Create function to calculate average photon energy

pvlib/spectrum/__init__.py

@@ -10,5 +10,6 @@
     spectral_factor_pvspec,
     spectral_factor_jrc,
     sr_to_qe,
-    qe_to_sr
+    qe_to_sr,
+    average_photon_energy
 )

pvlib/spectrum/mismatch.py

@@ -10,6 +10,7 @@
 import scipy.constants
 from scipy.integrate import trapezoid
 from scipy.interpolate import interp1d
+from scipy import constants
 
 from pathlib import Path
 from warnings import warn
@@ -1100,3 +1101,20 @@ def qe_to_sr(qe, wavelength=None, normalize=False):
         spectral_responsivity = normalize_max2one(spectral_responsivity)
 
     return spectral_responsivity
+
+
+def average_photon_energy(spectral_irr):
+    si = spectral_irr
+    hclambda = pd.Series((constants.h*constants.c)/(si.T.index*1e-9))
+    hclambda.index = si.T.index  # set wavelength as the index
+    pfd = si.div(hclambda)  # calculate the photon flux density
+
+    def integrate(e):  # define a helper function
+        return trapezoid(e, x=e.T.index, axis=-1)
+
+    int_si = integrate(si)  # integrate spectral irradiance wrt wavelength
+    int_pfd = integrate(pfd)  # integrate photon flux density wrt wavelength
+
+    ape = (1/constants.elementary_charge)*int_si/int_pfd
+
+    return ape

pvlib/tests/test_spectrum.py

@@ -576,3 +576,18 @@
     assert_allclose(sr, sr_and_eqe_fixture["spectral_response"])
     qe = spectrum.sr_to_qe(sr)
     assert_allclose(qe, sr_and_eqe_fixture["quantum_efficiency"])
+
+
+def test_average_photon_energy_series():
+    # test that the APE is calculated correctly with single spectrum
+    # series input
+
+    si = spectrum.get_reference_spectra()
+    si = si['global']
+
+    ape = spectrum.average_photon_energy(si)
+
+    expected = 1.45029
+
+    assert_allclose(ape, expected, rtol=1e-4)
+