Heat flux density — W/m² and BTU/(h·ft²)
W/m² is the SI. HVAC keeps BTU/(h·ft²). Solar, radiant heating and process both live on the same axis.
Heat flux density measures how fast energy crosses a surface per unit area. It appears in insulation ratings, solar power, radiant heaters and combustion analysis.
The exact factor
1 W/m² = 0.3170 BTU/(h·ft²) > 1 BTU/(h·ft²) = 3.1546 W/m² > 1 kW/m² = 316.9 BTU/(h·ft²)
Typical values
Solar (top of atmosphere): 1361 W/m². Solar at surface (clear noon): 1000 W/m². Human body: 100 W/m². Radiant floor heater: 100–200 W/m². Standard incandescent lamp bulb surface: 30 kW/m². Metal welding arc: 10 000 kW/m².
Worked example — solar array
A 20 m² array at 1000 W/m² intercepts 20 kW of raw solar flux. At 20% panel efficiency, DC output is 4 kW. Skip the flux density unit and confuse it with total incident W — you get 1000 W total (not per m²) and predict a 200 W array output, 20× too low.
Recap
W/m² is the currency of radiation and heat transfer. Multiply by 0.317 for BTU/(h·ft²).
Try the converters mentioned in this article
FAQ
Is 'irradiance' the same as heat flux?+
Same units, different context — irradiance is specifically for radiation (usually solar or thermal); heat flux includes all mechanisms.
How does emissivity affect heat flux?+
Radiative flux is proportional to ε × σ × T⁴. Emissivity ε < 1 reduces flux at a given temperature.
This article was written by The Turbo Unit Converter engineering desk (Applied metrology & engineering units) and last reviewed on 2026-07-07 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.