diff --git a/BuildingSync.xsd b/BuildingSync.xsd
index e3e577e9..7d0ed81a 100644
--- a/BuildingSync.xsd
+++ b/BuildingSync.xsd
@@ -13344,6 +13344,17 @@
   </xs:complexType>
   <xs:complexType name="OnsiteStorageTransmissionGenerationSystemType">
     <xs:sequence>
+      <xs:element name="AverageAnnualOperatingHours" minOccurs="0">
+        <xs:annotation>
+          <xs:documentation>The average operating hours per year of the onsite generation system.</xs:documentation>
+        </xs:annotation>
+        <xs:simpleType>
+          <xs:restriction base="xs:decimal">
+            <xs:minInclusive value="0"/>
+            <xs:maxInclusive value="8760"/>
+          </xs:restriction>
+        </xs:simpleType>
+      </xs:element>
       <xs:element name="EnergyConversionType" minOccurs="0">
         <xs:annotation>
           <xs:documentation>Type of energy conversion provided by the system.</xs:documentation>
diff --git a/proposals/2021/Add AverageAnnualOperatingHours.md b/proposals/2021/Add AverageAnnualOperatingHours.md
new file mode 100644
index 00000000..32b2bfa2
--- /dev/null
+++ b/proposals/2021/Add AverageAnnualOperatingHours.md	
@@ -0,0 +1,50 @@
+# Add AverageAnnualOperatingHours to System
+
+## Overview
+This proposal is to add the `AverageAnnualOperatingHours` element as a child element of `OnsiteStorageTransmissionGenerationSystem` under `System`.
+
+## Justification
+The average operating hours per year of the onsite generation system.
+
+## UDFs
+Currently this is conveyed in Audit Template via: `/auc:BuildingSync/auc:Facilities/auc:Facility/auc:Sites/auc:Site/auc:Buildings/auc:Building/auc:UserDefinedFields/auc:UserDefinedField[auc:FieldName/text() = Onsite Generation Operation Average Annual Hours]/auc:FieldValue`. Our proposal is to add it as a child element of `OnsiteStorageTransmissionGenerationSystem`.
+
+## Example
+```xml
+...
+    <auc:Facilities>
+        <auc:Facility ID="F1">
+            <auc:Systems>
+                <auc:System ID="S1">
+                    <auc:OnsiteStorageTransmissionGenerationSystems>
+                        <auc:OnsiteStorageTransmissionGenerationSystem ID="O1">
+                            <auc:AverageAnnualOperatingHours>4000</auc:AverageAnnualOperatingHours>
+                        </auc:OnsiteStorageTransmissionGenerationSystem>
+                    </auc:OnsiteStorageTransmissionGenerationSystems>
+                </auc:System>
+            </auc:Systems>
+        </auc:Facility>
+    </auc:Facilities>
+...
+```
+
+## Implementation
+```xml
+...
+  <xs:complexType name="OnsiteStorageTransmissionGenerationSystemType">
+    <xs:sequence>
+      <xs:element name="AverageAnnualOperatingHours" minOccurs="0">
+        <xs:annotation>
+          <xs:documentation>The average operating hours per year of the onsite generation system.</xs:documentation>
+        </xs:annotation>
+        <xs:simpleType>
+          <xs:restriction base="xs:decimal">
+            <xs:minInclusive value="0"/>
+            <xs:maxInclusive value="8760"/>
+          </xs:restriction>
+        </xs:simpleType>
+      </xs:element>
+...
+```
+
+## References
\ No newline at end of file