The power is defined as the energy used divided by the time :
[tex]P= \frac{E}{t} [/tex]
where E is the energy and t the time.
In our problem, the energy used is E=0.45 kWh=450 Wh while the time is t=6 h, so the power used by the fan is
[tex]P= \frac{E}{t}= \frac{450 Wh}{6 h}=75 W [/tex]
