Hi,
I think what the OP has in mind is similar to certain commercial offerings. There are a number of websites on this topic; here is one example from Germany:
There are various objectives a user might set. For me it is to optimize use of solar power when electric rates are high. For others, it could be optimization of home battery charging and discharging.
I would love to see a push by the Home Assistant dev team for Home Energy Management. It is likely a big challenge, but with a large payoff for users.
I agree that EMHASS is a great integration for this purpose, but I think it’s best for power users. I gave up on using EMHASS myself. (There is also a Blueprint for optimizing solar excess, but I haven’t tried it yet.)*
HEMS opportunities in rental flats may be limited, unfortunately. Perhaps commercial systems combined with regulations would be the answer for flat rentals. (?)