Heat density is total heat per area, not a transfer rate
Area-normalised heat values describe how much energy is associated with each unit of surface area over an event, exposure, or reporting period. Building-energy summaries may express seasonal solar gains or delivered loads in kWh/m², while radiant exposure references may use MJ/m² or cal/cm².
That differs from heat flux, which is a rate such as watts per square metre. The distinction matters because a total heat quantity can only be compared directly with other total heat-per-area quantities, not with a power or rate term that still depends on time.
1 kWh/m² = 3.6 MJ/m²
A kilowatt-hour is a fixed energy quantity, so the area-normalised form converts directly into megajoules per square metre.
1 BTU/ft² ≈ 11,356.53 J/m²
Imperial area-normalised heat can be translated into SI joules per square metre through the standard BTU and square-foot conversion factors.