You're right of course, the viability of the panels (system) needs to be considered. We had a feasibility study performed prior to installation and had *projected figures* presented to us to ensure that our system would return acceptable amounts of energy to justify the expense of installation. Obviously it worked out well. 

The new modern-panels will collect power even on cloudy days however full sunlight works best and being in Australia we have plenty of that! 

We can add panels, and storage also, which we may do. I think with 6 more panels we would be cost-neutral mostly however we may demolish the house at some stage and so are working through the viability of the cost of adding panels. It's all about finding the balance of cost versus benefit.