Fix consumer_power() not working certain configurations.

In microgrids without consumers and no main meter, the formula
would never return any values.

Signed-off-by: Mathias L. Baumann <mathias.baumann@frequenz.com>

RELEASE_NOTES.md

@@ -19,3 +19,6 @@
   component.
   So far we only had configurations like this: Meter -> Inverter -> PV. However
   the scenario with Inverter -> PV is also possible and now handled correctly.
+- Fix `consumer_power()` not working certain configurations.
+  In microgrids without consumers and no main meter, the formula
+  would never return any values.

src/frequenz/sdk/timeseries/_formula_engine/_formula_generators/_consumer_power_formula.py

@@ -5,14 +5,21 @@
 
 from __future__ import annotations
 
+import logging
 from collections import abc
 
 from ....microgrid import connection_manager
 from ....microgrid.component import Component, ComponentCategory, ComponentMetricId
 from ..._quantities import Power
 from .._formula_engine import FormulaEngine
 from .._resampled_formula_builder import ResampledFormulaBuilder
-from ._formula_generator import ComponentNotFound, FormulaGenerator
+from ._formula_generator import (
+    NON_EXISTING_COMPONENT_ID,
+    ComponentNotFound,
+    FormulaGenerator,
+)
+
+_logger = logging.getLogger(__name__)
 
 
 class ConsumerPowerFormula(FormulaGenerator[Power]):
@@ -121,4 +128,17 @@ def _gen_without_grid_meter(
             is_first = False
             builder.push_component_metric(successor.component_id, nones_are_zeros=False)
 
+        if len(builder.finalize()[0]) == 0:
+            # If there are no consumer components, we have to send 0 values at the same
+            # frequency as the other streams.  So we subscribe with a non-existing
+            # component id, just to get a `None` message at the resampling interval.
+            builder.push_component_metric(
+                NON_EXISTING_COMPONENT_ID, nones_are_zeros=True
+            )
+            _logger.warning(
+                "Unable to find any consumers in the component graph. "
+                "Subscribing to the resampling actor with a non-existing "
+                "component id, so that `0` values are sent from the formula."
+            )
+
         return builder.build()

tests/timeseries/test_logical_meter.py

@@ -240,6 +240,21 @@ async def test_consumer_power_no_grid_meter(self, mocker: MockerFixture) -> None
         await mockgrid.mock_resampler.send_meter_power([20.0, 2.0, 3.0, 4.0, 5.0])
         assert (await consumer_power_receiver.receive()).value == Power.from_watts(20.0)
 
+    async def test_consumer_power_no_grid_meter_no_consumer_meter(
+        self, mocker: MockerFixture
+    ) -> None:
+        """Test the consumer power formula without a grid meter."""
+        mockgrid = MockMicrogrid(main_meter=False)
+        mockgrid.add_batteries(2)
+        mockgrid.add_solar_inverters(2)
+        await mockgrid.start(mocker)
+
+        logical_meter = microgrid.logical_meter()
+        consumer_power_receiver = logical_meter.consumer_power.new_receiver()
+
+        await mockgrid.mock_resampler.send_non_existing_component_value()
+        assert (await consumer_power_receiver.receive()).value == Power.from_watts(0.0)
+
     async def test_producer_power(self, mocker: MockerFixture) -> None:
         """Test the producer power formula."""
         mockgrid = MockMicrogrid(main_meter=False)