Solar panels are mildly more efficient when colder, the same latitude in an area with similar cloud cover in north America is probably generally slightly better for solar than Europe because it is colder, not sure if it would ever be more than a rounding error though.

> To the point where rooftop solar stops being a great option.

Perhaps as a complete energy solution. But it is already the case today that a domestic rooftop solar in Europe (maybe not in the very north) has payback times <10 years. And that's without factoring in batteries which (as the OP describes) are rapidly approaching affordability.

This is a pretty late response, but even as "not too north" as Hamburg it gets close to 10%.

Hamburg is at 53.6 degrees North; Berlin is 52.5. Both are just farther north than Calgary, as a pretty reasonable comparison for the US.

To contrast, New York City is at 40.7 degrees, and Boston is at 42.3 degrees North.

In southernmost Sweden, just above Germany, solar production is only 5% in December compared to June. In northernmost Sweden the sun doesn't even rise above the horizon for most of December.