Correct day angle in ASCE extraterrestrial irradiance model

docs/sphinx/source/whatsnew/v0.6.2.rst

@@ -47,6 +47,8 @@ Bug fixes
   near horizon. (:issue:`656`)
 * Fixed numpy warnings in :py:func:`~pvlib.tracking.singleaxis` when
   comparing NaN values to limits. (:issue:`622`)
+* Fixed a bug in the day angle equation for the ASCE
+  extraterrestrial irradiance model. (:issue:`211`)
 * Silenced divide by 0 irradiance warnings in
   :py:func:`~pvlib.irradiance.klucher` and
   :py:func:`~pvlib.pvsystem.calcparams_desoto`. (:issue:`698`)

pvlib/irradiance.py

@@ -80,6 +80,10 @@ def get_extra_radiation(datetime_or_doy, solar_constant=1366.1,
 
     [4] Duffie, J. A. and Beckman, W. A. 1991. Solar Engineering of
     Thermal Processes, 2nd edn. J. Wiley and Sons, New York.
+
+    [5] ASCE, 2005. The ASCE Standardized Reference Evapotranspiration
+    Equation, Environmental and Water Resources Institute of the American
+    Civil Engineers, Ed. R. G. Allen et al.
     """
 
     to_doy, to_datetimeindex, to_output = \
@@ -88,7 +92,8 @@ def get_extra_radiation(datetime_or_doy, solar_constant=1366.1,
     # consider putting asce and spencer methods in their own functions
     method = method.lower()
     if method == 'asce':
-        B = solarposition._calculate_simple_day_angle(to_doy(datetime_or_doy))
+        B = solarposition._calculate_simple_day_angle(to_doy(datetime_or_doy),
+                                                      offset=0)
         RoverR0sqrd = 1 + 0.033 * np.cos(B)
     elif method == 'spencer':
         B = solarposition._calculate_simple_day_angle(to_doy(datetime_or_doy))

pvlib/solarposition.py

@@ -988,19 +988,21 @@ def nrel_earthsun_distance(time, how='numpy', delta_t=67.0, numthreads=4):
     return dist
 
 
-def _calculate_simple_day_angle(dayofyear):
+def _calculate_simple_day_angle(dayofyear, offset=1):
     """
     Calculates the day angle for the Earth's orbit around the Sun.
 
     Parameters
     ----------
     dayofyear : numeric
+    offset : int, default 1
+        For the Spencer method, offset=1; for the ASCE method, offset=0
 
     Returns
     -------
     day_angle : numeric
     """
-    return (2. * np.pi / 365.) * (dayofyear - 1)
+    return (2. * np.pi / 365.) * (dayofyear - offset)
 
 
 def equation_of_time_spencer71(dayofyear):

pvlib/test/test_irradiance.py

@@ -90,7 +90,7 @@ def test_deprecated_07():
 value = 1383.636203
 
 
-@pytest.mark.parametrize('input, expected', [
+@pytest.mark.parametrize('testval, expected', [
     (doy, value),
     (np.float64(doy), value),
     (dt_date, value),
@@ -103,9 +103,9 @@ def test_deprecated_07():
 ])
 @pytest.mark.parametrize('method', [
     'asce', 'spencer', 'nrel', pytest.param('pyephem', marks=requires_ephem)])
-def test_get_extra_radiation(input, expected, method):
-    out = irradiance.get_extra_radiation(input)
-    assert_allclose(out, expected, atol=1)
+def test_get_extra_radiation(testval, expected, method):
+    out = irradiance.get_extra_radiation(testval, method=method)
+    assert_allclose(out, expected, atol=10)
 
 
 def test_get_extra_radiation_epoch_year():