How to use the pvlib.pvsystem function in pvlib

fixed_sys = {'surface_tilt': 20,
                     'surface_azimuth': 180,
                     'albedo': 0.2}

        tracker_sys1 = {'axis_tilt': 0, 'axis_azimuth': 0,
                       'max_angle': 90, 'backtrack': True,
                       'gcr': 0.2, 'albedo': 0.2}

        tracker_sys2 = {'max_angle': 52, 'gcr': 0.3}

        fx_sys = pvc.pvlib_system(fixed_sys)
        trck_sys1 = pvc.pvlib_system(tracker_sys1)
        trck_sys2 = pvc.pvlib_system(tracker_sys1)

        self.assertIsInstance(fx_sys,
                              pvlib.pvsystem.PVSystem,
                              'Did not return instance of\
                               pvlib PVSystem')

        self.assertIsInstance(trck_sys1,
                              pvlib.tracking.SingleAxisTracker,
                              'Did not return instance of\
                               pvlib SingleAxisTracker')

        self.assertIsInstance(trck_sys2,
                              pvlib.tracking.SingleAxisTracker,
                              'Did not return instance of\
                               pvlib SingleAxisTracker')

def cell_temperature(poa, windspeed, ambient_temperature,
                     temperature_model):
    kwargs = dict(
        poa_global=poa,
        wind_speed=windspeed,
        temp_air=ambient_temperature
    )
    if temperature_model is not None:
        kwargs['model'] = temperature_model
    tcell = pvlib.pvsystem.sapm_celltemp(**kwargs)
    return tcell['temp_cell']

# preparing the weather data to suit pvlib's needs
# different name for the wind speed
my_weather.data.rename(columns={'v_wind': 'wind_speed'}, inplace=True)
# temperature in degree Celsius instead of Kelvin
my_weather.data['temp_air'] = my_weather.data.temp_air - 273.15
# calculate ghi
my_weather.data['ghi'] = my_weather.data.dirhi + my_weather.data.dhi
w = my_weather.data

# time index from weather data set
times = my_weather.data.index

# get module and inverter parameter from sandia database
sandia_modules = pvlib.pvsystem.retrieve_sam('SandiaMod')
sapm_inverters = pvlib.pvsystem.retrieve_sam('sandiainverter')

# own module parameters
invertername = 'ABB__MICRO_0_25_I_OUTD_US_208_208V__CEC_2014_'
yingli210 = {
    'module_parameters': sandia_modules['Yingli_YL210__2008__E__'],
    'inverter_parameters': sapm_inverters[invertername],
    'surface_azimuth': 180,
    'surface_tilt': 60,
    'albedo': 0.2,
    }

# own location parameter
wittenberg = {
    'altitude': 34,
    'name': 'Wittenberg',
    'latitude': my_weather.latitude,

ac_loss_factor : float
    poa_effective : pd.Series
    temp_air : pd.Series, default 20
    wind_speed : pd.Series, default 1

    Returns
    -------
    ac_power : pd.Series
    """
    pvtemps = pvlib.temperature.pvsyst_cell(poa_effective, temp_air,
                                            wind_speed=wind_speed)
    dc = pvlib.pvsystem.pvwatts_dc(poa_effective, pvtemps, dc_capacity,
                                   temperature_coefficient / 100)
    dc *= (1 - dc_loss_factor / 100)
    # set eta values to turn off clipping in pvwatts_ac
    ac = pvlib.pvsystem.pvwatts_ac(dc, dc_capacity, eta_inv_nom=1,
                                   eta_inv_ref=1)
    ac = ac.clip(upper=ac_capacity)
    ac *= (1 - ac_loss_factor / 100)
    return ac

model=transposition_model,
        dni_extra=dni_extra)

    if weather is None:
        weather = {'wind_speed': 0, 'temp_air': 20}

    cell_temperature = temperature.sapm_cell(
        total_irrad['poa_global'], weather['temp_air'], weather['wind_speed'],
        temperature_model_parameters['a'], temperature_model_parameters['b'],
        temperature_model_parameters['deltaT'])

    effective_irradiance = pvsystem.sapm_effective_irradiance(
        total_irrad['poa_direct'], total_irrad['poa_diffuse'], airmass, aoi,
        module_parameters)

    dc = pvsystem.sapm(effective_irradiance, cell_temperature,
                       module_parameters)

    ac = inverter.sandia(dc['v_mp'], dc['p_mp'], inverter_parameters)

    return dc, ac