diff --git a/city_model_structure/city.py b/city_model_structure/city.py
index bb9b8a7b..cf3e78b0 100644
--- a/city_model_structure/city.py
+++ b/city_model_structure/city.py
@@ -50,6 +50,7 @@ class City:
     self._parts_consisting_buildings = None
     self._city_objects_clusters = None
     self._city_objects = None
+    self._energy_systems = None
     self._fuels = None
 
   @property
@@ -200,6 +201,10 @@ class City:
       if self._subway_entrances is None:
         self._subway_entrances = []
       self._subway_entrances.append(new_city_object)
+    elif new_city_object.type == 'energy_system':
+      if self._energy_systems is None:
+        self._energy_systems = []
+      self._energy_systems.append(new_city_object)
     else:
       raise NotImplementedError(new_city_object.type)
 
@@ -341,6 +346,14 @@ class City:
     """
     return self._parts_consisting_buildings
 
+  @property
+  def energy_systems(self) -> Union[List[EnergySystem], None]:
+    """
+  Get energy systems belonging to the city
+  :return: None or [EnergySystem]
+  """
+    return self._energy_systems
+
   @property
   def city_objects_clusters(self) -> Union[List[CityObjectsCluster], None]:
     """
diff --git a/helpers/yearly_from_daily_schedules.py b/helpers/yearly_from_daily_schedules.py
index c7c3bac5..9c8431c9 100644
--- a/helpers/yearly_from_daily_schedules.py
+++ b/helpers/yearly_from_daily_schedules.py
@@ -32,7 +32,7 @@ class YearlyFromDailySchedules:
 
     values = []
     for month in range(1, 13):
-      for day in range(1, cal.monthlen(self._year, month)+1):
+      for day in range(1, cal._monthlen(self._year, month)+1):
         week_day = cal.weekday(self._year, month, day)
         values.extend(weekly_schedules[week_day])
     yearly_schedule.type = self._daily_schedules[0].type
diff --git a/imports/geometry/obj.py b/imports/geometry/obj.py
index 5c7fdd5b..6349921b 100644
--- a/imports/geometry/obj.py
+++ b/imports/geometry/obj.py
@@ -21,8 +21,8 @@ class Obj:
     with open(path, 'r') as file:
       self._scene = trimesh.exchange.load.load(file, file_type='obj', force='scene')
     self._corners = self._scene.bounds_corners
-    _bound_corner_min = []
-    _bound_corner_max = []
+    _bound_corner_min = None
+    _bound_corner_max = None
     for corner in self._corners:
       if _bound_corner_min is None:
         _bound_corner_min = corner
diff --git a/unittests/test_exports.py b/unittests/test_exports.py
index 5b25b4e1..b4c38be1 100644
--- a/unittests/test_exports.py
+++ b/unittests/test_exports.py
@@ -45,7 +45,7 @@ class TestExports(TestCase):
       else:
         file_path = (self._example_path / 'one_building_in_kelowna.gml').resolve()
         self._complete_city = self._get_citygml(file_path)
-        ConstructionFactory('ca', self._complete_city).enrich()
+        ConstructionFactory('nrel', self._complete_city).enrich()
         UsageFactory('ca', self._complete_city).enrich()
         SchedulesFactory('comnet', self._complete_city).enrich()
         cli = 'C:\\Users\\Pilar\\PycharmProjects\\monthlyenergybalance\\tests_data\\weather\\inseldb_Summerland.cli'
diff --git a/unittests/test_usage_factory.py b/unittests/test_usage_factory.py
index 82c692a1..e3c6b7ba 100644
--- a/unittests/test_usage_factory.py
+++ b/unittests/test_usage_factory.py
@@ -8,6 +8,7 @@ from unittest import TestCase
 
 from imports.geometry_factory import GeometryFactory
 from imports.usage_factory import UsageFactory
+from imports.construction_factory import ConstructionFactory
 from imports.geometry.helpers.geometry_helper import GeometryHelper
 
 
@@ -38,7 +39,7 @@ class TestUsageFactory(TestCase):
     city = self._get_citygml(file)
     for building in city.buildings:
       building.function = GeometryHelper.pluto_to_function[building.function]
-
+    ConstructionFactory('nrel', city).enrich()
     UsageFactory('hft', city).enrich()
     for building in city.buildings:
       self.assertIsNot(len(building.usage_zones), 0, 'no building usage_zones defined')
@@ -61,7 +62,7 @@ class TestUsageFactory(TestCase):
     # case 2: CA
     file = 'one_building_in_kelowna.gml'
     city = self._get_citygml(file)
-
+    ConstructionFactory('nrel', city).enrich()
     UsageFactory('ca', city).enrich()
     for building in city.buildings:
       self.assertIsNot(len(building.usage_zones), 0, 'no building usage_zones defined')