I burned as much as 10 cords per year in the old Jotuls, but I’m putting about 25% less thru the BK’s, so your guess is probably pretty close.
To be honest, I have not managed to have an “average” year of burning since I bought the BK’s, so I can’t say with any certainty. We have had a few freakishly warm years in a row, and last February I couldn’t burn at all, when everything melted and my yard turned to swamp (couldn’t access the stacks, until it re-froze in March).
I have a spreadsheet that tracks my oil usage and heating degree days, but until recently, I wasn’t recording my wood usage in the spreadsheet. Silly oversight, on my part.
i have a cousin who just recently decided to do an energy study on my house, and was trying to figure out what my total energy usage was (including wood consumption). He was very happy he could just take my spreadsheet, and after his analysis, we were able to come up with the conclusion that by burning wood, we are saving energy and money since our furnace is so inefficient.