If, say, a kettle is rated at 2kW that means it consumes 2kWh of electricity each hour. But if you install a solar PV station with 2kW installed (or peak) power it will not produce constant amount of electricity each hour because its generation depends on many factors like presence of clouds, position of the sun, shading, etc… All those factors influence the actual power of the PV station and at each given moment the power of the system will differ. You can see the actual power on the LCD screen of the inverter. In calculations we use annual production value which for a 2kW system is about 3000-3400kWh in Armenia because in different parts of the country annual production of 1kW installed power varies between 1500-1700kWh.