diff --git a/README.md b/README.md
index 19bb2a8..b32c669 100644
--- a/README.md
+++ b/README.md
@@ -41,6 +41,7 @@ This library is used by the [Opower integration in Home Assistant](https://www.h
 - Sacramento Municipal Utility District (SMUD)
 - Seattle City Light (SCL)
 - Southern Maryland Electric Cooperative (SMECO)
+- Southwest Gas
 
 ## Contributing
 
diff --git a/src/opower/utilities/swgas.py b/src/opower/utilities/swgas.py
new file mode 100644
index 0000000..02c0e4d
--- /dev/null
+++ b/src/opower/utilities/swgas.py
@@ -0,0 +1,105 @@
+"""Southwest Gas (SWG)."""
+
+from typing import Any
+
+import aiohttp
+
+# --- FIX: Import the USER_AGENT constant ---
+from ..const import USER_AGENT
+from ..exceptions import InvalidAuth
+from .base import UtilityBase
+
+
+class SouthwestGas(UtilityBase):
+    """Southwest Gas (SWG).
+
+    This utility uses the Opower portal at `swg.opower.com`.
+    Login is handled via the 'user-account-control-v1' API endpoint.
+    """
+
+    @staticmethod
+    def name() -> str:
+        """Return a distinct, human-readable name for this utility."""
+        return "Southwest Gas"
+
+    def subdomain(self) -> str:
+        """Return the opower.com subdomain for this utility."""
+        return "swg"
+
+    @staticmethod
+    def timezone() -> str:
+        """Return the timezone for this utility."""
+        return "America/Phoenix"
+
+    @staticmethod
+    def is_dss() -> bool:
+        """Indicate that this utility uses the DSS version of the portal."""
+        return False
+
+    async def async_login(
+        self,
+        session: aiohttp.ClientSession,
+        username: str,
+        password: str,
+        login_data: dict[str, Any],
+    ) -> str | None:
+        """Authenticate against the SWG Opower portal."""
+        # 1. Define URLs
+        base_url = f"https://{self.subdomain()}.opower.com"
+        login_page_url = f"{base_url}/ei/x/sign-in-wall?source=intercepted"
+        api_url = f"{base_url}/ei/edge/apis/user-account-control-v1/cws/v1/{self.subdomain()}/account/signin"
+
+        # 2. Define Headers
+        # --- FIX: Use the imported USER_AGENT constant instead of hardcoding one ---
+        headers = {
+            "User-Agent": USER_AGENT,
+            "Accept": "application/json, text/plain, */*",
+            "Accept-Language": "en-US,en;q=0.9",
+        }
+
+        # 3. Warm up the session
+        # We just call the context manager to get the cookies
+        async with session.get(login_page_url, headers=headers):
+            pass
+
+        # 4. Prepare Login Headers
+        login_headers = headers.copy()
+        login_headers.update(
+            {
+                "Content-Type": "application/json",
+                "Origin": base_url,
+                "Referer": login_page_url,
+                "X-Requested-With": "XMLHttpRequest",
+            }
+        )
+
+        # 5. Execute Login
+        payload = {"username": username, "password": password}
+
+        async with session.post(
+            api_url,
+            json=payload,
+            headers=login_headers,
+            raise_for_status=False,
+        ) as resp:
+            # --- HANDLE 204 SUCCESS ---
+            if resp.status == 204:
+                return "cookie-auth-success"
+
+            # If it's not 200 or 204, fail.
+            if resp.status != 200:
+                error_text = await resp.text()
+                raise InvalidAuth(f"Login failed: {resp.status} - {error_text}")
+
+            try:
+                result = await resp.json()
+            except Exception as exc:
+                raise InvalidAuth("Unexpected response from SWG login") from exc
+
+        # 6. Extract Token (Only if response was 200 JSON)
+        token = result.get("sessionToken") or result.get("accessToken")
+
+        if not token:
+            raise InvalidAuth(f"Login failed; token not found. Response keys: {list(result.keys())}")
+
+        return str(token)