In order to know how to choose an emergency radio, you first need to evaluate your current location. Make sure you consider the kind of emergencies which your radio will notify you with. Be aware of the specific details such as how often the power in your area goes out within your vicinity and if there are possible hazards in your location. Also, be aware if there are nearby areas which carry more weather updates.
You must also consider your needs. Search for a radio that gives constant weather-alerts and which vibrates (specially if you have problems with hearing). Do decide if you need to leave the radio powered on every time or only when you are expecting an emergency or danger.
Also available are Public Alert Receivers. This is a classification provided by the CEA or Consumer Electronics Association. As much as possible, do use radios which possess SMART technology (or Same Alert Messaging Coding). These are completely programmable in order to alert you every time there is a specific hazard within your county or city.