Understanding complex phenomena such as energy transitions, which bear technical, economic and social dimensions, requires a multi-directional approach. Expansion of the solar energy in the energy mix of a country is similarly complex, as its decentralized nature brings about a necessity of public approval and trust besides its economics. We therefore develop a combined socio-economic model, which is based on the sociodynamics framework, for the household-level adoption of photovoltaics (PV). We apply the model to the cases of German and Italian PV expansion and make a retrospective analysis regarding their dynamics, in order to identify the importance of various factors such as the profitability and the public opinion throughout their expansion timeline. We then project our model for the German PV expansion onto the near future and investigate the requirement of feed-in tariffs to maintain the expansion targets under various scenarios. Results of our projection point at the importance of the self-consumption of PV electricity; an average self-consumption ratio higher than 25% makes a phase-out of feed-in tariffs by 2030 possible, whereas 50% self consumption renders the feed-in tariff regulation obsolete even in today's economic conditions.