IMAGINE A TOY boat that might fit in the palm of your hand. At mid-ship add a squat spool of sewing thread lying on its side. Scale that up about a thousand-fold and the result is the 150-metre-long Nexans Aurora. The thread in question is kilometres of high-voltage power line ready to be deployed from the aft of the ship across the sea floor. Each cable, weighing a hefty 150kg per metre and thick as a tree trunk, is a woven mix of aluminium, steel, lead and insulating material. The single stretch loaded up in a bobbin nearly 30 metres across is as heavy as the Eiffel Tower.

The ways electricity is both consumed (more of it, notably by cars) and produced (also more, increasingly through renewable sources, see chart 1) are changing. Balancing energy supply and demand is never easy, as mayhem in European gas markets has shown. It is all the more complex for electricity, which is trickier to store than not just gas, but also coal, diesel or wood chips. Renewables add more wrinkles: wind blows haphazardly; the sun can be obscured by clouds or night. As a result, most of the power that is produced has to be consumed immediately, and mostly in the place that produces it.