diff --git a/artiq/language/units.py b/artiq/language/units.py
index 13f6f1535..569fb964b 100644
--- a/artiq/language/units.py
+++ b/artiq/language/units.py
@@ -19,4 +19,4 @@ _register_unit("Hz", "m_kMG")
 _register_unit("dB", "_")
 _register_unit("V", "um_k")
 _register_unit("A", "um_")
-_register_unit("W", "um_")
+_register_unit("W", "num_")