Presumably because Sweden is selling some of that cheap power to Germany.

The solution to which is to generate even more power in Sweden (so you can sell it off cheap and have it cheap too) or that Germany produces power more cheaply so that it's not giving Sweden so much money for electricity. Both of these should happen if the market is set up well.

In that case it sounds like "unification was bad" is an unfair characterization. Unification was bad by proxy, due to the bad decisions of Germany. If Germany had made better decisions, unification would've been good as Sweden would've had lower prices on a larger market.

”Bad or good by proxy” is how all policy plays out though, your ideas mean nothing if reality says otherwise. And Germany going coal was well known by time of unification (one might think it was because of that, tinfoil hat on).

Personally, I try to not think of the world in binary terms. I don't find "unification bad" useful.

That has nothing to do with the EU, it's just capitalism. Even if the EU wouldn't exist, the energy companies would have found a way to sell the cheap Swedish hydropower to Germany.