diff --git a/bmspy/ups.py b/bmspy/ups.py
index 7d59976..cea9ba0 100644
--- a/bmspy/ups.py
+++ b/bmspy/ups.py
@@ -9,6 +9,7 @@ import socket
 from collections import deque
 
 from bmspy import client
+from bmspy.classes import UPS, BMSScalarField
 
 DAEMON_UPDATE_PERIOD = 30
 
@@ -18,6 +19,46 @@ warning_sent = False
 alert_sent = False
 
 
+def _get_field_value(device: UPS, field_name: str) -> float | None:
+    """Return the raw_value of a named BMSScalarField from a UPS, or None if not found."""
+    for name, field in device.items():
+        if name == field_name and isinstance(field, BMSScalarField):
+            return field.raw_value
+    return None
+
+
+def _resolve_ups_device(data: dict[str, UPS], requested: str | None) -> str | None:
+    """Return the device name to monitor, or None and print an error if the selection is invalid.
+
+    Rules:
+    - No devices → error.
+    - requested is set and found → use it.
+    - requested is set but not found → error listing available devices.
+    - requested is None and exactly one device → use it.
+    - requested is None and multiple devices → error; user must specify one.
+    """
+    if not data:
+        print("Error: no UPS devices found")
+        return None
+    if requested is not None:
+        if requested not in data:
+            print(
+                "Error: UPS '{}' not found. Available: {}".format(
+                    requested, ", ".join(sorted(data))
+                )
+            )
+            return None
+        return requested
+    if len(data) > 1:
+        print(
+            "Error: multiple UPS devices found, specify one with --device: {}".format(
+                ", ".join(sorted(data))
+            )
+        )
+        return None
+    return next(iter(data))
+
+
 def handle_shutdown(action: str = "cancel", delay: int = 0, debug: int = 0) -> None:
     """Schedule or cancel a system shutdown; only schedules once per incident."""
     global scheduled_shutdown
@@ -157,6 +198,14 @@ def main() -> None:
         default="/run/bmspy/bms",
         help="Socket to communicate with daemon",
     )
+    parser.add_argument(
+        "--device",
+        dest="device",
+        action="store",
+        default=None,
+        metavar="NAME",
+        help="Name of the UPS device to monitor (required if multiple devices are available)",
+    )
     parser.add_argument(
         "--verbose",
         "-v",
@@ -173,10 +222,19 @@ def main() -> None:
     client.handle_registration(args.socket, "ups", debug)
     atexit.register(client.handle_registration, args.socket, "ups", debug)
 
+    initial_data = client.read_data(args.socket, "ups")
+    device_name = _resolve_ups_device(initial_data, args.device)
+    if device_name is None:
+        return
+
     history = deque()
     while True:
-        data = client.read_data(args.socket, "ups")
-        history.append(data)
+        data = client.read_data(args.socket, "ups", device=device_name)
+        device_obj = data.get(device_name)
+        if device_obj is None:
+            time.sleep(DAEMON_UPDATE_PERIOD)
+            continue
+        history.append(device_obj)
 
         # Remove the oldest data from the history
         while len(history) > 10:
@@ -192,19 +250,30 @@ def main() -> None:
             time.sleep(DAEMON_UPDATE_PERIOD)
             continue
 
-        current_amps = float(data["bms_current_amps"]["raw_value"])
-        charge_ratio = float(data["bms_capacity_charge_ratio"]["raw_value"]) * 100
-        comparison_1_current_amps = float(comparison_1["bms_current_amps"]["raw_value"])
+        current_amps = float(_get_field_value(device_obj, "bms_current_amps") or 0.0)
+        charge_ratio = (
+            float(_get_field_value(device_obj, "bms_capacity_charge_ratio") or 0.0) * 100
+        )
+        comparison_1_current_amps = float(
+            _get_field_value(comparison_1, "bms_current_amps") or 0.0
+        )
         comparison_1_charge_ratio = (
-            float(comparison_1["bms_capacity_charge_ratio"]["raw_value"]) * 100
+            float(_get_field_value(comparison_1, "bms_capacity_charge_ratio") or 0.0)
+            * 100
+        )
+        comparison_2_current_amps = float(
+            _get_field_value(comparison_2, "bms_current_amps") or 0.0
         )
-        comparison_2_current_amps = float(comparison_2["bms_current_amps"]["raw_value"])
         comparison_2_charge_ratio = (
-            float(comparison_2["bms_capacity_charge_ratio"]["raw_value"]) * 100
+            float(_get_field_value(comparison_2, "bms_capacity_charge_ratio") or 0.0)
+            * 100
+        )
+        comparison_3_current_amps = float(
+            _get_field_value(comparison_3, "bms_current_amps") or 0.0
         )
-        comparison_3_current_amps = float(comparison_3["bms_current_amps"]["raw_value"])
         comparison_3_charge_ratio = (
-            float(comparison_3["bms_capacity_charge_ratio"]["raw_value"]) * 100
+            float(_get_field_value(comparison_3, "bms_capacity_charge_ratio") or 0.0)
+            * 100
         )
 
         if debug > 1:
diff --git a/tests/test_ups.py b/tests/test_ups.py
index c7b0c8e..7cbfb25 100644
--- a/tests/test_ups.py
+++ b/tests/test_ups.py
@@ -4,7 +4,7 @@ from unittest.mock import patch, MagicMock
 
 import bmspy.ups as ups_mod
 from bmspy.classes import BMSScalarField, BMSMultiField, BMSInfoField, UPS
-from bmspy.ups import _get_field_value, handle_shutdown, handle_email
+from bmspy.ups import _get_field_value, _resolve_ups_device, handle_shutdown, handle_email
 
 
 # ---------------------------------------------------------------------------
@@ -204,6 +204,44 @@ class TestHandleEmail:
         assert args[1] == "root@myhost"
 
 
+# ---------------------------------------------------------------------------
+# _resolve_ups_device
+# ---------------------------------------------------------------------------
+
+def _simple_ups_data(*names: str) -> dict[str, UPS]:
+    return {name: UPS.from_dict({
+        "bms_current_amps": {"help": "A", "raw_value": 0.0, "value": "0.00", "units": "A"},
+    }) for name in names}
+
+
+class TestResolveUpsDevice:
+    def test_single_device_no_request_returns_it(self):
+        data = _simple_ups_data("myups")
+        assert _resolve_ups_device(data, None) == "myups"
+
+    def test_requested_device_found(self):
+        data = _simple_ups_data("ups1", "ups2")
+        assert _resolve_ups_device(data, "ups2") == "ups2"
+
+    def test_requested_device_not_found_returns_none(self, capsys):
+        data = _simple_ups_data("ups1")
+        result = _resolve_ups_device(data, "missing")
+        assert result is None
+        assert "missing" in capsys.readouterr().out
+
+    def test_multiple_devices_no_request_returns_none(self, capsys):
+        data = _simple_ups_data("ups1", "ups2")
+        result = _resolve_ups_device(data, None)
+        assert result is None
+        out = capsys.readouterr().out
+        assert "ups1" in out or "ups2" in out
+
+    def test_empty_data_returns_none(self, capsys):
+        result = _resolve_ups_device({}, None)
+        assert result is None
+        assert "no UPS" in capsys.readouterr().out
+
+
 # ---------------------------------------------------------------------------
 # ups main() - comprehensive loop testing
 # ---------------------------------------------------------------------------
@@ -229,27 +267,26 @@ def _make_ups(current_amps: float, charge_ratio: float) -> UPS:
 class TestUpsMain:
     """Test ups.main() loop behavior by running a limited number of iterations."""
 
-    def _run_main_with_data(self, data_sequence, argv=None, extra_patches=None):
-        """Run main() with a sequence of UPS data, stopping when data runs out."""
-        call_count = 0
+    def _run_main_with_data(self, data_sequence, argv=None):
+        """Run main() with a sequence of UPS data, stopping when data runs out.
+
+        The first call to read_data is the device-discovery call and returns
+        data_sequence[0].  Subsequent calls consume data_sequence in order.
+        """
+        # discovery call returns data_sequence[0], then loop calls follow
+        discovery_done = [False]
+        loop_count = [0]
 
         def _read_data(*args, **kwargs):
-            nonlocal call_count
-            if call_count >= len(data_sequence):
+            if not discovery_done[0]:
+                discovery_done[0] = True
+                return data_sequence[0]
+            if loop_count[0] >= len(data_sequence):
                 raise StopIteration("done")
-            result = data_sequence[call_count]
-            call_count += 1
+            result = data_sequence[loop_count[0]]
+            loop_count[0] += 1
             return result
 
-        patches = {
-            "bmspy.ups.client.read_data": _read_data,
-            "bmspy.ups.client.handle_registration": MagicMock(),
-            "bmspy.ups.time.sleep": MagicMock(),
-            "bmspy.ups.os.system": MagicMock(),
-        }
-        if extra_patches:
-            patches.update(extra_patches)
-
         with patch("sys.argv", ["bmspy-ups"] + (argv or [])):
             with pytest.raises(StopIteration):
                 with patch("bmspy.ups.client.read_data", side_effect=_read_data), \
@@ -257,7 +294,7 @@ class TestUpsMain:
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.os.system"):
                     ups_mod.main()
-        return call_count
+        return loop_count[0]
 
     def _make_data(self, current_amps, charge_ratio):
         return {"testups": _make_ups(current_amps, charge_ratio)}
@@ -269,7 +306,7 @@ class TestUpsMain:
 
         with patch("sys.argv", ["bmspy-ups"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -285,7 +322,7 @@ class TestUpsMain:
         data = [self._make_data(0.0, 0.95)] * 5
         with patch("sys.argv", ["bmspy-ups", "-v", "-v"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data + [StopIteration]),\
+                with patch("bmspy.ups.client.read_data", side_effect=[data[0]] + data + [StopIteration]),\
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.os.system"):
@@ -305,7 +342,7 @@ class TestUpsMain:
         mock_email = MagicMock()
         with patch("sys.argv", ["bmspy-ups", "--critical", "30"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown", mock_shutdown), \
@@ -325,7 +362,7 @@ class TestUpsMain:
         mock_email = MagicMock()
         with patch("sys.argv", ["bmspy-ups", "--warning", "75", "--critical", "30"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -352,7 +389,7 @@ class TestUpsMain:
         mock_email = MagicMock()
         with patch("sys.argv", ["bmspy-ups"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -381,7 +418,7 @@ class TestUpsMain:
         mock_shutdown = MagicMock()
         with patch("sys.argv", ["bmspy-ups"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown", mock_shutdown), \
@@ -394,7 +431,7 @@ class TestUpsMain:
         data_seq = [self._make_data(0.0, 0.95)] * 5
         with patch("sys.argv", ["bmspy-ups", "-v", "-v"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -409,7 +446,7 @@ class TestUpsMain:
         data_seq = [self._make_data(0.0, 0.60)] * 5
         with patch("sys.argv", ["bmspy-ups", "-v", "--warning", "75", "--critical", "30"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -430,7 +467,7 @@ class TestUpsMain:
         ]
         with patch("sys.argv", ["bmspy-ups", "-v"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -451,7 +488,7 @@ class TestUpsMain:
         ]
         with patch("sys.argv", ["bmspy-ups", "-v"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -465,7 +502,7 @@ class TestUpsMain:
         data_seq = [self._make_data(1.0, 0.80)] * 5
         with patch("sys.argv", ["bmspy-ups", "-v", "-v"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -480,7 +517,7 @@ class TestUpsMain:
         data_seq = [self._make_data(0.0, 0.20)] * 5
         with patch("sys.argv", ["bmspy-ups", "-v", "--critical", "30"]):
             with pytest.raises(StopIteration):
-                with patch("bmspy.ups.client.read_data", side_effect=data_seq + [StopIteration("done")]), \
+                with patch("bmspy.ups.client.read_data", side_effect=[data_seq[0]] + data_seq + [StopIteration("done")]), \
                      patch("bmspy.ups.client.handle_registration"), \
                      patch("bmspy.ups.time.sleep"), \
                      patch("bmspy.ups.handle_shutdown"), \
@@ -488,3 +525,68 @@ class TestUpsMain:
                     ups_mod.main()
         captured = capsys.readouterr()
         assert "critical" in captured.out.lower() or "threshold" in captured.out.lower()
+
+
+# ---------------------------------------------------------------------------
+# main() — device selection errors and --device flag
+# ---------------------------------------------------------------------------
+
+class TestUpsMainDeviceSelection:
+    def _make_data(self, name, current_amps=0.0, charge_ratio=0.8):
+        return {name: _make_ups(current_amps, charge_ratio)}
+
+    def test_main_no_devices_returns_early(self, capsys):
+        """main() exits cleanly when no devices are found."""
+        with patch("sys.argv", ["bmspy-ups"]), \
+             patch("bmspy.ups.client.read_data", return_value={}), \
+             patch("bmspy.ups.client.handle_registration"):
+            ups_mod.main()
+        assert "no UPS" in capsys.readouterr().out
+
+    def test_main_multiple_devices_no_flag_returns_early(self, capsys):
+        """main() exits with an error when multiple devices exist and --device is not set."""
+        two_devices = {
+            "ups1": _make_ups(0.0, 0.9),
+            "ups2": _make_ups(0.0, 0.8),
+        }
+        with patch("sys.argv", ["bmspy-ups"]), \
+             patch("bmspy.ups.client.read_data", return_value=two_devices), \
+             patch("bmspy.ups.client.handle_registration"):
+            ups_mod.main()
+        out = capsys.readouterr().out
+        assert "ups1" in out or "ups2" in out
+
+    def test_main_ups_flag_selects_device(self):
+        """--device selects the correct device from multiple available ones."""
+        data_seq = [self._make_data("ups2")] * 6
+        with patch("sys.argv", ["bmspy-ups", "--device", "ups2"]):
+            with pytest.raises(StopIteration):
+                with patch("bmspy.ups.client.read_data",
+                           side_effect=[{"ups1": _make_ups(0.0, 0.9), "ups2": _make_ups(0.0, 0.8)}]
+                                       + data_seq + [StopIteration("done")]), \
+                     patch("bmspy.ups.client.handle_registration"), \
+                     patch("bmspy.ups.time.sleep"), \
+                     patch("bmspy.ups.handle_shutdown"), \
+                     patch("bmspy.ups.handle_email"):
+                    ups_mod.main()
+
+    def test_main_ups_flag_unknown_device_returns_early(self, capsys):
+        """--device with an unknown device name exits with an error."""
+        with patch("sys.argv", ["bmspy-ups", "--device", "ghost"]), \
+             patch("bmspy.ups.client.read_data", return_value={"ups1": _make_ups(0.0, 0.9)}), \
+             patch("bmspy.ups.client.handle_registration"):
+            ups_mod.main()
+        assert "ghost" in capsys.readouterr().out
+
+    def test_main_device_disappears_in_loop_sleeps(self):
+        """When the named device is missing from a loop response, main() sleeps and retries."""
+        initial = {"testups": _make_ups(0.0, 0.8)}
+        # Second call returns empty dict (device gone), third raises StopIteration
+        with patch("sys.argv", ["bmspy-ups"]):
+            with pytest.raises(StopIteration):
+                with patch("bmspy.ups.client.read_data",
+                           side_effect=[initial, {}, StopIteration("done")]), \
+                     patch("bmspy.ups.client.handle_registration"), \
+                     patch("bmspy.ups.time.sleep") as mock_sleep:
+                    ups_mod.main()
+        mock_sleep.assert_called()