Couple of rules of thumb - not always dead-on, but usually close: Average heat load is typically about 50% of calculated peak load Average boiler output over the course of a fire is about 75% of rated output In my case, a peak load of 30kBTU per hour and an average load of 15k. I have an Eko 25 rated at 80kBTU/hr with an average output of around 60k. Average winter day: 24 hours at 15kBTU/hr = 360,000 BTU Average winter fire: 7 hours at 60k = 420,000 BTU Where does the extra 60kBTU go? Domestic hot water and hot tub. For me, 7 hours of burn time per day works well - 7 hours heating from the boiler and 17 hours from storage. I don't have enough storage, so if I had a bigger boiler I'd be all done after a 3 hour burn and I'd have a hard time heating the house from storage for the remaining 21 hours.