A fan rated in kilowatts tells you what the motor draws, not what air reaches the compost. Resistance rises with fill depth, with moisture and with any compaction, so the same fan delivers different airflow on different batches.
So judge the fan by the mass, not the label. Watch plenum pressure, watch how quickly the three temperatures converge, and watch whether the far end of the tunnel behaves like the near end. Advance a batch on verified compost condition, not because one air sensor touched a set point while the compost stayed cold.