A inverter ensures that the DC what comes out of your solar panels, is converted into an alternating current that could (possibly) be sold to your electricity supplier. A single solar cell produces a voltage of approximately 0.5V DC. The current from the same cell depends to a large extent on the UV index at that time. For solar panels are often chosen for a circuit where the total panel supplies either 12V or 24V DC. For autonomous systems this often is good, since most batteries and accessories work with 12V. For grid connected systems an inverter (also: inverter) is needed, ensuring that you produce power is a “recast” into suitable AC power. In this article we briefly discuss what types of inverters there are available and how an inverter actually works.

Buy an Inverter?
For non-synchronous inverters (see section “which inverter to choose?”), you can contact the Conrad Electronics shop in Europe.

AC and DC?
There are two kinds of power, AC and DC. DC is a stream with a constant flow direction. DC is supplied by batteries and solar panels. Major disadvantage of DC is that it can not be transformed to high voltage. So it can not be transported over long distances without significant losses. AC, however, can be transformed, however with some (small) losses. AC, as shown in the figure, with a periodically varying flow downstream.

To flow over long distances, it is worthwhile for an energy company to transform the voltage to up to 300,000 volts at a frequency of 50 Hz using a transformer. The higher the voltage, the lower current, which results in a lower loss. In several steps (transformer houses), this voltage is then transformed back to electricity with a voltage of 230V. The flow we then have entering the house, is a pure AC (AC) of 230V/50Hz.

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Figure: AC (black line) and DC (light green line)

Types of inverters
There are two main types of inverters: the pure sine wave inverter and the modified sine inverter. The difference between the two lies in the current waveform produced. The flow of electricity that we buy has a pure sine wave (see red line in figure below). This is the waveform produced by the expensive pure sine wave inverter. The modified sine wave inverter produces a square wave (blue line) with the same frequency as the AC. Because a square wave has, by the Fourier theory, actually an infinite number of sine forms, in practice often gives no problems. Problems that arise are usually caused by the harmonic components of the square wave (read: all non-ground components of the Fourier series).

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Under Dimensioning
Contrary to popular belief, it is sometimes possible to use a inverter with an input power lower than the peak power of solar panels. This is because the peak power of solar panels is almost never achieved. Ideally, you can assume that the maximum power reached at about 80% of peak power. This is because solar cells are in practice much warmer than the laboratory conditions, where the cells held at a comfortable 25° C. See the performance of solar panels for information about the inverse relation between yield and temperature. In other words, an inverter with a maximum input power of 1000 watts is a good choice when solarpanel set of 1250Wp is connected to it. If the input power is still exceeded, then the voltage inverter switch back so that the solar power temporarily decrease. An inverter for this purpose is called ins acetate under honorable dimension.

Which inverter to choose?
If your solar system is not grid connected, your choice depends on the devices you want to provide power to. If it only contains one electric device, a pure sine wave is practically mandatory. Additionally, for many audio-video equipment the rule is that an impure wave can cause noise. For example, for a laptop it makes no difference and the cheaper modified sine wave is a good solution. It must be said, that a square-wave inverter signal, often is buzzing because of the adjustable conversion. A pure sine wave inverter is always better, but the value is often low.

In case of grid connected PV systems, you are required to use a special so called synchronous inverter. This ensures that you provide current in phase (ie the wave is similar) with the AC grid. If this is not the case, your inverter will immediately fail. A non-synchronous inverter is not working! Synchronous inverters provide a clean pure sine wave that exactly matches the power of the grid. In combination with a current flow back to you this way you are providing power to sell and carry back the net. What synchronous inverter to choose, is entirely dependent on the power (in watts) of your PV system.

Serial or in parallel?
An inverter works at a fixed voltage, which is usually a multiple of 12V. It is important that your solar panels are connected so that the voltage across the circuit is equal to the  inverter input voltage. To achieve this, there are often a combination of serial and parallel connections needed. See video below for more info: