automatic water saving toilet

[NiHaoMike]

"(Proper circuit design can ensure the controller standby losses are negligible.) "

Controls ?

And when the electricity goes out ?

Repeat after me....A toilet should not have a P.L.C. on it,
nor electricity wired to it.

Unless it uses recycled water, in which case there will be a pump to fill the toilet tank from a larger storage tank. And since the pump will most likely be electric, wouldn't you need a controller to control its operation?
Something like the following, but with more features:
12 Volt Toilet Tank Refiller
It is straightforward to add a second water level sensor and a weight sensor. Don't even need any fancy logic circuits for it.

If it uses tap water that is already under pressure, an electronic control would indeed be overkill. But it is possible to adjust water level based on weight using purely mechanical methods.