We run a wood fired cooker (french made rayburn ish machine) with 10kW output and another 10kW stove in our winter living/dining room. (The main sitting room is too expensive to heat, so we only use it in summer).
Notable experiences to date :
1 - Quality of wood is critical. It must be dry or efficiency is poor.
2 - Chimney needs cleaning twice a year or you risk a fire and, believe me, you do not want to see the pipe from the stove to the chimney masonry glowing cherry red! It is frightening!
3 - All (dry) wood has roughly the same calorific value per unit weight. However, as all us woodworkers know, the density is very variable and lighter wood will fill the stove with only half the weight, therefore needs filling much more often.
4 - Heavier is better, the more iron there is, the more surface area to transfer energy and the more energy is stored in the metal of the stove itself.
Watch out for output figure massage. Some quote peak output, some quote average output with a fill every 2 hours etc. It is not always a case of comparing like with like. Our Godin in the dining room is quoted as 10kW, based on 20kW peak output 15mins after a fill, then falling off to around 5kW as the fuel is expended until refilled when it rises back to 20kw.