Run pvlib modelchain with pvwatts model

Question

I want to run the pvlib modelchain with a pvwatts model:

pvwatts_module={'pdc0':250,'gamma_pdc':-0.005}
pvwatts_inverter={'pdc0':4600,'eta_inv_nom':0.93}

location=Location(latitude,longitude, tz, altitude)
system = PVSystem(surface_tilt = surface_tilt, surface_azimuth = surface_azimuth, albedo = albedo,\
                           surface_type = None, module = None, module_parameters = pvwatts_module,\
                           modules_per_string = 21, strings_per_inverter = 1,\
                           inverter = None, inverter_parameters = pvwatts_inverter,\
                           racking_model = 'roof_mount_cell_glassback', name = None)

mc = ModelChain(system, fx_model.location, dc_model='pvwatts',ac_model='pvwatts',aoi_model='physical',spectral_model='no_loss',temp_model='sapm')

mc.run_model(fx_data.index, weather=fx_data);

modules_per_string = 21 has no effect on the resulting ac-power. Does the modelchain not support this parameter with pvwatts or is there a mistake in my code? My workaround is now

pvwatts_module={'pdc0':5250,'gamma_pdc':-0.005}

Thank you

Answer 1

ModelChain does not support those parameters for the pvwatts model.

For the other models, pvlib python uses modules_per_string and strings_per_inverter to scale voltage and current. But PVWatts doesn't have a concept of voltage and current, so the same scaling cannot be applied. Feel free to open an issue and pull request on GitHub if you'd like to see a power scaling applied that is equal to the product of the parameters.