diff --git a/openelectricity/models/timeseries.py b/openelectricity/models/timeseries.py
index dbe7a45..ce93468 100644
--- a/openelectricity/models/timeseries.py
+++ b/openelectricity/models/timeseries.py
@@ -31,10 +31,17 @@ def value(self) -> float | None:
 
 
 class TimeSeriesColumns(BaseModel):
-    """Column metadata for time series results."""
+    """Column metadata for time series results.
+
+    Populated according to the ``secondary_grouping`` used in the request:
+    ``fueltech`` / ``fueltech_group`` / ``renewable`` / ``unit_code`` /
+    ``network_region``.
+    """
 
     unit_code: str | None = None
+    fueltech: str | None = None
     fueltech_group: str | None = None
+    renewable: bool | None = None
     network_region: str | None = None
 
 
diff --git a/tests/models/test_timeseries.py b/tests/models/test_timeseries.py
index 61b4efe..e6471f5 100644
--- a/tests/models/test_timeseries.py
+++ b/tests/models/test_timeseries.py
@@ -195,3 +195,36 @@ def test_invalid_interval():
                 "network_timezone_offset": "+10:00",
             }
         )
+
+
+def test_columns_fueltech_populated_when_grouping_by_fueltech() -> None:
+    """secondary_grouping=fueltech sets the fueltech column (issue surfaced
+    while validating #8)."""
+    result = TimeSeriesResult.model_validate(
+        {
+            "name": "energy_solar_rooftop",
+            "date_start": "2026-05-22T00:00:00+10:00",
+            "date_end": "2026-05-23T00:00:00+10:00",
+            "columns": {"fueltech": "solar_rooftop"},
+            "data": [["2026-05-22T00:00:00+10:00", 48000.0]],
+        }
+    )
+    assert result.columns.fueltech == "solar_rooftop"
+    assert result.columns.fueltech_group is None
+    assert result.columns.renewable is None
+
+
+def test_columns_renewable_populated_when_grouping_by_renewable() -> None:
+    """secondary_grouping=renewable sets the renewable boolean column."""
+    result = TimeSeriesResult.model_validate(
+        {
+            "name": "energy_True",
+            "date_start": "2026-05-22T00:00:00+10:00",
+            "date_end": "2026-05-23T00:00:00+10:00",
+            "columns": {"renewable": True},
+            "data": [["2026-05-22T00:00:00+10:00", 250000.0]],
+        }
+    )
+    assert result.columns.renewable is True
+    assert result.columns.fueltech is None
+    assert result.columns.fueltech_group is None