Not sure you can calculate it by the hour.
You are supposed to allow about 500 extra calories per day in your diet, so maybe time your feeds multiplied by how many to get a total time for the day and figure out from there?
So let's say you feed 6x 20 minutes or approx 2 hours.
That would give a rough 250 calories per hour.
That's pretty rough, though - really it would differ (quite a lot) with the individual.