Stuff like your fridge can use a fair chunk of power continuously and a small solar setup can offset that when the sun is out. Same with routers, chargers, standby devices, TVs, etc.
For a 800W setup, 4–5 kWh on a very good summer day is plausible. Over a full year, it's going to be something like 600–900 kWh depending on orientation, shading, and location. So in strong summer months you might get something like 80–120 kWh. But you won't be able to use all of that unless you have a battery.
However, A typical apartment in Germany is not using that much electricity. Roughly speaking, a one-person place might use around 160 kWh a month and a two-person place around 270 kWh. Finding a use for 20–30 kWh a month during sunny periods. That's how you get to 10%. 25% might be harder but doable if you could somehow have a battery soak up the excess power. I don't think there are a lot of plug and play solutions for that yet. But it should not be that hard to do technically.
Power in Germany is relatively expensive 160*0.40 is about 80/month for me. I pay a bit less than that because I use less power somehow. But still, that's is close to 1000 euro per year. Saving 100 per year means the whole setup would earn itself back in 2-4 years (most plug and play setups you find on amazon are between 200 and 400 euro). And depending on where you live you can actually get some of that back via subsidy. But it basically pays for itself even if you don't. Unless like me your balcony faces east and you only get a few hours of sunlight in the morning.