[{meter:"MeterA",date:"2021-05-01",hourlyConsumption:[0,0,1,1,1,2,2,2,4,4,4,4,3,3,3]},{meter:"MeterB",date:"2021-05-01",hourlyConsumption:[10,10,10,10,10,10,10,10,10,10,10,10,10,10,10]}]

db.collection.aggregate([{$group:{_id:"$date",hourlyConsumption:{$push:"$hourlyConsumption"}}},{$set:{hourlyConsumption:{$reduce:{input:"$hourlyConsumption",initialValue:[],in:{$map:{input:{$range:[0,15]},as:"h",in:{$sum:[{$arrayElemAt:["$$value","$$h"]},{$arrayElemAt:["$$this","$$h"]}]}}}}}}}])