FIDDLY cables, incompatible plugs and sockets, and the many adaptors needed to fit them all together used to be the travellers’ bane. But the USB (Universal Serial Bus) has simplified their life. Most phones and other small gadgets can charge from a simple USB cable plugged into a computer or an adaptor. Some 10 billion of them are already in use. Hotel rooms, aircraft seats, cars and new buildings increasingly come with USB sockets as a standard electrical fitting.

Now a much bigger change is looming. From 2014, a USB cable will be able to provide power to bigger electronic devices. In the long term this could change the way homes and offices use electricity, cutting costs and improving efficiency.

The man who invented the USB, Ajay Bhatt of Intel, a chipmaker, barely thought about power. His main aim was to cut the clutter and time-wasting involved in plugging things into a computer. The keyboard, mouse, speakers and so forth all required different cables, and often drivers (special bits of software) as well. The USB connection’s chief role was to help computers and devices negotiate and communicate.

Mr Bhatt did not think he was creating a new charging system. Indeed, the trickle of electricity (up to ten watts on the existing standard) is still barely enough for devices such as an iPad. Yet USB charging is now the default for phones, e-readers and other small gadgets. Some mobile-phone manufacturers are already shipping their products without a power adaptor. Ingenious inventors have eked out the slender USB power supply to run fans, tiny fridges and toy rocket-launchers.

The big change next year will be a new USB PD (Power Delivery) standard, which brings much more flexibility and ten times as much oomph: up to 100 watts. In his London office Simon Daniel, founder of Moixa, a technology company, charges his laptop from a prototype souped-up USB socket. The office lighting, which uses low-voltage LED (light-emitting diode) lamps, runs from the same circuit. So do the monitors, printers and (with some fiddling) desktops. Mains power is only for power-thirsty microwaves, kettles and the like.

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That could presage a much bigger shift, reviving the cause of direct current (DC) as the preferred way to power the growing number of low-voltage devices in homes and offices. DC has been something of a poor relation in the electrical world since it lost out to alternating current (AC) in a long-ago battle in which its champion Nikola Tesla (pictured, left) trounced Thomas Edison (right). Tesla won, among other reasons, because it was (in those days) easier to shift AC power between different voltages. It was therefore a better system for transmitting and distributing electricity.

But the tide may be turning. Turning AC into the direct current required to power transistors (the heart of all electronic equipment) is a nuisance. The usual way is through a mains adaptor. These ubiquitous little black boxes are now cheap and light. But they are often inefficient, turning power into heat. And they are dumb: they run night and day, regardless of whether the price of electricity is high or low. It would be better to have a DC network, of the kind Mr Daniel has rigged up, for all electronic devices in a home or office.

This is where USB cables come in. They carry direct current and also data. That means they can help set priorities between devices that are providing power and those that are consuming it: for example, a laptop that is charging a mobile phone. “The computer can say ‘I need to start the hard disk now, so no charging for the next ten seconds’,” says Mr Bhatt. The new standard, with variable voltage and greater power, enlarges the possibilities. So does another new feature: that power can flow in any direction.

This chimes with another advantage. A low-voltage DC network works well with solar panels. These produce DC power at variable times and in variable amounts. They are increasingly cheap, and can fit in windows or on roofs. Though solar power is tricky to feed into the AC mains grid, it is ideally suited to a low-voltage local DC network. When the sun is shining, it can help charge all your laptops, phones and other battery-powered devices.

Such a set-up would benefit an individual home or office. It works even better if the network has a biggish central battery hooked up to the mains grid, which can charge itself up at night when power is cheap. But the real prize comes when several buildings combine such DC networks. Pooling supply, demand and storage gives you the makings of a “smart grid”—an electricity supply system which is more resilient and thrifty than the existing set-up.

Emergency planners like the idea: in a power cut, devices such as phones are vital. Those trying to manage ageing power grids welcome anything that flattens the peaks in electricity consumption.

Mr Daniel’s company has already set up a dozen prototypes in Britain, including at a London theatre and in a neighbourhood in Southend-on-Sea. Another project is at Nupharo, a technological park in the Czech Republic. A conference held there this month looked at how to bring low-voltage DC power networks to people in poor countries who have scant chance of hooking up to the AC mains grid. A system that stores solar power and shares it among those needing to charge mobile phones or read at night meets a big need.

Electricity-lovers are excited. Low-voltage DC power is cheap, safe and green. Big companies are working on rejigging chips and logic to fit the new standard. The first USB PD devices will come to market in 2014, with a “big roll-out” in 2015, says Brad Saunders of Intel. Gregory Reed, of the Swanson School of Engineering at the University of Pittsburgh, calls the new USB standard a “game-changer”. Big data centres, with their huge, humming arrays of servers, are already using DC circuits. Homes and offices will follow, he says.

The shift comes just in time for the “internet of things”—the idea that devices and gadgets can talk intelligently and automatically to each other online. That will mean many millions of new bits of equipment, all needing their own power supply and means of communication. The new USB standard provides both.

Mr Bhatt, who invented it 20 years ago, is delighted. His next plan is to make the USB cable “flippable”—so that the plug fits the socket whichever way it is inserted (for now it works only one way round). That tiresome flaw is because an original design priority was to make manufacture as cheap as possible: few believed his idea would really catch on.