7/30 ~= 22 cents. Or about half a kwh per day here in Germany. Per day. I think that should be feasible with a well positioned panel. You might do better on sunny days. But if you balance that against all the lesser days, I don't think it's a strange average. All back of the envelope of course. If you get six sun hours on your 800w setup, you get almost 5 kwh. That's sort of the upper range probably. Maybe on really well positioned panels you might get 10 sun hours in the summer months. Or 8 kwh. I think few people would get that.

But an average 0.5kwh per day is fairly modest and adds up to about 7 euro per month. Probably too conservative but that was on purpose. I think the official figures project higher yields.

Since setups are capped at 800w, the percentage depends on your monthly usage. Easier to work in absolute numbers. I think most people could shave between 7 and 20 euros per month off their bills depending on how much sun they manage to get on their panels.

But this assumes that you consume the power as it's produced, right? Otherwise you need a battery as well.

I imagine, for example, that the power produced during weekdays while at work would go mostly unused.