Q&A: How to calculate the capacity of an inverter for a solar system?
As for the winter months, the average accumulated production of the panels corresponds to ca. 10–15% of its summer production. However, during the bright and freezing winter days, peak output can easily exceed 120 % of the nominal power.
Thus you need to size the inverter according to the nominal power of the panels + minimally 30 % of the safety reserve.
In the picture, you can see the production of 3,5 kWp solar array and consumption of a family house in Middle Europe (Czech Republic) over one year. Despite the production during winter months is less than 10 % of summer times, for peaks in winter we need at least 4,5kW inverter.