Fix fuentes error for tz-aware inputs (#1072)

* swap out timedelta calculation; add tz test

* whatsnew

* suggestion from review

Author: kandersolar

docs/sphinx/source/whatsnew/v0.8.1.rst

@@ -17,7 +17,8 @@ Enhancements
 
 Bug fixes
 ~~~~~~~~~
-
+* Fix issue with :py:func:`pvlib.temperature.fuentes` with timezone-aware
+  inputs. (:issue:`1071`, :pull:`1072`)
 
 Testing
 ~~~~~~~

pvlib/temperature.py

@@ -599,8 +599,9 @@ def fuentes(poa_global, temp_air, wind_speed, noct_installed, module_height=5,
     # n.b. the way Fuentes calculates the first timedelta makes it seem like
     # the value doesn't matter -- rather than recreate it here, just assume
     # it's the same as the second timedelta:
-    timedelta_hours = np.diff(poa_global.index).astype(float) / 1e9 / 60 / 60
-    timedelta_hours = np.append([timedelta_hours[0]], timedelta_hours)
+    timedelta_seconds = poa_global.index.to_series().diff().dt.total_seconds()
+    timedelta_hours = timedelta_seconds / 3600
+    timedelta_hours.iloc[0] = timedelta_hours.iloc[1]
 
     tamb_array = temp_air + 273.15
     sun_array = poa_global * absorp

pvlib/tests/test_temperature.py

@@ -190,3 +190,17 @@ def test_fuentes(filename, inoct):
     night_difference = expected_tcell[is_night] - actual_tcell[is_night]
     assert night_difference.max() < 6
     assert night_difference.min() > 0
+
+
+@pytest.mark.parametrize('tz', [None, 'Etc/GMT+5'])
+def test_fuentes_timezone(tz):
+    index = pd.date_range('2019-01-01', freq='h', periods=3, tz=tz)
+
+    df = pd.DataFrame({'poa_global': 1000, 'temp_air': 20, 'wind_speed': 1},
+                      index)
+
+    out = temperature.fuentes(df['poa_global'], df['temp_air'],
+                              df['wind_speed'], noct_installed=45)
+
+    assert_series_equal(out, pd.Series([47.85, 50.85, 50.85], index=index,
+                                       name='tmod'))