Answer
Do not justify the replacement.
Work Step by Step
$E_L=(40-18)W\times 60 h/year$
$E_L=1.32\ kWh$
Since this heat will not need to be removed, with COP=1.3
$E_R=E_L/COP$
$E_R=1.02\ kWh$
$C_s=(E_R+E_L)p,\ p=\$0.08/kWh$
$C_s=\$0.19/year$
Since the buying cost is $25, it would take 132 years for it to pay off, so it's not justified.
