Fix sign conversion

flexmeasures/utils/tests/test_unit_utils.py

@@ -25,7 +25,12 @@
         ("m³", "m³/h", 4, None),
         ("MW", "kW", 1000, None),
         ("kWh", "kW", 4, None),
+        ("-W", "W", -1, None),
+        ("l/(100km)", "l/km", 0.01, None),
         ("°C", "K", None, [273.15, 283.15, 284.15]),
+        # no support for combining an offset unit with a scaling factor, but this is also overly specific
+        # ("-°C", "K", None, [273.15, 263.15, 262.15]),
+        # ("l/(10°C)", "l/(°C)", 0.1, None),
     ],
 )
 def test_convert_unit(

flexmeasures/utils/unit_utils.py

@@ -3,6 +3,7 @@
 
 from moneyed import list_all_currencies
 import importlib.resources as pkg_resources
+import numpy as np
 import pandas as pd
 import pint
 
@@ -161,25 +162,31 @@ def convert_units(
     """Updates data values to reflect the given unit conversion."""
 
     if from_unit != to_unit:
+        from_magnitudes = (
+            data.to_numpy() if isinstance(data, pd.Series) else np.asarray(data)
+        )
         try:
-            if isinstance(data, pd.Series):
-                data = pd.Series(
-                    pint.Quantity(data.values, from_unit)
-                    .to(pint.Quantity(to_unit))
-                    .magnitude,
-                    index=data.index,
-                    name=data.name,
-                )
+            from_quantities = ur.Quantity(from_magnitudes, from_unit)
+        except ValueError as e:
+            # Catch units like "-W" and "100km"
+            if str(e) == "Unit expression cannot have a scaling factor.":
+                from_quantities = ur.Quantity(from_unit) * from_magnitudes
             else:
-                data = list(
-                    pint.Quantity(data, from_unit).to(pint.Quantity(to_unit)).magnitude
-                )
+                raise e  # reraise
+        try:
+            to_magnitudes = from_quantities.to(ur.Quantity(to_unit)).magnitude
         except pint.errors.DimensionalityError:
+            # Catch multiplicative conversions that use the resolution, like "kWh/15min" to "kW"
             multiplier = determine_unit_conversion_multiplier(
                 from_unit, to_unit, event_resolution
             )
-            if isinstance(data, pd.Series):
-                data = multiplier * data
-            else:
-                data = [multiplier * value for value in data]
+            to_magnitudes = from_magnitudes * multiplier
+        if isinstance(data, pd.Series):
+            data = pd.Series(
+                to_magnitudes,
+                index=data.index,
+                name=data.name,
+            )
+        else:
+            data = list(to_magnitudes)
     return data