The square footage of a space can be determined be measuring its length and width and multiplying the two figures together. A typical 14-foot by 12-foot room has a square footage of 168 square feet.
Heat loss from a space is important because older spaces with little or no insulation and single, rather than double, glazed windows will lose heat more quickly than newer, energy efficient spaces. It stands to reason that if more heat is lost, a more powerful wall heater is required to maintain a comfortable temperature in the space.
As a general rule of thumb, 10 watts of heat output are required to heat each square foot of a properly insulated space. So, assuming that a 14-foot by 12-foot room is above average for energy efficiency, a wall heater with an output of 1,680 watts or 1.68 kilowatts (kW) would be adequate to heat it comfortably.