diff --git a/custom_components/tibber_prices/services/helpers.py b/custom_components/tibber_prices/services/helpers.py
index 44f0c9c..d6036e7 100644
--- a/custom_components/tibber_prices/services/helpers.py
+++ b/custom_components/tibber_prices/services/helpers.py
@@ -717,6 +717,15 @@ def check_min_distance_from_avg(
     For cheapest searches: window mean must be at least X% BELOW range average.
     For most expensive searches: window mean must be at least X% ABOVE range average.
 
+    CRITICAL: Uses abs(range_avg) to scale the distance so the direction of the
+    threshold shift is always correct, even when range_avg is negative (Tibber
+    prices can go negative during grid oversupply). Multiplying a negative
+    range_avg directly by (1 ± ratio) flips the intended direction (e.g. avg *
+    1.05 makes a negative average MORE negative, i.e. cheaper, which is the
+    wrong direction for a "most expensive" threshold). This mirrors the
+    abs()-based normalization already used for the period system's
+    min_distance_from_avg handling (see coordinator/period_handlers/level_filtering.py).
+
     Args:
         window_mean_base: Window mean price in BASE currency (not display unit).
         range_avg: Search range average price in BASE currency.
@@ -731,10 +740,11 @@ def check_min_distance_from_avg(
         return True  # Cannot calculate percentage difference from zero
 
     distance_ratio = min_distance_pct / 100
+    distance_amount = abs(range_avg) * distance_ratio
     if reverse:
-        # Most expensive: window mean must be >= avg * (1 + distance)
-        threshold = range_avg * (1 + distance_ratio)
+        # Most expensive: window mean must be >= avg + distance
+        threshold = range_avg + distance_amount
         return window_mean_base >= threshold
-    # Cheapest: window mean must be <= avg * (1 - distance)
-    threshold = range_avg * (1 - distance_ratio)
+    # Cheapest: window mean must be <= avg - distance
+    threshold = range_avg - distance_amount
     return window_mean_base <= threshold
diff --git a/tests/services/test_service_helpers.py b/tests/services/test_service_helpers.py
new file mode 100644
index 0000000..50eb152
--- /dev/null
+++ b/tests/services/test_service_helpers.py
@@ -0,0 +1,71 @@
+"""Tests for small pure helper functions in services/helpers.py."""
+
+from __future__ import annotations
+
+from custom_components.tibber_prices.services.helpers import check_min_distance_from_avg
+
+
+class TestCheckMinDistanceFromAvgPositiveAverage:
+    """Regression coverage for the common case: positive range average."""
+
+    def test_cheapest_passes_when_far_enough_below(self) -> None:
+        """Window 10% below a positive average passes a 5% requirement."""
+        assert check_min_distance_from_avg(18.0, 20.0, 5.0, reverse=False) is True
+
+    def test_cheapest_fails_when_too_close(self) -> None:
+        """Window only 2% below a positive average fails a 5% requirement."""
+        assert check_min_distance_from_avg(19.6, 20.0, 5.0, reverse=False) is False
+
+    def test_most_expensive_passes_when_far_enough_above(self) -> None:
+        """Window 10% above a positive average passes a 5% requirement."""
+        assert check_min_distance_from_avg(22.0, 20.0, 5.0, reverse=True) is True
+
+    def test_most_expensive_fails_when_too_close(self) -> None:
+        """Window only 2% above a positive average fails a 5% requirement."""
+        assert check_min_distance_from_avg(20.4, 20.0, 5.0, reverse=True) is False
+
+
+class TestCheckMinDistanceFromAvgNegativeAverage:
+    """Regression coverage for GH-reported sign bug: negative range average.
+
+    Tibber prices can go negative during grid oversupply. The threshold must
+    be computed as `avg ± abs(avg) * ratio` (not `avg * (1 ± ratio)`), since
+    multiplying a negative average directly flips the intended direction.
+    """
+
+    def test_cheapest_passes_when_more_negative_than_avg(self) -> None:
+        """A window that is 10% cheaper (more negative) than a -5 avg passes 5% req."""
+        # -5 - abs(-5)*0.05 = -5.25 → window must be <= -5.25
+        assert check_min_distance_from_avg(-5.6, -5.0, 5.0, reverse=False) is True
+
+    def test_cheapest_fails_when_less_negative_than_avg(self) -> None:
+        """A window that is actually more expensive (less negative) than avg must fail.
+
+        Regression: the old buggy formula (avg * (1 - ratio)) computed a
+        threshold of -4.75 here, incorrectly letting -5.1 pass even though
+        it is only ~2% below avg (needs 5%).
+        """
+        assert check_min_distance_from_avg(-5.1, -5.0, 5.0, reverse=False) is False
+
+    def test_most_expensive_passes_when_less_negative_than_avg(self) -> None:
+        """A window 10% above (less negative than) a -5 avg passes a 5% req."""
+        # -5 + abs(-5)*0.05 = -4.75 → window must be >= -4.75
+        assert check_min_distance_from_avg(-4.4, -5.0, 5.0, reverse=True) is True
+
+    def test_most_expensive_fails_when_actually_cheaper_than_avg(self) -> None:
+        """A window that is cheaper (more negative) than avg must fail the most-expensive check.
+
+        Regression: the old buggy formula (avg * (1 + ratio)) computed a
+        threshold of -5.25 here, incorrectly letting -5.1 pass even though
+        it is actually cheaper than the average, not more expensive.
+        """
+        assert check_min_distance_from_avg(-5.1, -5.0, 5.0, reverse=True) is False
+
+
+class TestCheckMinDistanceFromAvgEdgeCases:
+    """Edge cases: zero average."""
+
+    def test_zero_average_always_passes(self) -> None:
+        """A zero average makes percentage distance undefined; always pass."""
+        assert check_min_distance_from_avg(5.0, 0.0, 5.0, reverse=False) is True
+        assert check_min_distance_from_avg(-5.0, 0.0, 5.0, reverse=True) is True