The heat output of the radiator is the most important purchasing consideration, but it's also the most complicated because the output value of the radiator is largely controlled by your home's heating systems. You can find details about your boiler by going to the manufacturers website or reading the manual provided with the boiler. However, your gas engineer is the one who will have configured the boiler to its flow temperature (which impacts the delta heat output).

The delta heat output calculation of a radiator is:
(System Flow Temperature + System Return Temperature) / 2 - Room Temperature (20°C).
For example:
Flow 75°C + Return 65°C = 140°C / 2 = 70°C - Room 20°C = Delta T(Δ)50.