The smallest unit of energy in the physical world is the Planck unit. This is a metric unit that is based on the reduced Planck constant. It can also be applied to mass, speed, frequency, and length.
In the physical world, atoms are the smallest units of matter, but they combine to form molecules. A molecule is a complex structure composed of at least two atoms held together by a chemical bond. Most biologically important molecules are macromolecules, such as deoxyribonucleic acid, which has instructions for its function and structure. Typically, a macromolecule is formed through polymerization.
One cubic centimeter of space contains enough energy to boil the oceans. Similarly, a kilowatt hour is the smallest unit of electrical energy in the commercial sector. Electricity usage is often quoted in units of kilowatt-hours per year. For instance, a kilowatt-hour is equivalent to 3.6 megajoules.
Another unit of energy is the British thermal unit (BTU). This is a measure of energy required to heat one gram of water to a temperature of one degree Celsius. Other examples include the watt hour, which is the amount of energy required to operate a single watt appliance for one hour. And, of course, there’s the ton of oil equivalent. These are all examples of the energy-based units that are widely used in the modern world.
If you’re interested in learning more about the Planck unit, watch this PBS video. It’ll explain everything you need to know about this metric unit.