|Two depictions of electrical energy: the top diagram showing mechanical work being converted into light via electromagnetic induction; the bottom diagram shows sun light being converted into the rotative movement of an electric motor.|
In circa 580BC, Greek thinker Thales, noted that when amber was rubbed, straw attracted to it, and therein made the first type of observations on the properties of electricity.
In circa 1670, German engineer Otto Guericke and built his electrostatic generator (c.1670).
In 1744, the Leyden jar, a type of charge storage device, was invented in Leiden.
In 1776, Italian physician Luigi Galvani discovered the ability of circuit-induced dead twitching frog legs by led to the hypothesis of "animal electricity" to explain this phenomenon, a theory which led to the invention of the batter or voltaic pile by Italian physicist Alessandro Volta in 1800. 
In 1802, Humphry Davy passed the current of an extremely powerful battery through a thin strip of platinum, heating the filament enough to make incandescent light, wherein electrical energy was transformed into heat by passing an electric current though a resistance. 
In the 1840s, experimental mechanical equivalent of heat measurement of electrical energy was done by English scientist James Joule.
1. Fermi, Enrico. (1936). Thermodynamics. Prentice Hall.
2. Thims, Libb. (2007). Human Chemistry (Volume One) (pg. 214). Morrisville, NC: LuLu.
● Berg, Ernst J. (1907). Electrical Energy: its Generation, Transmission, and Utilization. McGraw Publishing Co.
● Electrical energy – theFreeDictionary.com.