.. just wondering where and when the universal “3W” standard power size, commonly used for bicycle generators, did it come from. With a few exceptions indeed, e.g. 2.4W Shimano and 6W-12V B&M, most of generators on the market today are rated 3W (nominal power).
Anybody knows why?

I know why.

There was an old german Law of the for traffic.
in this law, all bicycle components where rated at 15 km/h.

A 3W Bicycle Dynamo had to power the 2,4 w front bulb and the 0,6 W rear bulb (3W) at 15 km/h at full Power.

Today, with LED Equipment, it would be more interesting to know the mAmpere, that are delivered.
Because the Volt are depending on the Speed.

Very clear, thank you.