BTU (British Thermal Unit) is merely a formula for measuring heat. Different climate zones will have different heat requirements per square footage ranges. The Pacific Northwest is considered a moderate temperature zone. A general guideline for BTU requirements in our zone, is as follows:
20,000 BTU = 500-750 square feet
30,000 BTU = 1,200-1,400 square feet
40,000 BTU = 1,800-2,000 square feet
These numbers are based on appliances 75%- 80% in efficiency, and are based on averages. Other considerations should be insulation levels of home, amount and quality of windows, and layout of home. An on-site evaluation, by a hearth product professional is recommended, before your purchase, to insure that your stove is properly sized for your home and needs.