It was the failure to fulfil one brief that set the creative wheels in motion for another.

London-based product design consultants Martin Riddiford and Jim Reeves were asked by charity SolarAid to develop a light with a photovoltaic solar panel and battery at a cost of under US$6 (S$7.60). The goal was to give people in the developing world an alternative to kerosene lamps, which are expensive, toxic, dirty and dangerous.

After racking their brains, the duo had to concede that they couldn't fulfil the task. Using solar energy after dark required storing it in battery form, making any product expensive and inefficient.

What was needed was energy that could be produced effortlessly and used instantly. And this was when the two men had their lightbulb moment.

"Realising that energy generated by gravity could replace expensive alternatives as an energy store was definitely one of those moments," Mr Reeves said.

"The idea that something as simple as a bag of stones or dirt could be what produced the tension needed to generate kinetic energy was another."

The result was the GravityLight. Weighing just under 1kg, it contains a few basic components: a main casing, a weight strap that feeds through the generator, and two bags with hooks.

When filled with sand or rocks, the bigger bag weighs in at around 10kg. Once filled, the smaller bag attaches to the other end and works as a counterweight, sending the strap through a series of small gears that convert the kinetic energy into fuel for an LED light.

The GravityLight quickly generates energy - a three-second lift of a bag gives up to 28 minutes of low light. Producing 0.1W of power, GravityLight can also run and recharge other devices such as torches and radios.

The GravityLight is set to hit the market next year, with an initial price of around US$10 per unit in the developing world.