What is the illuminance of Tau Ceti?
You can easily convert flux to luminosity and vice versa using the spherical nature of the propagation of light. The equation you need is:

$$
L=4\pi D^{2}f
$$
Where $f$ will be your flux, and $D$ is the distance to the object, Tau Ceti in this case.

From Pijpers (2003) Tau Ceti's luminosity is $0.52\pm0.03L_{\odot}$. You should have everything you need there. $L_{\odot}$ is of course the luminosity of the Sun.