Getting the heating capacity you need

The rule of thumb for calculating adequate heating capacity is 10W/sq. ft. (107.64W/sq. m). These calculations are based on a room with an 8 foot ceiling, R12 wall insulation, and R20 ceiling insulation.
The suggested heating capacity could then maintain room temperature at 21°C (70°F) when the outdoor temperature is at –25°C (–13°F). The heating capacity required will vary depending on the area and volume of the room, the layout and orientation of the residence, its geographic location, type of construction, insulation and airtightness levels, as well as the size, type, and orientation of the windows, etc. You may also wish to consult a specialist to assess heat loss.
It is important not to install an underpowered heating system: this will cause the unit to run continuously and its parts to wear out prematurely.