diff --git a/homeassistant/components/sensor/dsmr.py b/homeassistant/components/sensor/dsmr.py
index 8a24793a15754a..6319a68b0c865f 100644
--- a/homeassistant/components/sensor/dsmr.py
+++ b/homeassistant/components/sensor/dsmr.py
@@ -351,7 +351,8 @@ async def async_update(self):
                 # Recalculate the rate
                 diff = current_reading - self._previous_reading
                 timediff = timestamp - self._previous_timestamp
-                self._state = diff / timediff * 3600
+                total_seconds = timediff.total_seconds()
+                self._state = round(float(diff) / total_seconds * 3600, 3)
 
             self._previous_reading = current_reading
             self._previous_timestamp = timestamp
diff --git a/tests/components/sensor/test_dsmr.py b/tests/components/sensor/test_dsmr.py
index 69e05df1d9297d..c2ea61e5bb4fa4 100644
--- a/tests/components/sensor/test_dsmr.py
+++ b/tests/components/sensor/test_dsmr.py
@@ -6,6 +6,7 @@
 """
 
 import asyncio
+import datetime
 from decimal import Decimal
 from unittest.mock import Mock
 
@@ -104,8 +105,8 @@ def test_derivative():
 
     entity.telegram = {
         '1.0.0': MBusObject([
-            {'value': 1551642213},
-            {'value': 745.695, 'unit': 'm3'},
+            {'value': datetime.datetime.fromtimestamp(1551642213)},
+            {'value': Decimal(745.695), 'unit': 'm3'},
         ])
     }
     yield from entity.async_update()
@@ -115,13 +116,13 @@ def test_derivative():
 
     entity.telegram = {
         '1.0.0': MBusObject([
-            {'value': 1551642543},
-            {'value': 745.698, 'unit': 'm3'},
+            {'value': datetime.datetime.fromtimestamp(1551642543)},
+            {'value': Decimal(745.698), 'unit': 'm3'},
         ])
     }
     yield from entity.async_update()
 
-    assert abs(entity.state - 0.03272) < 0.00001, \
+    assert abs(entity.state - 0.033) < 0.00001, \
         'state should be hourly usage calculated from first and second update'
 
     assert entity.unit_of_measurement == 'm3/h'