You will need to measure the size of the room, i.e. the floor area in square metres and ceiling height, before deciding which model you need. If the room is very big, you may need several heaters to spread the heat evenly between each source. In mild climates, use approx. 60-70 watts per square metres for calculating requirement, but up to 70-85 watts per square metres should be the benchmark used for colder climates.
The age of your home and how well insulated it is will also influence heating requirement. Fewer watts per square metres are required for newer houses and apartments with heat inflow, whilst older and less well-insulated homes will require proportionately more watts per square metres.
Avoid cold draughts
Even though modern windows are airtight, they do not usually insulate as well as walls. The air inside them will cool rapidly and sink towards the floor. This is called a cold draught, and can be avoided by installing a panel heater or wall strip panel heater under each window.
Go for more watts than you need
Once you have found how many watts you need, avoid buying a heater that just meets that need. Buy one that gives you a little extra capacity, so that it does not need to run at full power. The surface temperature of the heater will also be lower.