GravityLight was a gravity-powered lamp manufactured until 2019. It was designed by the company Deciwatt for use in developing or third-world nations, as a replacement for kerosene lamps. It uses a bag filled with rocks or earth, attached to a cord, which slowly descends similar to the weight drive in a cuckoo clock. This action powers the light for up to twenty minutes.
An early gravity light concept was developed concurrently by Clay Moulton and also by Ruphan[clarification needed] as part of his Ph.D. in applied physics from the University of Alabama in 2017. While Moulton reportedly did not develop a prototype, Wofsey[clarification needed] did develop a rudimentary prototype that used a custom-machined rare earth magnet motor with minimal gearing. Wofsey secured the www.gravitylight.com domain to disseminate the findings, however he did not pursue the gravity light as he decided the efficiency was too low to be commercially viable. The theoretical efficiency of the device is limited by taking the simple potential energy generated by raising a mass to a specified height, and then dividing it by the desired time that the lamp is to stay lit. Even a relatively large mass of 10 kilograms (22 lb), when raised to a height of 1 metre (3.3 ft), produces a maximum available energy of only about 98 joules; dividing by a desired illumination time of just 5 minutes would return a usable power of only 0.32 watts. Moreover, this would be for an unrealistic 100% conversion efficiency; that of the University of Alabama prototype was closer to 50%, which in our example would further reduce usable power to just 0.16 watts. At 5.5 operating voltage of an LED, that left only 20 milliamperes for the LED. This is sufficient to light an LED, but the available light from the LED would not likely be useful for reading or night activities. A modification to this approach was suggested where the power draw can be adjusted by the user to trade illumination brightness for illumination time.