The 4.75 kBtu/ft²/yr requirement is the mathematical conversion of 15 kWh/m²a, which is the fundamental energy limit established by the Passive House Institute (PHI) through a combination of physics-based constraints and cost optimization during the late 1980s and early 1990s. 

The standard was not chosen arbitrarily; it was derived from the maximum amount of heat that can be delivered through a standard ventilation system’s supply air. 

The 10 W/m² limit: To maintain air quality, there is a limit to how hot supply air can be before it starts to smell of “burnt dust.” This limit restricts the heating capacity to approximately 10 watts per square meter of floor area.

The annual result: In the central-European climate where the standard originated, a building that never requires more than 10 W/m² of peak heating power will typically consume no more than 15 kWh/m² of energy over an entire year.

The goal: By staying under this threshold, a building can completely eliminate traditional heating systems like radiators or underfloor pipes, using only the fresh air ventilation system for warmth. This “passive” approach provides the standard its name.

The originators, including Dr. Wolfgang Feist, sought a sweet spot in which the extra cost of high-performance windows and insulation was offset by the savings from removing the furnace and boiler. The 15 kWh/m²a (4.75 kBtu/ft²/yr) limit was identified as the threshold where these capital costs balanced out, making ultra-high efficiency economically viable:

15 kWh/m²a × 3.412 (kBtus per kWh) ÷ 10.76 (sq.ft. per m²) ≈ 4.75 kBtu/ft²/yr.