Fix ZeroDivisionError in simple_efficiency when load_loss=0

docs/sphinx/source/whatsnew/v0.13.3.rst

@@ -14,6 +14,9 @@ Deprecations
 
 Bug fixes
 ~~~~~~~~~
+* Fix a division-by-zero condition in
+  :py:func:`pvlib.transformer.simple_efficiency`
+  when ``load_loss = 0``. (:pull:`2646`)
 
 
 Enhancements

pvlib/transformer.py

@@ -111,7 +111,8 @@ def simple_efficiency(
     b = 1
     c = no_load_loss - input_power_normalized
 
-    output_power_normalized = (-b + (b**2 - 4*a*c)**0.5) / (2 * a)
+    disc = (b*b - 4*a*c)**0.5
+    output_power_normalized = 2*c / (-b - disc)
 
     output_power = output_power_normalized * transformer_rating
     return output_power

tests/test_transformer.py

@@ -1,7 +1,7 @@
 import pandas as pd
+import pytest
 
 from numpy.testing import assert_allclose
-
 from pvlib import transformer
 
 
@@ -58,3 +58,24 @@ def test_simple_efficiency_known_values():
                                       *args),
         rating,
     )
+
+
+@pytest.mark.parametrize(
+    "input_power, no_load_loss, load_loss, transformer_rating, expected",
+    [
+        (1000.0, 0.01, 0.0, 1000.0, 990.0),
+    ],
+)
+def test_simple_efficiency_zero_load_loss(
+    input_power, no_load_loss, load_loss, transformer_rating, expected
+):
+    # for load_loss = 0, the model reduces to:
+    # P_out = P_in - L_no_load * P_nom
+    result = transformer.simple_efficiency(
+        input_power=input_power,
+        no_load_loss=no_load_loss,
+        load_loss=load_loss,
+        transformer_rating=transformer_rating,
+    )
+
+    assert_allclose(result, expected)