Add new power utility (#78867)

* Add power utility

* Fix tests

tests/util/test_power.py

@@ -0,0 +1,41 @@
+"""Test Home Assistant power utility functions."""
+import pytest
+
+from homeassistant.const import POWER_KILO_WATT, POWER_WATT
+import homeassistant.util.power as power_util
+
+INVALID_SYMBOL = "bob"
+VALID_SYMBOL = POWER_WATT
+
+
+def test_convert_same_unit():
+    """Test conversion from any unit to same unit."""
+    assert power_util.convert(2, POWER_WATT, POWER_WATT) == 2
+    assert power_util.convert(3, POWER_KILO_WATT, POWER_KILO_WATT) == 3
+
+
+def test_convert_invalid_unit():
+    """Test exception is thrown for invalid units."""
+    with pytest.raises(ValueError):
+        power_util.convert(5, INVALID_SYMBOL, VALID_SYMBOL)
+
+    with pytest.raises(ValueError):
+        power_util.convert(5, VALID_SYMBOL, INVALID_SYMBOL)
+
+
+def test_convert_nonnumeric_value():
+    """Test exception is thrown for nonnumeric type."""
+    with pytest.raises(TypeError):
+        power_util.convert("a", POWER_WATT, POWER_KILO_WATT)
+
+
+def test_convert_from_kw():
+    """Test conversion from kW to other units."""
+    kilowatts = 10
+    assert power_util.convert(kilowatts, POWER_KILO_WATT, POWER_WATT) == 10000
+
+
+def test_convert_from_w():
+    """Test conversion from W to other units."""
+    watts = 10
+    assert power_util.convert(watts, POWER_WATT, POWER_KILO_WATT) == 0.01