The power dissipation of a phototransistor refers to the amount of power that the phototransistor can handle without overheating or getting damaged. It is measured in watts or milliwatts.

Phototransistors have a maximum power dissipation rating, often denoted as "P_d". This rating represents the maximum amount of power that the phototransistor can safely dissipate as heat without exceeding its specified operating temperature limits.

The power dissipation of a phototransistor is primarily determined by its construction, size, thermal properties, and the ambient conditions in which it operates. Exceeding the specified power dissipation can lead to the phototransistor getting too hot, which can negatively affect its performance, reliability, and potentially lead to permanent damage.

It is important to calculate or estimate the power dissipation when working with phototransistors in a circuit, based on the voltage, current, and other parameters in the circuit.