Stefan–Boltzmann law
where σ is the Stefan-Boltzmann constant, which is equal to 5.670 373(21) x 10-8 W m-2 K-4, and where R is the energy radiated per unit surface area and per unit time. T is temperature, which is measured in Kelvin scale. Although this law is accurate and helpful, it is only usable for the energy radiated by blackbodies.