A 5w panel probably wouldn't keep your cell phone charged
If you lived in the desert, it might maintain one with no load on it. If you're in Wash State, forget it.
I bought a 15w panel to replace a non-working 10w factory original. It was a big waste of money here in the northern latitudes of Mich, especially in the winter.
Anyway, you can leave it hooked up 24/7/365 without any worry and it is better than none at all.