Calculating the Proper Wattage Size for Electric Baseboard Heaters
Installing electric baseboard heating is often the easiest and most effective way to add heating to a room addition or when you are converting an unheated space, such as an attic or basement. Ideally, it's always best to extend existing ductwork from your central furnace/air conditioning system, but sometimes it's just not possible to route additional ductwork. At these times, installing electric baseboard heaters is by far the easiest solution.
Baseboard Heater Basics
Baseboard heaters are mounted at the bottom of walls and are powered by electrical circuits via wiring that is usually routed through wall cavities to the main service panel. The wiring may route through a wallmounted thermostat, or the thermostats may be built into the heaters themselves. While portable baseboard heaters are available that can plug into standard wall outlets, these are best for temporary use only. For most efficient heating, it's best to install permanent baseboard heaters mounted to the wall.
Permanent heaters are available in both 120volt and 240volt models. Wherever possible, install 240volt heaters, as these are more efficient in terms of energy use.
When shopping for baseboard heaters, you will notice many different specifications, including the length of the baseboard heater, wattage, amperage, and voltage. For the sake of choosing heaters that are sufficient for the space you need to heat, it is the wattage rating that is the most critical. In general, longer baseboard heaters will produce more wattage. Here is an example from one manufacturer:
Cadet 240Volt Baseboard Heater Wattages  

30 inches  500 watts 
36 inches  750 watts 
48 inches  1000 watts 
72 inches  1500 watts 
96 inches  2000 watts 
The immediate practical question is how much wattage do you need to heat the room, and how many baseboard heaters should you install. The answer to this will involve calculating the heating needs of the space.
Calculating Heater Wattage: A Quick and Easy Method
A very simple method for determining how much total heating wattage you need can be found by calculating the square footage of the room, then multiplying this by 10 watts to produce a baseline wattage requirement.
For example, if you are heating a 12foot x 12foot bedroom, it will have 144 square feet. Multiplying this by 10 watts tells you that the necessary heater wattage for the room is 1440.
This base wattage calculation method presumes that the room uses modern construction methods with the typical wall, ceiling, and floor insulation and that it has 8ft.high ceilings. If the room differs from these specifications, it's recommended that you make the following adjustments:
 Add 25% more wattage if the ceilings are 10 feet high rather than 8 feet.
 Add 50% more wattage if the ceilings are 12 feet high rather than 8 feet
 In an older home, multiply the square footage by 12.5 watts, not 10.
 In an ultrainsulated home, multiply the square footage of the room by 7.5 watts, not 10.
For the sake of our example, let's assume that the room has normal specifications. With 144 square feet, the required wattage is 1440 watts, which means you could heat the room with a single 1500 watt baseboard heater, or two 750watt heaters.
Calculating by Length of Baseboard Heater
In this method, it's assumed that 240volt baseboard heaters typically produce about 250 watts per linear foot of length. This calculation is designed to tell you how long your baseboard heater should be.
 Begin by measuring the width and length of the room to find the square footage.
 Multiply the square footage by 9.
 Using this base wattage number, add 10% for EACH of the following, as applicable:

 Each window
 Each exterior door
 Each exterior wall
 An uninsulated space below the room
 Poorly insulated walls
 Ceiling more than 8 feet tall
The resulting number will be the total wattage required for baseboard heaters to heat the room. Now, divide by 250 to arrive at the length of baseboard heater required.
Using the same size room as described in the first calculation method, we'll assume that our 144squareft. bedroom has one window and two exterior walls but is otherwise typical. The calculation runs like this:
 144 square feet multiplied by 9 watts equals 1296 watts
 Adding 10% for a window equals 1425.6 watts
 Adding 20% for two exterior walls equals 1710.72 watts
 Dividing by 250 (the normal wattage per linear foot) equals 6.84 feet of baseboard heater required
 Rounding up, this means that 7 feet, or 84 inches, of heater is needed. Standard heaters are not available in this length, so in this case, a single 8foot or two 4foot heaters would be the likely choice.
Manufacturer's Recommended Heating Needs
It is always best to slightly oversize when selecting baseboard heaters. There is no loss of efficiency by heating with baseboard heaters that are slightly larger than minimum requirements.
Total Area of Room (sq. ft.)  Recommended Heater Rating (watts)  Electrical Circuit Size Needed (240 volts) 

100  900  15 amps 
150  1350  15 amps 
200  1800  15 amps 
300  2700  15 amps 
400  3600  20 amps 
500  4500  30 amps 
800  7200  40 amps 
1000  9000  50 amps 