The size of the system is the DC power rating of the photovoltaic array in kilowatts. PVWatts® assumes that the array consists of crystalline silicon modules rated at standard test conditions (STC).
The default PV system size is 4 kW. For a system with 16% efficient PV modules, this corresponds to an array area of approximately 25 m2(269 ft2): 4 kW ÷ 1 kW/ m2 ÷ 16% = 25 m2.
The array area is the total module area, not the total area required by the system that might include space between modules and space for inverters and other parts of the system.
You can either estimate the system size based on the area available for the array, or calculate it from the module nameplate size at STC and number of modules in the array:
Size (kW) = Array Area (m2) × 1 kW/m2 × Module Efficiency (%)
or
Size (kW) = Module Nameplate Size (W) × Number of Modules × 1,000 W/kW
Important Note: PVWatts® assumes that the modules in the array are made of mono- or poly-crystalline silicon photovoltaic cells, and that the module nameplate size is for standard test conditions (STC): Solar irradiance of 1,000 W/m2 , cell temperature of 25°C (77°F), and air mass of 1.5. You should not use PVWatts® to model a system with thin-film modules, or with a nameplate size for other test conditions.