When does the SightSensor's heater turn on?

The window heater, which is powered by 24V A/C only, automatically starts when the temperature is between 5°C and 0°C  and ramps up to 20 watts (100%) by the time it gets to -20°C.

Note that you can  disable the heater from starting automatically (it's enabled by default) by changing the  Enable Heater option  in the Device page in WebConfig.