An application note from NXP on how to calculate the power dissipated by the triac:
This Application Note describes how to calculate the power dissipation for triacs and Silicon Controlled Rectifiers. Thermal calculations are also included to help the circuit designer to predict the maximum junction temperature or calculate the required heatsink thermal resistance. Four worked examples ensure that all the power and thermal questions that arise during the design process are covered.
Triacs – How to calculate power and predict Tjmax – [Link]