Could just be as easy as putting some food grade barrels up on the roof, running them in series, cutting a drain spout on the bottom edge of one and run it into a hose. I've used rain barrels on downspouts before and the water got hot; haven't set up at our new place yet.
OIF we left 1.5l water bottles out in the sun for hot showers and shaving, even on cold days. You can get a lot done with 1.5l of water if you plan accordingly ... 2l would be like Christmas!