More generally, you need to be aware of the ratings and proper installation methods for the equipment and materials you are using. Nearly all PV modules today are rated for a 600V system voltage, as are most combiner boxes and common wire types. Not all breakers are, though! Higher voltage requires higher-rated and sometimes more expensive equipment as well as more careful work, but it allows longer wire runs with less copper which saves money, and it may enable you to use more efficient equipment as well. All depends on what you're trying to accomplish. If you aren't comfortable doing a 600V (or 300V, or 150V) wiring job, you need to weigh the disadvantages of lower system voltages (less flexible, more costly to wire, limited options for batteries and other equipment) against the cost of hiring someone qualified to do the work.

I don't subscribe to the idea that a rooftop system should be limited to 800W. An 800W system is not viable for grid-tied use (with modern modules you're looking at 3 modules in series which will not even give you 120V much less the required 340 or 480 to do proper grid-tied installation), and will not be cost-effective for an off-grid application unless you have a very small house, minimal power needs, and have a sunny climate year-round.

Know your needs. Know your abilities. Know your equipment. Safety first should not be in conflict with building a cost-effective system that meets your needs; if you can't figure out how to do so properly, hire someone who knows!