In physics, Planck energy, denoted by EP, is the unit of energy in the system of natural units known as Planck units.
where c is the speed of light in a vacuum, ћ is the reduced Planck's constant, and G is the gravitational constant. EP is a derived, as opposed to basic, Planck unit.
An equivalent definition is:
where tP is the Planck time.
Also:
where mP is the Planck mass.
The ultra-high-energy cosmic ray observed in 1991 had a measured energy of about 50 joules, equivalent to about 2.5×10−8 EP. Most Planck units are fantastically small and thus are unrelated to "macroscopic" phenomena (or fantastically large, as in the case of Planck temperature). Energy of 1 EP, on the other hand, is definitely macroscopic, approximately equaling the energy stored in an automobile gas tank (57.2 L of gasoline at 34.2 MJ/L of chemical energy).
Planck units are designed to normalize the physical constants G, ћ and c to 1. Hence given Planck units, the mass-energy equivalence E = mc² simplifies to E = m, so that the Planck energy and mass are numerically identical. In the equations of general relativity, G is often multiplied by 8π. Hence writings in particle physics and physical cosmology often normalize 8πG to 1. This normalization results in the reduced Planck energy, defined as: