A powered valve actuator assembly converts a manually operated valve without removal of the valve handle for automated valve operation in response to detection of a preselected condition. The design incorporates a motor, motor control circuit, drive shaft, valve handle engaging member and anti-rotation member. The motor is supported by its drive shaft, which is mounted on the valve handle via an adaptor. The motor, when operated, applies rotational torque to the drive shaft, and is secured from counter rotation about the drive shaft by the anti-rotation member, which extends between the motor and a fixed reference point with respect to the valve body such as the conduit or an extension thereof. The control circuit may incorporate a monitoring sub-circuit and power regulator, which operates at reduced power levels depending on the sensed state of the sensor or sensors.