Add simple_efficiency tests for numpy array inputs

docs/sphinx/source/whatsnew/v0.15.1.rst

@@ -28,7 +28,9 @@ Documentation
 
 Testing
 ~~~~~~~
-
+* Add test to verify that :py:func:`~pvlib.transformer.simple_efficiency`
+  produces consistent results for vectorized and scalar inputs.
+  (:pull:`2661`)
 
 Benchmarking
 ~~~~~~~~~~~~

tests/test_transformer.py

@@ -5,6 +5,8 @@
 
 from pvlib import transformer
 
+import numpy as np
+
 
 def test_simple_efficiency():
 
@@ -61,6 +63,27 @@ def test_simple_efficiency_known_values():
     )
 
 
+def test_simple_efficiency_vector_equals_scalar():
+    input_power = np.array([200.0, 600.0, 900.0])
+    no_load_loss = 0.005
+    load_loss = 0.01
+    rating = 1000.0
+
+    vector_result = transformer.simple_efficiency(
+        input_power=input_power,
+        no_load_loss=no_load_loss,
+        load_loss=load_loss,
+        transformer_rating=rating
+    )
+
+    scalar_result = np.array([
+        transformer.simple_efficiency(p, no_load_loss, load_loss, rating)
+        for p in input_power
+    ])
+
+    assert_allclose(vector_result, scalar_result)
+
+
 @pytest.mark.parametrize(
     "input_power, no_load_loss, load_loss, transformer_rating, expected",
     [