I suppose you are looking from the resource tab (top right corner) in there rum production p/h is shown after deducting the feed of your army.
so maybe your production total is 15000 PH but 14958 is being consumed by your army everyhour. Rest is 42 PH
yeah you got it right, and also when you hover mouse over rum it shows you your rum consumption as well.
Something to keep in mind. Some units like dragon units aren't worth the rum they consume.
All 4 have no higher than a def stat of 99 with prestige at level20. However they each use 4 rum per hour. Now the next def unit that uses the same rum value are mercenaries but none of those exceed 250 in defense. So in defense for instance the units you want drinking all your rum would be either fleet or armada units
