Yeah so depends on your cost per kWh, and usage time versus production time. My 800W of balcony panels can theoretically produce 3-4 kWh per day (they peak at about 550w of actual production), but they generally produce a lot less than that. This is because my usage during production time is both less than that and also very spiky (my power usage is often 0w or 100-200w base load, with 2kw spikes as the compressor in the refrigerator kicks on, or some other device kicks on). So the savings for me isn't 3 years payoff, it's probably more like 6. But yes, these panels will product for 25-30 years. So yeah, they can pay for themselves very quickly depending on your usage patterns.
I'm in the US, and over here we also have some other complicating factors (here we have 2 sets of breakers that are 180 degrees out of phase and so the solar panels can only feed into one half of the breakers without extra complications. I only sort of understand this, someone else can explain better), so solar panels plugged into the balcony that don't backfeed straight to the grid can only cover a subset of the usage. In Germany you have 240v power so I would assume you would hit payoff very quickly.
I'm in the US, and over here we also have some other complicating factors (here we have 2 sets of breakers that are 180 degrees out of phase and so the solar panels can only feed into one half of the breakers without extra complications. I only sort of understand this, someone else can explain better), so solar panels plugged into the balcony that don't backfeed straight to the grid can only cover a subset of the usage. In Germany you have 240v power so I would assume you would hit payoff very quickly.