So far we have counted heat in lump sums. But through a window, heat runs as a steady rate — joules every second, which is to say watts, the same watts as any power. The machine is $P = \dfrac{kA\,\Delta T}{d}$ (read aloud: P equals k A delta-T over d): conductivity $k$ for the material, area $A$ for how much wall is leaking, $\Delta T$ for how hard the heat is pushed — and thickness $d$ underneath, because a thicker wall resists.

That placement of $d$ is the whole physics of insulation: double the thickness, halve the leak — the double-pane logic. And a field note: thickness arrives in millimetres, but the machine runs on metres. A 4 mm pane entered as 4 leaves your answer a thousand-fold wrong, in the embarrassing direction.