pure sine wave inverter, there is no doubt that it will well preserve your electronics and save you a lot of money over time.

Dr Metaye has a PhD in chemistry from Ecole Polytechnique, France. He is a renewable energy expert with more than 10 years of experience within the research world. During his career, he supervised more than 150 projects on clean energy. Off-grid smart systems, solar energy, battery and the hydrogen economy are among his specialties.

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