The obvious choice would be solar, except for the fact that at this time of year we only get around 6 hours of daylight in Copenhagen. Winter solstice was yesterday.

Wind might work, a windmill for boats might be able to give us the power needed. At least it would extend the battery time. Only problem with that is that they're quite expensive, and we didn't have much of a budget to begin with.

Maybe next year... There were so many things we'd like to have done, but didn't find the time and money for. One of them was an app or a dedicated webpage where you control the lights on the tree. Maybe even some kind of game where you play some game on the tree against other players.

Lots of possibilities, but limited time and money...