What is the productivity of the panels? What does a 6 kW system, for example, mean in practice?

A “6 kW system” refers to the nominal power, i.e., output of the panels in standard conditions (corresponds to a somewhat cool and sunny spring day in Southern Finland with a solar power of 1,000 W/m2). Over one hour, this kind of system generates 6 kWh (six kilowatt hours). In ideal circumstances, the system outputs over 6 kWh. A typical energy consumption of a detached house is about 15,000 kWh per year. A 6 kW system, optimally installed in the south-western coast of Finland, can generate max. 5,500 kWh per year.