What is undersizing? Some solar installers like to buy a smaller inverter than the maximum power of the PV system, e.g. an inverter with 3KW for a 3.5KW PV system.
Why undersizing? Saving money was one reason. In the past, inverters were much more expensive, so the savings from undersizing were correspondingly much greater, at that time you could save € 200-300. Nowadays the inverter is already that there is no longer a big difference between 3KW and 4KW inverters, often a difference of only 10 € -50 €. Some solar installers also deliberately take a size smaller in order to keep more profit for themselves.
Above is an example from the flash list of a 365Wp solar module from Canadian Solar, which has a measured output of approximately 6-8W more than 365Wp on average. Due to tough competition, manufacturers are forced to give more power than stated, already 364.99W falls into the lower category of 360Wp modules. That is also one reason why the inverters should not be undersized. In a large system with 50-100 modules, the discrepancy can be even bigger than you think. So it's better to be one size larger than smaller.
With a 3 KW inverter, the output power is limited to 3KW, i.e. although the maximum input PV power is often 4KW, the output will always be 3KW. This means that you could have installed fewer modules if you had taken the smaller inverter, because the output of a 4KW PV system and 3kW system is the same if you use the 3KW inverter in both systems.
Undersizing the inverter is not only financially meaningless, it also poses a great technical risk. Solar modules are nowadays more and more powerful. The STC output below 1000W / m² and 25 ° C of a module is not achieved in most times, but there are always peak times. In winter, for example, the voltage of the module is significantly higher than in summer, although the solar radiation is weaker, even in winter on good days at low temperatures, 100% PV output and even more can be achieved. Snow also reflects a lot of light onto the module. In systems with 70% regulation, this is limited, you only lose valuable energy. But there are also systems where they are not regulated to 70% or the regulation does not work, in this case undersized systems are particularly at risk. Without overvoltage protection, the inverter would be damaged. If you insist on a warranty claim in the event of defects due to undersizing, you do not have a chance. In case of an undersizing, the manufacturer will not accept any guarantee.
So it is not worthwhile to undersize, rather to oversize it a little. If you have a 3.8KW system, you prefer to use a 4KW WR instead of a 3.5KW, you are on the safe side.