The smallest energy unit is joule (J) followed by British thermal unit (Btu), Calorie (kcal) and Kilowatt0hour (kWh). 1 joule is equivalent to 1 newton meter; 1 BTU is equivalent to 1055 joules; 1 calorie is equivalent to 0.0003969 BTU and 1 kWh is equivalent to 3,413 BTU.