To extract hydrogen from natural compounds such as water, methane, or coal requires very large inputs of energy. Most commonly, hydrogen is produced from methane gas using heat and steam, or by electrolysis of water using large quantities of electricity. Hydrogen can be used to power rockets, cars, and engines of all types. However, the energy used to produce the hydrogen can never be fully recovered from the energy in the hydrogen. It is thus not a source of energy. It is merely a storehouse for energy – a battery.