that's a very hard question to answer but ill try , I hooked my 400 HE late last year ( December) , I bought it specifically for my greenhouse and bird aviary , I was using a rural king wood furnace and had to fill it up every 4 hours , still had wide temp swings and uneven heating , I hooked up my 400 HE and the temp was a stable 55 in the greenhouse and no temp swings and more importantly no excessive moisture , better yet was the fill times , I could fill it at 1000 pm and come back out at 700 am and still burning good , so that was a win anyway. I probably burnt a little more wood that I did with the rural king furnace but I am happy with the way it worked so far , I did run out of wood so had to burn a lot of green and a lot of old stuff that was laying in the woods for the last month
next season ill have the house hooked up, 5800 sq ft , I don't expect to heat the whole thing but supplement as much as possible. Ill also be better prepared with wood , cutting now for next year, I would suspect what you burn ( what type wood and seasoned or not ) will make a difference also ,
bottom line is I am very happy with it and I don't have to get up at 3 am to fill it up it was worth every penny
the last 3 years we lost a lot of stuff in the greenhouse due to freezing, lost NOTHING this year , again worth every penny
if I'm comparing it to the rural king furnace it was very very efficient , but that's not a real comparison , my greenhouse is 1500 sq ft with nothing but 2 layers of 6 mil plastic, so no R value to speak of , temps go down to the 20's at night highs 35- 40 most of the winter
hope that helps