I'm planning on building a 2 story deck (about 15' X 20') with a roof composed of photo voltaic solar panels. I have to build the 2 story deck so I can get high enough in order to take advantage of sunlight for the PV panels. I'm trying to decide on what type of footing/supports to use. I was thinking of simply using 6x6 pressure treated perimeter posts sunk 3'-4' in the ground. The ground is sand. Should I just sink the posts in the sand, pack the bottom and sides with gravel for drainage, set the posts in concrete, etc. ?

Thanks!