diff --git a/homeassistant/components/growatt_server/__init__.py b/homeassistant/components/growatt_server/__init__.py
index abf9118c8fa5..9bb94e9ea58f 100644
--- a/homeassistant/components/growatt_server/__init__.py
+++ b/homeassistant/components/growatt_server/__init__.py
@@ -23,7 +23,6 @@ Error handling pattern for reauth:
   → raise ConfigEntryAuthFailed
 - All other errors → ConfigEntryError (setup) or UpdateFailed (coordinator)
 """
-# pylint: disable=home-assistant-use-runtime-data  # Uses legacy hass.data[DOMAIN] pattern
 
 from collections.abc import Mapping
 import datetime
@@ -49,7 +48,6 @@ from homeassistant.helpers.typing import ConfigType
 from .const import (
     AUTH_API_TOKEN,
     AUTH_PASSWORD,
-    CACHED_API_KEY,
     CONF_AUTH_TYPE,
     CONF_PLANT_ID,
     DEFAULT_PLANT_ID,
@@ -70,6 +68,11 @@ _LOGGER = logging.getLogger(__name__)
 
 CONFIG_SCHEMA = cv.config_entry_only_config_schema(DOMAIN)
 
+# Temporary handoff of authenticated Classic API sessions from async_migrate_entry
+# to async_setup_entry. Avoids a second login() during the same startup, which
+# would hit Growatt's 5-minute per-endpoint rate limit. Popped after first use.
+_CACHED_APIS: dict[str, growattServer.GrowattApi] = {}
+
 
 async def async_setup(hass: HomeAssistant, config: ConfigType) -> bool:
     """Set up the Growatt Server integration."""
@@ -196,8 +199,7 @@ async def async_migrate_entry(
                 )
 
                 # Cache the logged-in API instance for reuse in async_setup_entry()
-                hass.data.setdefault(DOMAIN, {})
-                hass.data[DOMAIN][f"{CACHED_API_KEY}{config_entry.entry_id}"] = api
+                _CACHED_APIS[config_entry.entry_id] = api
 
                 _LOGGER.info(
                     "Migrated config entry to use specific plant_id '%s'",
@@ -347,9 +349,7 @@ async def async_setup_entry(
         # Check if migration cached an authenticated API instance for us to reuse.
         # This avoids calling login() twice (once in migration, once here) which
         # would trigger rate limiting.
-        cached_api = hass.data.get(DOMAIN, {}).pop(
-            f"{CACHED_API_KEY}{config_entry.entry_id}", None
-        )
+        cached_api = _CACHED_APIS.pop(config_entry.entry_id, None)
 
         if cached_api:
             # Reuse the logged-in API instance from migration (rate limit optimization)
diff --git a/homeassistant/components/growatt_server/const.py b/homeassistant/components/growatt_server/const.py
index 02c40a17aaef..a3478c418ed4 100644
--- a/homeassistant/components/growatt_server/const.py
+++ b/homeassistant/components/growatt_server/const.py
@@ -55,11 +55,6 @@ BATT_MODE_LOAD_FIRST = 0
 BATT_MODE_BATTERY_FIRST = 1
 BATT_MODE_GRID_FIRST = 2
 
-# Internal key prefix for caching authenticated API instance
-# Used to pass logged-in session from async_migrate_entry to async_setup_entry
-# to avoid double login() calls that trigger API rate limiting
-CACHED_API_KEY = "_cached_api_"
-
 # Supported device types for coordinator creation
 SUPPORTED_DEVICE_TYPES = ["inverter", "tlx", "storage", "mix", "min", "sph"]
 
diff --git a/tests/components/growatt_server/test_init.py b/tests/components/growatt_server/test_init.py
index 2519ea019d32..f2499de6fd28 100644
--- a/tests/components/growatt_server/test_init.py
+++ b/tests/components/growatt_server/test_init.py
@@ -10,10 +10,10 @@ import pytest
 import requests
 from syrupy.assertion import SnapshotAssertion
 
+from homeassistant.components.growatt_server import _CACHED_APIS
 from homeassistant.components.growatt_server.const import (
     AUTH_API_TOKEN,
     AUTH_PASSWORD,
-    CACHED_API_KEY,
     CONF_AUTH_TYPE,
     CONF_PLANT_ID,
     DEFAULT_PLANT_ID,
@@ -733,10 +733,8 @@ async def test_setup_reuses_cached_api_from_migration(
     # This confirms setup did NOT resolve plant_id again (optimization working)
     mock_growatt_classic_api.plant_list.assert_called_once_with(123456)
 
-    # Verify the cached API was removed after use (should not be in hass.data anymore)
-    assert f"{CACHED_API_KEY}{mock_config_entry.entry_id}" not in hass.data.get(
-        DOMAIN, {}
-    )
+    # Verify the cached API was removed after use (one-time handoff)
+    assert mock_config_entry.entry_id not in _CACHED_APIS
 
 
 async def test_migrate_failure_returns_false(