The DC system size is the DC (direct current) power rating of the photovoltaic array in kilowatts (kW) at standard test conditions (STC). PVWatts® can model any size of array, from small residential rooftop systems to large ground-mounted power generation systems.
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. This 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.
By default, PVWatts® uses a DC-to-AC size ratio of 1.1 so that the array's DC nameplate size at STC is 1.1 times the inverter\s AC (alternating current) size. For example, the default 4 kW system has an array size of 4 DC kW and an inverter size of 3.63 AC kW. The default DC-to-AC ratio value is appropriate for most analyses, but you can can change it under Advanced Parameters.
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
If you are unsure of the number and size of modules or array area, you can System Size Estimator to get a rough estimate based on the area available for the photovoltaic array.
Important Note: PVWatts® makes assumptions about the module performance based on the Module Type you choose, and assumes 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 other types of modules, or with a nameplate size for other test conditions.