Heater Rough guide
A useful guide to calculating the size you need is:

to allow about 44 watts per cubic metre of room volume.
Add another 10 percent for a large window area and another 10 to 20 percent for partial or no insulation.
If it's a lounge, multiply by 1.5. If it's a bedroom, make it 1.2, and for other areas multiply by 0.8.
Example: A well-insulated bedroom 3m x 4m x 2.4m high has a volume of 28.8 cubic metres. Multiply by 44 to get 1267 watts, and again by 1.2 to get 1520 watts (1.5kW).

Calculator
Our calculator allows you to estimate more accurately the capacity of heater you'll need to maintain a comfortable temperature.

Measure the different elements (ceiling and walls, for example) of your room as required in the calculator, and enter their size in square metres (multiply length by height or width to calculate square metres).