In a location where feed-in to the grid is not rewarded, it makes sense to somehow consume all the "oversupply" electricity from photovoltaics by yourself.
How can I implement this in an automated way (means, not having to switch or do something manually every day)? I am looking for solutions for the cases (1) where feed-in is not rewarded and (2) where feed-in is forbidden. (For an explanation of the different cases, see this question.)
Maybe my answer is naive, but have you thought about a battery like the Tesla Powerwall? Storing energy during daylight and using at night increases dramatically the auto-consumption rate (~98%).
For the case where feed-in is forbidden, you need a zero export mechanism (see this answer for options). In addition, since you want to consume all your electricity locally, it must include a demand management feature.
I know of the following devices that fulfill both requirements:
Circutor Dynamic Power Controller CDP-G. Includes an electricity meter, three relays to switch non-critical electrical consumers ("demand management") and, as ultima ratio to achieve zero export, a mechanism that can throttle inverters of various brands. Ca. 800 EUR. According to the datasheet, the three load-management relays can switch up to 6 A * 230 V AC = 1380 W each. (To drive even higher loads, it should be possible to connect a secondary relay to a relay output.) For each relay output, a connected power consumption can be configured so that the device can choose what device to switch on when depending on the available surplus power.
elgris ZERO EXPORT. Same mode of function as the Circutor CDP-G.
Fronius SnapINverter and Victron Venus. This combines a Fronius SnapINverter with a Victron Venus controller and a Victron smart meter. Export limiting and zero feed-in is possible with this setup (source).
And fortunately, Venus is open source software and even runs on a Raspberry Pi. So in principle, it should be possible to build a sophisticated DIY solution that can adjust local demand to consume all locally produced energy. It's not an off-the-shelf solution though, but a flexible one.
Examples of loads that can be controlled by these devices, most according to the list by Circutor:
The Honeywell Hot Water Timer, the ST9100C is an inexpensive way to send solar power to your hot water cylinder. You can switch it so that it only uses power during the day and turns off at night. It has 3 on/off periods per day so you can use the mains to heat up in the early hours of the morning if necessary.
