[{"sensorId":101,"sensorType":"s101","BuildingId":"b101","CalcItems":[{"Ts":"101","Value":2}]},{"sensorId":102,"sensorType":"s102","BuildingId":"b102","CalcItems":[{"Ts":"102","Value":6}]},{"sensorId":101,"sensorType":"s101","BuildingId":"b101","CalcItems":[{"Ts":"101","Value":5}]},{"sensorId":102,"sensorType":"s102","BuildingId":"b102","CalcItems":[{"Ts":"102","Value":3}]},{"sensorId":103,"sensorType":"s103","BuildingId":"b103","CalcItems":[{"Ts":"103","Value":2}]}]

db.collection.aggregate([{"$unwind":"$CalcItems"},{$group:{_id:{"sensorType":"$sensorType","BuildingId":"$BuildingId","Ts":"$CalcItems.Ts"},"total":{"$sum":"$CalcItems.Value"}}},{"$project":{"_id":0,"sensorType":"$_id.sensorType","BuildingVal":"$_id.BuildingId","CalcItems.Ts":"$_id.Ts","CalcItems.Value":"$total"}}])