fix derived rate, fixes #20097 (#21620)

* fix derived rate, fixes #20097

* fix derived rate, fixes #20097

* Fix typo

thnx @amelchio

* Make the test more realistic

Took values from my own smart meter for the test

* Update test to ignore rounding issues

homeassistant/components/sensor/dsmr.py

@@ -350,7 +350,8 @@ class DerivativeDSMREntity(DSMREntity):
             else:
                 # Recalculate the rate
                 diff = current_reading - self._previous_reading
-                self._state = diff
+                timediff = timestamp - self._previous_timestamp
+                self._state = diff / timediff * 3600
 
             self._previous_reading = current_reading
             self._previous_timestamp = timestamp

tests/components/sensor/test_dsmr.py

@@ -104,8 +104,8 @@ def test_derivative():
 
     entity.telegram = {
         '1.0.0': MBusObject([
-            {'value': 1},
-            {'value': 1, 'unit': 'm3'},
+            {'value': 1551642213},
+            {'value': 745.695, 'unit': 'm3'},
         ])
     }
     yield from entity.async_update()
@@ -115,14 +115,14 @@ def test_derivative():
 
     entity.telegram = {
         '1.0.0': MBusObject([
-            {'value': 2},
-            {'value': 2, 'unit': 'm3'},
+            {'value': 1551642543},
+            {'value': 745.698, 'unit': 'm3'},
         ])
     }
     yield from entity.async_update()
 
-    assert entity.state == 1, \
-        'state should be difference between first and second update'
+    assert abs(entity.state - 0.03272) < 0.00001, \
+        'state should be hourly usage calculated from first and second update'
 
     assert entity.unit_of_measurement == 'm3/h'