Are you wondering what BTU's are? Wondering why they are included and what they even mean?
What is a BTU?
To start, and simply put, a BTU is a British Thermal Unit. It is used as a unit of measurement for the heat required to raise one pound of water one degree Fahrenheit. Translated into laymen terms it is the heat output by the gas appliance. Most gas units post their potential heat output not considering the loss through operation, the vents, windows, and other insulating factors within your heated room.
How many BTU's do you need?
The average home (1300sqft) in Ontario during the cooler months would need about 10,000-20,000 BTU per hour to heat their home. In order to maintain that temperature, you would need a consistent 5,000-10,000 BTU output. The fluctuations in this heating will be adjusted due to considerations mentioned below. It would be up to you whether you like to keep your home warmer or if you happen to have bad insulation.
Considerations of what would make you want more or less BTU's.
Additional things you will want to keep in mind when selecting your fireplace are mostly regarding the conditions of your selected room. You will want to pay attention to the rooms ventilation, the insulation level, and quality of windows. If your home has modern and up-to-date insulation then you won't have to worry about getting a higher BTU rated fireplace as less heat will go further for you.