You won't be able to do a heck of a lot with a single 120w panel but it's a start.
You need an inverter of course.
device that checks when power is below a certain point then switching the appliance to my Mains power.
I don't remember seeing such a gadget but maybe, you could certainly make one.
Most large/good inverters do something similar ie switch from inverting to mains when mains is available then switch back again when not, but you really should sync the waveforms.
Rob