<P> Some equipment uses human power . It may directly use mechanical power from muscles, or a generator may convert energy generated by the body into electrical power . </P> <P> Human - powered equipment consists of electrical appliances which can be powered by electricity generated by human muscle power as an alternative to conventional sources of electricity such as primary batteries and the power grid . Such devices contain electrical generators or an induction system to recharge their batteries . Separate crank - operated generators are now available to recharge battery - powered portable electronic devices such as cell phones . Others, such as mechanically powered flashlights, have the generator integrated within the device itself . </P> <P> An alternative to rechargeable batteries for electricity storage is supercapacitors, now being used in some devices such as the mechanically powered flashlight shown here . Devices that store the energy mechanically, rather than electrically, include clockwork radios with a mainspring which is wound up by a crank and turns a generator to power the radio . </P> <P> An early example of regular use of human - powered electrical equipment is in early telephone systems; current to ring the remote bell was provided by a subscriber cranking a handle on the telephone, which turned a small magneto generator . Human - powered devices are useful as emergency equipment, when natural disaster, war, or civil disturbance make regular power supplies unavailable . They have also been seen as economical for use in poor countries, where batteries may be expensive and mains power unreliable or unavailable . They are also an environmentally preferable alternative to the use of disposable batteries, which are wasteful source of energy and may introduce heavy metals into the environment . Communications is a common application for the relatively small amount of electric power that can be generated by a human turning a generator . </P>

How many watts does a human body produce