[{"id":"DE","sites":[{"name":"paper_factory","energy_consumption":1000},{"name":"chair_factory","energy_consumption":2000}]},{"id":"FR","sites":[{"name":"pizza_factory","energy_consumption":3000},{"name":"foo_factory","energy_consumption":4000}]}]

db.collection.aggregate([{"$set":{"sites":{"$map":{"input":"$sites","in":{"$setField":{"field":"new_field","input":"$$this","value":"$$this.energy_consumption"}}}}}}])