Calculation of poa from ghi,dhi,dni

Question

I am try to calculate POA using pvlib library using these parameters (GHI,DNI,DHI,AZIMUTH,ZENITH). I am getting good result during clear-sky but in cloudy day variation is huge. Please help me if anyone has idea related to this.

i=24
for j in range(i):
    Y = X[j]
    M = ['ghi','dni','dhi','zenith','azimuth','period_end','air_temp']
    values = list( map(Y.get, M) )
    df_poa = pvlib.irradiance.get_total_irradiance(
        surface_tilt=10,  # tilted 10 degrees from horizontal
        surface_azimuth=0,  # facing South                                           
        dni=values[1],
        ghi=values[0],
        dhi=values[2],
        solar_zenith=values[3],
        solar_azimuth=values[4],
        model='isotropic')
    OUT = dict(df_poa)
    Final_out = list(OUT.values())
    POA_Global = Final_out[0]