db.measurements.aggregate([
    { "$match" : { "timestamp": { "$gte": ISODate('2014-03-12 00:00:00'), "$lt": ISODate('2014-03-12 23:59:59') } } },
    { "$project": {
        "wh": 1,
        "hourly": { "$hour": "$timestamp" }
    }},
    { "$sort": { "hourly": 1 } },
    { "$group": {
       "_id": "$hourly",
       "firstReading": { "$first": "$wh" },
       "lastReading": { "$last": "$wh" }
    }},
    { "$project": {
       "hourReading": { "$subtract": [ "$lastReading" - "$firstReading" ] }
    }
])