Such radios are often designed to run on minimal or renewable power sources, such as solar.
Commonly, such radios are designed to use minimal battery power, and a common feature (especially on units sold in less-developed countries where electrical supplies may be unreliable or nonexistent) is a hand-cranked electrical generator along with a rechargeable battery to store the generated energy (the device is sometimes known as a clockwork radio).
A few, such as the Blackout Buddy from Eton Corporation, and the Adventurer from Ambient Weather are designed to recharge from wall current.
This is especially useful in areas where sudden storms, tornadoes, tsunamis or other fast-breaking emergencies can occur.
Some emergency radios are designed to also charge other devices, such as cell phones or mp3 players, but this can vary widely.