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In physics, the unit of energy in the system of natural units known as Planck units is called the Planck energy, denoted by EP.
 1.956 × 109 J  1.22 × 1019 GeV 0.5433 MWh
where c is the speed of light in a vacuum,  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  is the Planck time.
The ultra-high-energy cosmic rays 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. One Ep, on the other hand, is definitely macroscopic, approximately equaling the energy of a bolt of lightning, or two weeks' per capita consumption of electricity in the United States in 2001.[citation needed]
Even so, Ep is a meaningful quantity in particle physics when gravitation is taken into account. The Planck energy is not only the energy needed (in principle) to probe the Planck length, but is probably also the maximum possible energy that can fit into a region of that scale. A sphere 1 Planck length in diameter, containing 1 unit of Planck energy, will result in a tiny (and very hot) black hole.
Planck units are designed to normalize the physical constants , G, and c to 1. Hence given Planck units, the mass-energy equivalence E = mc˛ 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 8p. Hence writings in particle physics and physical cosmology often normalize 8pG to 1. This normalization results in the reduced Planck energy, defined as:
 0.390 × 109 J 2.43 × 1018 GeV.
See also
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