How do I calculate the wattage of my baseboard heater?

There are several factors to take into consideration when shopping for a baseboard heater. The specifications of importance are the voltage, wattage, amperage, and the length of the heater. To achieve the desired heat for the size of the room in question, you want to focus on the wattage and the length of the heater. The longer the heater, the higher the wattage and vice versa.

If you are trying to determine the heating that your room would require before you install a baseboard heater, you can calculate that as shown below:

  • You can calculate the square footage and multiply by 10 watts. This method assumes that the ceiling is 8 feet high. If it is higher than 8 feet, you multiply the square footage by 1.25. For example, a 15’ by 10’ size room with 8’ high ceilings would be 15 x 10 x 10 = 1500 W. If the ceilings are more than 8 feet high, then the required efficient wattage would be  15 x 10 x 1.25 x 10 = 1875W. However, this is assuming that it is a modern building with adequate insulation. 

If you have a heater already and you want to determine the wattage of the heater, you can do that in the following ways:

  • If you can locate the manufacturer’s service plate on your heater, this usually shows the wattage of the heater.
  • If you can only find the amperage and the voltage, then you can determine the wattage by merely multiplying these values. For example, if the heater is drawing 9 amps at 120V, multiply 9 by 120 to get 1080 watts. 
  • You can measure the length of your baseboard heater; each linear foot of the heater produces, on average, about 250 watts. If your heater is 5 feet long, then it yields about 1250 watts. Generally, for 120V and 240V heaters, 3 feet will produce 750 watts, 4 feet will produce 1000 watts, 6 feet will produce 1500 watts, and 8 feet will produce 2000 watts. 

Manufacturers have determined effective heating based on the square footage of your room as shown in the table below. 

Size of the Room (Square feet)

Recommended Heater Wattage

Amperage required (240V)

100

900

15

150

1350

15

200

1800

15

300

2700

15

400

3600

20

 

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