Inventions that take the breath away with their simplicity and elegance are rare. The new rechargeable batteries from USBCell qualify for this honour. As their name indicates, they are AA batteries that are recharged by the USB port on a laptop or other powered device. They are not cheap, but will repay the investment by being far easier to recharge than conventional rechargeable AAs. The carbon savings from these batteries are not large. My calculation is that they might save 10kg of CO2 a year in a household full of portable devices. But they will, of course, reduce the waste going into landfill.

The company that makes the batteries has won some important awards for its innovation. More importantly, it also has some extremely interesting views on the evolution of home electricity demand. It correctly points out that a larger and larger fraction of home energy is used in 12V, not 240V appliances. We waste a lot of energy switching 240V AC down to 12V DC. Its next products include a box that will allow all DC devices (phones, handheld consoles, laptops) to be efficiently charged. Eventually, it will be possible to use cheap(-ish) solar power collectors to charge all the battery DC devices in the home. The savings in carbon would be worthwhile (but probably outweighed by the purchase of one extra TV).

***

These elegant batteries are sold by Moixa, a UK company that specialises in innovations that reduce home energy consumption. They are now widely available in UK stores. People in the UK throw away 600 million batteries a year (10 per person) so the market is large enough. This means 22,000 tonnes of toxic metal is disposed of every year.

The typical UK household has about 25 devices using batteries, ranging from remote controls, torches and games consoles. Some of the batteries are barely used and last for years, others fade within a few hours of heavy use.

Conventional rechargeable batteries have never really succeeded. They need a separate recharging device, and the performance of the batteries tends to disappoint. Moixa told me that the average rechargeable battery is only actually charged 8 times before it is lost, accidentally thrown away or the recharger is broken.

Moixa reckons its USBCell batteries can be recharged 500 times and that the ease of plugging the battery into a USB connection will mean that users cherish the product. They believe that each battery, which has a power equivalent to a disposable battery of the same size, will be reused at least 50 times. This means that the cost of around £11 for two batteries will turn out to be less than 25p each time it is recharged.

The electricity cost of recharging a battery is a fraction of a penny, and doesn’t really affect the economics. A laptop is a very good converter of 240V AC into 12V DC and little energy is wasted. For heavy battery users, it clearly makes sense to switch to the new USBCells.

What are the carbon implications? Moixa told me that an Australian study had shown that a USBCell reused 50 times would save 7kg of CO2. A house throwing away 25 batteries a year would therefore only save 3.5kg a year from buying USBcell AA batteries for all of its appliances. In the context of five or six tonnes of emissions yearly from a typical house, this is a tiny fraction of the total carbon footprint.

But the company was eager to explain the broader strategy. Thirty years ago, the only devices in the house using low-voltage electricity might have been a torch and a 12V children’s train set. Now the average house has scores of little devices, though all with quite low power use. Phone chargers, powered toothbrushes, iPods, laptops, and portable lights are all growing users of home electricity.

When the electricity supply system was coming into being in the 1880s in the US, Edison’s direct current system lost out to Nikola Tesla’s alternating current. AC was far better for long distance transmission. All our major appliances are designed around 240V or 110V AC. But most new appliances don’t need much power and operate at safe and cheap 12V DC. The AC power is still probably better for appliances with large motors, such as the washing machine, but for many other home devices it would be better if the supply was 12V DC.

Moixa’s next device, which it expects to start selling in late 2008, is a box that allows tens of devices all to be charged from it. It centralises the 12V transformer, reducing energy losses to a very low level. Importantly, this box could run from the mains, or could easily be powered by a small solar panel left in daylight. The savings aren’t likely to be huge, perhaps 200 kWh a year, but the new box could be a relatively cheap way of saving 5% of the electricity bill.

Eventually, all lighting will be switched to 12V. A large fraction of the lighting in new houses is already run at low voltage, with the power to each light going through an AC to DC transformer. It would be better to run a single 12V lighting circuit with a single, and very efficient, transformer. Moixa is working on this now.