Thomas Edison and his direct current, or DC, technology lost the so-called War of the Currents to alternating current, or AC, in the 1890s after it became clear that AC was far more efficient at transmitting electricity over long distances.

Today, AC is still the standard for the electricity that comes out of our wall sockets. But DC is staging a roaring comeback in pockets of the electrical grid.

Alstom, ABB, Siemens and other conglomerates are erecting high-voltage DC grids to carry gigawatts of electricity from wind farms in remote places like western China and the North Sea to faraway cities. Companies like SAP and Facebook that operate huge data centers are using more DC to reduce waste heat. Panasonic is even talking about building eco-friendly homes that use direct current.

In a DC grid, electrons flow from a battery or power station to a home or appliance, and then continue to flow in a complete circuit back to the original source. In AC, electrons flow back and forth between generators and appliances in a precisely synchronized manner — imagine a set of interlocking canals where water continually surges back and forth but the water level at any given point stays constant.