Operating electrical power grids requires a very careful balance between production and consumption. Long term trends are regulated on the consumption side through pricing, short term jitter is regulated on the production side through switching power plants on and off.
Traditionally, a power grid only had medium and large power plants, and they are not only connected to the power grid but also a control network. Adding many small power generators poses a problem in how you communicate and coordinate with them.
800w is the maximum being produced, and not all of it will be fed into the grid. And it is unlikely that millions of households have all installed powerplants which are all oriented in the same direction and not shaded.
Germany is 84 million people. If 1% have solar and they currently average to 400 watts (half capacity from their 800 watts because of positional and shading issues) that is 33 Gigawatts. That much power if not managed will cause problems for any grid.
Traditionally, a power grid only had medium and large power plants, and they are not only connected to the power grid but also a control network. Adding many small power generators poses a problem in how you communicate and coordinate with them.
"Connecting Solar to the Grid is Harder Than You Think" https://www.youtube.com/watch?v=7G4ipM2qjfw