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DC Voltage Range for Nominal Power

· 5 min read
Marc Hauser
Marc Hauser
Founding and data Engineer

Many new solar plants commissioned in recent years fail to meet performance expectations. The reason? Strings that are too short. How is it possible that many well-established and experienced installers fall into this trap? In this article, I will first delve into the technical issue and then offer my honest opinion on why we are suddenly observing many new sizing issues in 2025.

Solar Inverters Trend​

New solar inverter models enter the market every year. A notable trend is the move toward larger string inverters with multiple Maximum Power Point Trackers (MPPTs), some featuring a very wide MPPT Operating Voltage Range. This trend is highly positive. A large number of MPPTs enable smooth performance optimization and offer many advantages in situations with local shading or different solar module orientations, such as those found on many rooftop plants. Having one or two strings connected to a single MPPT also allows for accurate string-level monitoring. Additionally, a wide MPPT Operating Voltage Range makes plant sizing very flexible. But what exactly does MPPT Operating Voltage Range mean?

MPPT Operating Voltage Range​

As explained in the blog post about boosterless inverters, inverters with a wide MPPT Operating Voltage Range rely on an internal booster that increases the input voltage to a level required by the inverter stage. By definition, the MPPT Operating Voltage Range defines the absolute minimum and maximum voltage that allows the booster stage and inverter stage to function together. Any voltage below the minimum or above the maximum may prevent the inverter from working. Voltages above the Max. Input Voltage may even damage the device. But is a voltage that allows the inverter to work sufficient for correct sizing? Not really. We don't just want a working inverter; we want the inverter to operate at maximum power. This makes a significant difference for a plant owner expecting to get the maximum power from their solar plant.

For many years, adhering to the MPPT Operating Voltage Range stated in the datasheet was perhaps enough to ensure the inverter extracts maximum power from the DC generator field. However, this is no longer the case for new inverter types that are becoming very popular. Let's consider a real-world example:

  • Datasheet Information: MPPT Operating Voltage Range: 200–1000 V
  • User Manual Information: Full-load MPPT Voltage Range: 540–800 V

What happens if the voltage is between the minimum MPPT operating voltage and the minimum full-load MPPT voltage?

The answer can be found in the inverter user manual or in separate documents, sometimes called Output Characteristics Curve. In our numerical example, operation at maximum power cannot be ensured for voltages between 200 and 540 V.

The output characteristics curve document of the related inverter contains the following chart:

Output characteristic curve

This chart clearly shows that the MPPT might run in power limitation for input voltages below 540 V. And this is what happens in reality! The chart below shows the DC current of two MPPTs. One has a high enough voltage to operate at full power, while the other is clearly limited to around 60% of its nominal current.

MPPT comparison

Why is the Power Limited at Lower Voltage?​

A booster is essentially composed of a power switch, an inductor, a diode, a capacitor and some control electronics. A key component is the inductor, which is also probably the heaviest and most expensive component in a booster circuit. The size of an inductor is proportional to the output current and to the voltage difference between the input and output voltage. This means: the lower the input voltage, the bigger the inductor needs to be. Therefore, it's understandable that some inverter types with a wide MPPT Operating Voltage Range might not be sized to guarantee Full MPPT Load for the same voltage range.

Personal Opinion and Recommendations​

The fact that many recently built solar plants suffer from the same issue indicates that information sharing between inverter manufacturers, sellers, and installers is lacking. This is why I wrote this blog post, and I hope it will help raise awareness of this important topic. In my personal opinion, problems occur at different stages:

Inverter Manufacturers and Sellers​

Some inverter manufacturers are presumably intentionally omitting the difference between MPPT Operating Voltage Range and Full-load MPPT Voltage Range in published datasheets. This makes a quick evaluation of solar inverter models for a specific project harder for installers, who must delve into user manuals and additional documents to ensure the chosen inverter is suitable for a specific use case. Additionally, we might question whether the training of installers in the sizing and usage of specific inverters by manufacturers and sellers is adequate.

Installers​

This sizing issue is one of many typical sizing and layout issues observed in existing plants regularly. Even though inverters are now sold as plug-and-play and easy-to-install devices, they remain complex devices requiring a certain level of technical understanding to ensure the choice of a matching device for a specific solar plant. It is regrettable to see many solar plants built in a way that the installed components cannot provide their rated power simply because the publicly available equipment documentation was not read carefully or understood during the planning phase. We recommend installers always read all relevant parts of the documentation, including sometimes decisive footnotes.

Plant Owners​

We generally recommend plant owners to inspect and approve new plants very carefully. Ensure that the documentation received corresponds to the plant built. Define a detailed final acceptance test, including the evaluation of the first available monitoring data, to ensure that each part of the plant performs according to expectations. Novasense would be happy to support you in defining this process.