In addition to what @adrianN says about the laws (which is very true, these things are extremely easy to setup, install, and even to register despite the reputation of German bureaucracy), PV is also absurdly cheap.
We've got one. As per the law, limited to 800 W. €350 (of which €50 was delivery), including the inverter and the stands. As it happens, the stands weren't too useful for us (didn't fit our balcony so the panels are now on the driveway) and we could've reduced that to €250 if we'd gone for a model without stands and had been able to pick the kit up in person rather than getting delivered.
But even at €350, assuming 10% capacity factor, €350/(800W*24h*10%*€0.3/kWh*365) = 1.665 years = 1y8m.
Even if electricity was a third of the price, even with the €350 we spent, these things would still be no-brainers because they would still pay for themselves five to seven times over in their expected lifespans. As is, 15-21 times over.
