Stefan-Boltzmann Power Calculator

How it is calculated

The tool applies P = ε × σ × A × T⁴, with σ = 5.670374×10⁻⁸ W/(m²·K⁴). ε is the emissivity, a number from 0 to 1 (1 for an ideal black body), A is the radiating surface area in square metres and T is the surface temperature in kelvin. It first finds the power per unit area, flux = ε σ T⁴ in W/m², then multiplies by the area to get watts and divides by 1000 for kilowatts. Emissivity outside 0 to 1, or a temperature or area of zero or less, gives no result. This is the power the surface emits. It is not the net heat loss, because the surroundings radiate back; for net exchange you would use the difference T⁴ − T_surroundings⁴.

Common mistakes

• Entering the temperature in °C or °F. It must be in kelvin: add 273.15 to Celsius first. Entering 33 instead of 306 shifts the result by a factor of about 7,400.
• Entering the emissivity as a percentage such as 90. The valid range is 0 to 1, so 90 gives no result; type 0.9.
• Reading the answer as heat actually lost. The tool gives the emitted power only; the surroundings send radiation back, so the net loss is smaller.
• Using the total surface area of an object when only part of it faces open space or radiates freely. Faces that touch another surface or are shielded should be left out of the area.
• Expecting a doubling of power for a doubling of temperature. Power scales with T⁴, so doubling the kelvin temperature increases the power by 16 times.