diff --git a/custom_components/powercalc/sensors/group.py b/custom_components/powercalc/sensors/group.py
index b4fc01b5d..2449ffded 100644
--- a/custom_components/powercalc/sensors/group.py
+++ b/custom_components/powercalc/sensors/group.py
@@ -580,7 +580,10 @@ def calculate_new_state(self, member_states: list[State]) -> Decimal:
         for entity_state in member_states:
             prev_state = self._prev_state_store.get_entity_state(entity_state.entity_id)
             cur_state = self._get_state_value_in_native_unit(entity_state)
-            prev_state = self._get_state_value_in_native_unit(prev_state) or Decimal(0)
+            if prev_state:
+                prev_state = self._get_state_value_in_native_unit(prev_state)
+            else:
+                prev_state = cur_state if self.state else Decimal(0)
             self._prev_state_store.set_entity_state(
                 entity_state.entity_id, entity_state
             )
@@ -614,9 +617,12 @@ async def async_get_instance(hass: HomeAssistant) -> PreviousStateStore:
             stored_states = None
 
         if stored_states is None:
-            instance.last_states = {}
+            instance.states = {}
         else:
-            instance.states = stored_states
+            instance.states = {
+                entity_id: State.from_dict(json_state)
+                for (entity_id, json_state) in stored_states.items()
+            }
 
         instance.async_setup_dump()
 
diff --git a/tests/sensors/test_group.py b/tests/sensors/test_group.py
index 37a207a26..4079e7581 100644
--- a/tests/sensors/test_group.py
+++ b/tests/sensors/test_group.py
@@ -58,6 +58,7 @@
     SensorType,
     UnitPrefix,
 )
+from custom_components.powercalc.sensors.group import PreviousStateStore
 
 from ..common import (
     create_input_boolean,
@@ -988,7 +989,7 @@ async def test_ignore_unavailable_state(hass: HomeAssistant) -> None:
     assert hass.states.get("sensor.testgroup_power").state == "0.00"
 
 
-async def test_energy_sensor_delta_updates(hass: HomeAssistant) -> None:
+async def test_energy_sensor_delta_updates_new_sensor(hass: HomeAssistant) -> None:
     config_entry_group = MockConfigEntry(
         domain=DOMAIN,
         data={
@@ -1022,3 +1023,46 @@ async def test_energy_sensor_delta_updates(hass: HomeAssistant) -> None:
     await hass.async_block_till_done()
 
     assert hass.states.get("sensor.testgroup_energy").state == "5.3000"
+
+
+async def test_energy_sensor_delta_updates_existing_sensor(hass: HomeAssistant) -> None:
+    config_entry_group = MockConfigEntry(
+        domain=DOMAIN,
+        data={
+            CONF_SENSOR_TYPE: SensorType.GROUP,
+            CONF_NAME: "TestGroup",
+            CONF_GROUP_ENERGY_ENTITIES: ["sensor.a_energy", "sensor.b_energy"],
+        },
+    )
+    config_entry_group.add_to_hass(hass)
+    assert await hass.config_entries.async_setup(config_entry_group.entry_id)
+    await hass.async_block_till_done()
+
+    hass.states.async_set("sensor.testgroup_energy", "5.00")
+    await hass.async_block_till_done()
+
+    hass.states.async_set("sensor.a_energy", "2.00")
+    hass.states.async_set("sensor.b_energy", "3.00")
+    await hass.async_block_till_done()
+
+    assert hass.states.get("sensor.testgroup_energy").state == "5.0000"
+
+    hass.states.async_set("sensor.a_energy", "2.50")
+    await hass.async_block_till_done()
+
+    assert hass.states.get("sensor.testgroup_energy").state == "5.5000"
+
+
+async def test_storage(hass: HomeAssistant):
+    state = State("sensor.dummy_power", "20.00")
+
+    store = PreviousStateStore(hass)
+    store.set_entity_state("sensor.dummy", state)
+    await store.persist_states()
+    await hass.async_block_till_done()
+
+    # Retrieving singleton instance should retrieve and decode persisted states
+    store: PreviousStateStore = await PreviousStateStore.async_get_instance(hass)
+    store_state = store.get_entity_state("sensor.dummy")
+
+    assert state == store_state