Double the absolute temperature of a surface and it radiates sixteen times as much power.
The Stefan–Boltzmann law: power per square metre is emissivity times 5.67×10⁻⁸ times the temperature in kelvin to the fourth, and two to the fourth is 16. A surface at 600 K radiates 7 kW per square metre at emissivity 1; at 1,200 K it radiates 118 kW.
The temperature must be absolute: doubling 20 °C is not doubling the temperature. What matters for cooling is the difference of fourth powers with the surroundings, so a surface only slightly hotter than its surroundings loses far less than the bare formula suggests, and near room temperature convection and conduction usually carry more heat than radiation does. Emissivity also changes with temperature and surface.
Estimate with the rule, then check it against the calculator that models it properly.
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