Heat Flux Converter: W/m², BTU/(hr·ft²), kcal/(hr·m²)

Heat flux (also called heat flux density or surface heat flux) is the rate of heat energy transfer per unit area, measured in watts per square metre (W/m²). Heat transfer rate (or thermal power) is simply the total power flowing through a surface, measured in watts (W) without accounting for area. Heat flux equals total heat transfer rate divided by the area through which the heat is flowing. For example, if 500 W flows through a 2 m² wall, the heat flux is 250 W/m². Heat flux is more useful for comparing materials and surfaces, independent of size.

The SI unit for heat flux density is the watt per square metre (W/m²). It is a derived unit: watts are joules per second (J/s), so W/m² equals joules per second per square metre (J/(s·m²)). In practice, multiples such as kW/m² (kilowatts per square metre) and MW/m² (megawatts per square metre) are common in industrial and nuclear engineering contexts where heat fluxes are very high. The NIST and ASHRAE standards use W/m² as the base unit for heat flux in building and thermal engineering.

The solar constant is the total solar irradiance at the mean Earth-Sun distance (1 AU) measured above the atmosphere. The current best value, as published by NASA and the WMO, is 1,361 W/m² (as of 2019 satellite measurements from the SORCE mission). This value varies by about plus or minus 0.1% with the solar cycle. At Earth's surface under clear skies at solar noon, the peak irradiance is about 1,000 W/m² due to atmospheric absorption. The global average solar irradiance reaching the surface, accounting for night and clouds, is approximately 340 W/m².

In building science, U-value (or thermal transmittance) is the heat flux per degree of temperature difference across a building element, measured in W/(m²K). A lower U-value means better insulation: less heat flows per unit area per degree of temperature difference. For example, a wall with U-value 0.25 W/(m²K) with a 20-degree temperature difference between inside and outside will lose heat at a rate of 5 W/m² (0.25 times 20). ASHRAE Standard 90.1 in the United States and the UK Building Regulations both specify maximum U-values for new construction.

The Stefan-Boltzmann law states that the total radiant heat flux from a blackbody is proportional to the fourth power of its absolute temperature: q = sigma times T to the fourth power. The Stefan-Boltzmann constant sigma equals 5.670374419 x 10^-8 W/(m²K^4), as defined by NIST from fundamental constants. For a real surface, the actual emission is multiplied by the emissivity (a number between 0 and 1). At room temperature (293 K), a blackbody emits about 418 W/m². At the Sun's surface temperature of about 5,778 K, it emits about 63.3 MW/m².