From b1cb161f8ea16849c5060b697c4542db85c33b7b Mon Sep 17 00:00:00 2001
From: Aman Srivastava <amansri345@gmail.com>
Date: Sun, 18 Jan 2026 15:26:54 +0530
Subject: [PATCH 1/2] Fix division-by-zero in transformer.simple_efficiency

---
 pvlib/transformer.py      |  3 ++-
 tests/test_transformer.py | 22 ++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/pvlib/transformer.py b/pvlib/transformer.py
index 3b66b0beb3..3fea282e2b 100644
--- a/pvlib/transformer.py
+++ b/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
diff --git a/tests/test_transformer.py b/tests/test_transformer.py
index 0739a9e95a..7d7a106a3b 100644
--- a/tests/test_transformer.py
+++ b/tests/test_transformer.py
@@ -1,4 +1,5 @@
 import pandas as pd
+import pytest
 
 from numpy.testing import assert_allclose
 
@@ -58,3 +59,24 @@ 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)    

From 0028bb6294358c989c392f87ffaa571b3f0dc52e Mon Sep 17 00:00:00 2001
From: Aman Srivastava <amansri345@gmail.com>
Date: Sun, 18 Jan 2026 15:29:42 +0530
Subject: [PATCH 2/2] fix flake8

---
 tests/test_transformer.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tests/test_transformer.py b/tests/test_transformer.py
index 7d7a106a3b..48f11e0a45 100644
--- a/tests/test_transformer.py
+++ b/tests/test_transformer.py
@@ -79,4 +79,4 @@ def test_simple_efficiency_zero_load_loss(
         transformer_rating=transformer_rating,
     )
 
-    assert_allclose(result, expected)    
+    assert_allclose(result, expected)