How many kW does my solid fuel boiler need to be?

I have not used Kw as the measure of energy in such systems where fuel is being "burned" (?), so am thinking more in terms of BTU's. I have a 2700 sq/ft home and it uses a 175000 BTU gas fired furnace and nicely heats the entire house. Once upon another time, I had a 1650 sq/ft home and it used an electric furnace to heat the house. It employed 15Kw (3 5Kw strips within the forced air housing). Using these two examples, I can derive that I am using 64 BTU's per square foot currently and formerly was using 9 Kw per square foot in the other house. You might try the formula to see if your heated space (square feet in air conditioned space) times 9 Kw yields anything that sounds plausible.