Thermal energy measures the movement of atoms or molecules within a substance. It’s considered less a measurement and more a process or an amount of internal energy. Thermal energy is often associated with heat because as molecules move faster, they transfer energy as heat.
Thermal energy itself is expressed in British thermal units (Btu), calories and joules. One Btu is the amount of heat necessary to raise 1 lb. of water through 1 degree Fahrenheit. A calorie is the amount of thermal energy needed to raise the temperature of 1 gram of water by 1 degree Celsius.
Heat is widely measured in Btu’s or calories. 1 Calorie (with a big C) is 1,000 calories and measures thermal energy, which derives from the movement of molecules within a liquid, gas, or solid.