diff --git a/esphome/core/config.py b/esphome/core/config.py
index d40b693b81..d4a839cb79 100644
--- a/esphome/core/config.py
+++ b/esphome/core/config.py
@@ -224,7 +224,8 @@ else:
 FRIENDLY_NAME_MAX_LEN = 120
 
 # Max device class string length (47 chars + null = 48-byte PROGMEM buffer)
-# Keep in sync with MAX_DEVICE_CLASS_LENGTH in esphome/core/entity_base.h
+# Keep in sync with MAX_DEVICE_CLASS_LENGTH in esphome/core/entity_base.h:
+# DEVICE_CLASS_MAX_LENGTH == MAX_DEVICE_CLASS_LENGTH - 1 (C++ includes the null)
 DEVICE_CLASS_MAX_LENGTH = 47