Solar heat load (solvärmelast)

Solar heat load (Swedish solvärmelast, SVL) is a measure of instantaneous solar gain through the windows of a room, expressed in W/m² of floor area. It is a thermal comfort / overheating indicator, not a daylight metric — but it shares its inputs (window geometry, orientation, glazing properties, external shading) with a daylight model, which is why it appears alongside daylight requirements in Swedish practice.

Definition

SVL is the peak solar power entering the room through the glazing, divided by the room floor area:

SVL = (Σ window area × g-value × incident solar irradiance × shading factor) / floor area

evaluated for a defined design condition (a reference date, hour and sky) during the summer half-year. The g-value (solar heat gain coefficient) of the glazing is the dominant material input; the SVL formula itself does not use light transmittance — though some schemes attach a separate LT condition alongside the SVL threshold (see Miljöbyggnad).

The room assessed is normally the least-favourable one in the building — typically a small room with a large west- or south-facing window and no external shading.

Typical target values

Voluntary schemes that use SVL set thresholds by ambition level and building type. In Miljöbyggnad 4.1, for example, residential thresholds run from ≤ 38 W/m² (Brons) down to ≤ 18 W/m² (Guld); see Miljöbyggnad for the full table and the scheme's own manual for the authoritative numbers and calculation conditions.

How LUX handles it

LUX computes daylight, not thermal load. SVL is reference-only for now; whether it is brought into scope (it reuses the same geometry and glazing inputs) is undecided.