The following explains the terms heating load and standard output.
Heating Load:
The heating load of a room refers to the amount of heat required to maintain a specified indoor temperature. It is expressed in watts (W). The heating load depends on several factors, including the size of the room, insulation quality, type and size of windows, outdoor temperature, and other climatic conditions.
Standard Output:
The standard output of heating surfaces indicates how much heat a specific heating element—such as a radiator or underfloor heating—can emit under standardized conditions. This output is typically expressed in watts (W).
The default standard used is 75/65, which refers to the flow and return temperatures of the heating water. A flow temperature of 75 °C is the temperature of the water as it leaves the heat source (e.g., a boiler) and enters the heating surface. The return temperature of 65 °C is the temperature of the water as it exits the heating surface and flows back to the heat source.
Therefore, the standard output at 75/65 flow and return temperatures indicates how much heat a heating surface can emit when the water enters at 75 °C and exits at 65 °C.
Source: Recknagel (2020): Handbook for Heating and Air Conditioning Technology