The Stefan-Boltzmann Law
The total energy radiated by an object per unit time is proportional to the fourth power of its absolute temperature T (in Kelvin, 0°C≈273K):
where A is the surface area of the object, σ is the Stefan-Boltzmann constant, and ε is the emissivity of the object's material. This is known as the Stefan-Boltzmann Law.
The emissivities of materials vary. If the object is a perfect black body, ε=1. Metals tend to have a very low emissivity. Water and paper, on the other hand, have high emissivity (0.95).