EP is a derived, as opposed to basic, Planck unit. It is defined by:
Substituting values for the various components in this definition gives the approximate equivalent value of this unit in terms of other units of energy:
An equivalent definition is:
where tP is the Planck time.
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 2.5×10-8EP. Theoretically, the highest energy photon carries about 1 EP of energy (see Ultra-high-energy gamma ray). Most Planck units are extremely small, as in the case of Planck length or Planck time, or extremely large, as in the case of Planck temperature or Planck acceleration. For comparison, the Planck energy is approximately equal to 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, ? and c to 1 (See General relativity and the Einstein Field Equations). 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 4?. Hence writings in particle physics and physical cosmology often normalize 8?G to 1. This normalization results in the reduced Planck energy, defined as: