Measure the area length and width, baseboard to baseboard.
Multiply the length by width. This gives the square footage. For an area 31 feet by 16 feet, for instance, the area is 496 square feet.
Multiply the area by 6 for homes constructed since 1980. Here the result is 2,976. Round that up to the nearest thousand and you have 3,000 watts required for the area. Multiply by 10 for homes built prior to 1980. Here, that's 4,960. Round up to get 5,000 watts required. Add 2 to the multiplication factor for chilly climates or high altitude residences. Thus, for a 496 square foot area in a newer home at high elevation multiply 496 by 8, yielding 3,968 watts, which rounds to 4,000.