Inverter for your PV system: what you need to know

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When hit by light from the sun, photovoltaic modules on roofs convert free radiant energy into direct current. However, since domestic systems and public grids run on alternating current, an additional component is needed to actually make this self-generated energy usable. The component in question is an inverter –– a voltage converter that changes direct current into alternating current. But how do these devices work? When are they used and what should be considered during selection and installation? We provide the answers to these questions below.

What is an inverter? Function and tasks explained

When photovoltaic systems convert solar radiation into electrical energy, direct current is produced. The free charge carriers always move in one direction for this –– from the positive pole to the negative pole. The only possible differences are in the voltage level and current strength. Alternating current, meanwhile, is a current that periodically changes direction. This is used in heating systems and public electricity grids, as it is easier to generate in large power plants. Current or voltage converters are needed in order to operate alternating current consumers with a direct current source. These are known as inverters and are a component of most photovoltaic systems.

Power electronics produce alternating current from self-generated energy  

Photovoltaic inverters "chop up" incoming DC voltage in order to then periodically reverse the polarity. While this used to be performed mechanically, transistors and special electrical circuits carry out this function today. They use what is known as pulse width modulation and generate a sinusoidal voltage that is important for many electrical appliances. Unlike the simpler square-wave voltage, where the polarisation is abruptly reversed again and again, the current strength and direction are alternated much more smoothly.

Inverters for PV systems have a variety of other functions

Besides making the electricity generated by a photovoltaic system usable, inverters also ensure efficiency and safety. Here is an overview of their most important functions:

Convert direct current to alternating current

Primarily, inverters convert (or "invert", hence the name) direct current into alternating current or vice versa (for example in battery inverters). This is necessary so that the electricity can be used or fed into a storage unit.

Optimise photovoltaic system efficiency

Photovoltaic inverters monitor the power of the connected modules and ensure that this is always optimal. Experts refer to this as maximum power point (MPP) tracking, which is necessary due to different irradiation and temperature values.

Ensure system safety

These compact components continuously monitor the mains frequency and adjust the self-generated power to match it. They cool themselves to prevent overheating and disconnect the PV modules from the grid if it is down or a dangerous situation occurs. Inverters can also reduce the photovoltaic power.

Increase self-consumption and savings

Integrated monitoring makes it possible to consume self-generated power according to demand and thus increase self-consumption. This way, more electricity stays in the home. You save money by having to buy less.

Provide emergency power in case of power failure

If the public grid fails, you are at an advantage when you have a photovoltaic system installed – with an emergency power-capable inverter, you can supply yourself independently of the grid. If you also have a power storage unit to supplement your PV system, your home can still be supplied with electricity at night when the sun has long since set.

If you want to buy a voltage converter, you have a great deal of choice

Inverters for photovoltaic systems are available in various designs. A distinction can be made between string, module and central inverters. In addition, there are battery, hybrid and standalone inverters. Here is a summary of the differences:

String inverters are usually used when installing all modules on one roof surface. They are located at a central point in the home (such as the mechanical room) and are equipped with an MPP tracker. This ensures that the connected modules work efficiently as long as they are all exposed to similar conditions.  

This type of inverter is not suitable if your photovoltaic modules are installed on differently oriented roof surfaces. In this case, either several string inverters or what are known as multi-string inverters are used for the PV system. The latter are equipped with multiple MPP trackers and ensure high efficiency even with different irradiation values.

Good to know: A string describes the totality of several individual photovoltaic modules connected in series.

Inverters that ensure the function of individual modules are referred to as module inverters. These are connected to individual panels. They react quickly to different irradiation values and thus ensure high efficiency. However, module inverters only make sense for small systems, given that the use of many such devices greatly increases the acquisition and maintenance costs. The same applies to the risk of failure and malfunction –– this too rises when a large number of units are working in parallel.

If all photovoltaic modules are subject to similar conditions, central inverters are used to feed power into the home or grid. These handle multiple strings and reduce installation and maintenance costs. The devices are typically used for large scale systems on roofs or for freestanding PV systems with a power generation capacity of around 30  kilowatts peak or more. If the output is significantly higher, some of the electricity generated can also be fed directly into the medium voltage network and distributed efficiently.  

There are also special types of inverters for particular applications. Take, for instance, battery inverters. These convert alternating current into direct current so that electrical energy can be fed into a power storage unit if it cannot (or can no longer) be injected into the DC network. Hybrid inverters, such as those used in the Viessmann Vitocharge  VX3 power storage unit, combine photovoltaic and battery inverters. In this way, they ensure savings and in some cases even allow wind turbines, CHP units or emergency generators to be connected.  

Another special form is the standalone inverter, which supplies power to standalone systems without a grid connection. These devices usually work more flexibly. They do not have to match the grid frequency and are generally combined with storage units.

Easily find the right inverter for your solar PV system

Are you looking for a photovoltaic inverter that will allow you to feed power into your home? Then it is important to choose the right device. After all, this will not only influence how your system works, but also its efficiency and thus cost effectiveness. We generally recommend that you seek advice from a trade partner.  The following overview shows you what should be considered when making your selection.  

  1. What type of inverter do I need? It depends on the installation. String inverters are often used for single-family houses. If your system is installed on several differently oriented roof surfaces or there is a big difference between the amount of sun and shade exposure across individual modules, a multi-string inverter may also be worthwhile. In addition, a battery inverter is often required –– for example, if you install a power storage unit in the AC network.
  2. How big does the inverter need to be for my solar PV system? The size or capacity depends on the connected modules. It is usually expressed in kilo-volt-amperes (kVA) or kilowatts (kW) and should roughly correspond to the module output of the connected strings. Individual simulations can help to determine this.
  3. How do I choose the right inverter for my system? Once you know the type and capacity of inverter you need, you are usually still faced with a wide choice. You can select the best devices on the basis of their efficiency, among other things. The higher this is, the less power is lost. The yield increases and the cost effectiveness of the entire system is improved.

Wide range of options to suit your needs

The  Vitovolt  300 photovoltaic packages  from Viessmann consist not only of PV modules including mounting system, but also an inverter and the necessary connecting cable. As all components are perfectly matched to each other, you get great peace of mind and a high level of efficiency.  If you opt for an individual solution, you can combine  Vitovolt  300 photovoltaic modules  with matching inverters.

Things to remember during installation and operation

You have identified the right inverter. Now where should it be installed? Heat is a key consideration here, as this is generated during system operation. Good heat transfer is needed to prevent performance degradation and faults. Therefore, it is important to install a photovoltaic inverter on a wall where it will not be obstructed by other devices or by furnishings. The inverter should be located as close as possible to the export meter. The manufacturer's installation specifications must be observed, and usually set out minimum clearances to ceilings, walls and other objects.  

Correct cabling with large DC cables  

The inverter is connected to the modules of the PV system using DC cables. Since these usually have to bridge very long distances, you should use sufficiently large copper cables. As a rule, these are included in system solutions such as the PV packages from Viessmann. For individually designed connections, our partners select the appropriate cabling.  

Regular maintenance ensures a long service life and efficient operation

Inverters are among the most sensitive components of a PV system and usually last ten years. Regular maintenance of your PV system will ensure that everything functions reliably and efficiently all the time. It is important to check the cable connections at regular intervals. In addition, you should always keep the inverter software up to date in order to maximise efficiency.

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