Since the most important source of thermal energy is the Sun, the ratio of glazing to thermal mass is an important factor to consider. Various formulas have been devised to determine this. As a general rule, additional solar-exposed thermal mass needs to be applied in a ratio from 6:1 to 8:1 for any area of sun-facing (north-facing in Southern Hemisphere or south-facing in Northern Hemisphere) glazing above 7% of the total floor area. For example, a 200 m house with 20 m of sun-facing glazing has 10% of glazing by total floor area; 6 m of that glazing will require additional thermal mass. Therefore, using the 6:1 to 8:1 ratio above, an additional 36–48 m of solar-exposed thermal mass is required. The exact requirements vary from climate to climate.