We try to give a guide on many heaters however, in reality this is very difficult as everyone has a different idea of exactly how much warmth is desirable to them. Another factor is the ambient air temperature; so a heater that seems great all summer during cool evenings, suddenly seems inadequate as the colder months draw in. As a rule of thumb, for shortwave infrared heaters, we recommend 1kW of power per 4M2 in sheltered areas and 3M2 in more exposed areas. This should be enough heat to cope for the warmer 8 months of the year.
Example: you have a sheltered patio of 3m x 4M. 3 x 4 = 12M2. Divide this by 3 or 4 and it tells you that between 3kW and 4kW is needed. This might be one big heater or perhaps 2 x 2kW heaters?