Advice???

I'll see what others say but offhand a 20 watt panel when lit up will drive the fan flat out and charge the battery too. If the sun drops to half that then it should still run the fan but not charge the battery and if the sun goes out the battery should run the fan for (18 amps for an hour =say 1 amp for 18 hours ) 18 hours to dead flat.

So if run on the battery plan on probably 6 hours running while the battery is new and less as it dies .They all die in probably 3 years if regularly used and charged and a lot less if just spasmodic.