54V 5A DC thru "regular" coax "TV" cable for use as a "poor man's" solar power cable?
Your voltage is high enough that the electrical codes require you to use  mains  type power cable.  This is made in mass quantity, and so, is cheap.  However there is no need for silly "solar wire".  They already make mains rated electrical wire for use outside. 

14 AWG outdoor rated cable such as UF-B should suffice.  Do not use NM. 

You can also use common THWN-2 individual wires in Conduit, either EMT or outdoor rated electrical PVC.  The PVC conduit will start to degrade from UV and turn white after a year or two in the sun. At that point, give it a quickie swiff sand, and paint it with Rustoleum white metal primer then a topcoat of your choice.  

If you want to give yourself headroom for future expansion, use 12 AWG or 10 AWG.