When converting or adding solar electric, a windmill, or a water wheel, one should use DC whenever possible and AC if necessary. This was stressed
20 years ago and still holds true today. The 300,000+ for a system would typically be for a 'utility interface' system where you're still hooked to the grid. If the power goes down, your system has to go down, too, so as not to electricute a lineman attempting repairs....in essence the utility is your "battery storage", if you will. In this system the utility companies are required by law to 'buy back' any excess energy you produce...at their cost.
The lower cost systems (with batteries) are referred to as a stand-alone system, as they can operate 'alone' without the utilities.
The general rule of thumb was to provide enough power for the essentials (lighting, entertainment, ect) and the higher 'draw' items would be powered by a generator ( washing machine, ect). One would have to be conscious of their power requirements and allow for enough battery storage to get them over the hump of 'x' number of cloudy days( houses/cabins, usually plan for 3 days). RoadKingMoe is right about the amout of power one can expect from a flat mounted panel. The only way to increase the output is to have a 'tilt mount ( I think rv-solar electric has these). Of course, you could only tilt the panel(s) when parked, and then they would have to be facing south.