Before the days of electricity the first types of clocks were mechanically driven, with the energy required to power them from a wind up internal spring which the owner would wind up each day, if you forgot to wind it up the clock would stop and you would have to reset the time and the alarm would not go off. Electricity has been used to power these devices since the 1950’s and there are 2 kinds of electric designs, mains powered and battery powered.
When most people are looking for an electric alarm there are normally referring to mains powered clocks, many people purposely look for these as they feel more secure that they will not loose time because they are constantly on, many have backup batteries as well just in case the main electricity goes off.
Most modern electric clocks that are powered by the mains and plugged into the socket have digital displays and have a large number of features, such as radio, Ipod docking stations. The extra current required for these extra features is why they do not run off batteries.
Most of these electric alarms have larger more prominent displays, which use led’s to show the time, the led’s can come in green, red and blue and its a lot easier to read the time than say an lcd display.
With the advent of mp3 players, mobile phones, dab radio, ipods and other new devices many of the devices that people buy these days act as a dual alarm, in the sense that they can be used at night to charge up your mp3 players, be a docking station for an iPhone as well as other features such as dab or fm radio, simply battery operated alarms would not have enough power for these extra features, as many also have speakers to play music. Many of the mains powered devices can also power brighter and larger displays which is great if you want an easy to read and reliable way to tell the time.