Assembled piecemeal over the last 100 years or so, power-grid networks now span entire continents, with regional distribution grids stitched together through interconnects. Improving the ability of operators to balance variations in supply and demand will require the construction of new high-capacity transmission lines to share power over larger areas. Because Europe has a more uniform population density than the United States, fewer long-distance lines are required. In the United States, these lines are necessary to transfer renewable power from the windy heartland and sunny Southwest to the heavily populated coasts.