How to fit the power of the inverter to the power of photovoltaic panels?

A photovoltaic inverter, often also called an inverter, is the second most important component of a PV installation (right after the modules themselves). Therefore, one of the key challenges faced by the designer of a photovoltaic system is to choose a device with appropriate parameters. Even when investing in excellent quality PV modules, improperly selected inverter power can contribute to a significant performance degradation, and also lead to situations such as underload or overload of the inverter. So how do you fit an element to your system that will guarantee reliability and optimal operation for many years? You will find all the answers in the article.

What is a PV inverter and how does it work?

Before we move on to the actual part of the discussed topic, let’s explain how a PV inverter works and why it is such an important element of a PV installation. The most important function of the PV inverter is the conversion of the power generated in photovoltaic panels in the form of direct current into alternating current, i.e. electricity in accordance with the specificity of the low voltage network (frequency 50 Hz, voltage 230/400 V). However, this is not his only role. The other most important tasks for which a single-phase inverter or a three-phase inverter for photovoltaic panels in the installation are responsible, also synchronization with the power grid, logging information on operation, monitoring the maximum point that the power of photovoltaic panels reaches, as well as overall management of the entire PV installation. . Additionally, the device serves as a fully automatic disconnector. Due to all of the aforementioned considerations, many people call the inverter the heart of a PV system.

Inverter power and the power of photovoltaic panels

Now that we know how a photovoltaic inverter works, let’s move on to outline what the power of the inverter should be in relation to the power of the photovoltaic panels. A fairly common problem regarding the relationship between the power of inverters and the power of modules are strongly different opinions of people interested in the subject of photovoltaics. To help you analyze the relationship, let’s start by defining a certain concept, i.e. the ratio of PV panels power (SM) to inverter power. This value is determined by dividing the power of the modules by the nominal power of the inverter, and then multiplying the result by the efficiency of the inverter expressed as a percentage. As a result of the operation, we can get three different kinds of values:

• inverter underload – the nominal power of PV panels is less than the power of the inverter,
• load the inverter with nominal power – the nominal power of PV modules is equal to the power of the inverter,
• inverter overload on the DC side – the nominal power of PV panels is higher than the power of the inverter.

As for the latitude of Central Europe, including Poland, the generally accepted indication for systems facing south is to obtain the value of the inverter overload power ratio on the DC side oscillating in the range of 80-125%. On the other hand, in the case of East-West installations, the upper limit of the inverter overload range can be as high as 160%. Of course, these are only approximate values, not taking into account specific, important factors related to the specifics of a specific work area, but we use them for the purpose of general explanation of the issue.

A question arises here: why do most designers of PV installations recommend targeting SM at a level greater than 100%, i.e. striving for a situation where the power of the photovoltaic panels exceeds the nominal power of the inverter? On the surface, it would seem that such guidelines contradict the principle according to which the system converting the energy generated by the generator should have a higher nominal power than they are. In practice, however, it should be borne in mind that PV systems are custom designed.

At the latitude of Poland, the number of hours of sunshine is about 1600. Only 15% of them are hours of full sunshine. At the same time, PV panels, as a result of being exposed to radiation with an intensity of 1000 W / m2, heat up strongly, reaching a temperature exceeding 25 ° C, which significantly reduces their efficiency (even a possible 20% decrease in power). Therefore, the nominal power of photovoltaic panels is generated for a maximum of several dozen hours throughout the year, remaining much lower in the remaining period. Moderate overload of the inverter results in its more effective operation during days with low insolation values. In addition, one more factor to make the SM greater than 100% should be taken into account, namely the fact that incorrect inverter dimensioning causes unnecessary increase in system costs – remember that the price of the inverter can reach up to 30% of the entire investment.

Finally, it is worth mentioning that in the case of relatively small PV installations, generating a power of several watts, single-phase inverters are used as standard. A three-phase inverter for photovoltaic panels, on the other hand, is worth considering when we are dealing with a greater power of the system. Contrary to its single-phase counterpart, it has not three, but five wires (N and PE, and L1, L2, L3 instead of L). The three-phase inverter for solar panels significantly increases the stability within the local grid.