Calculating heat gain: surface, k value and temperature difference

The heat entering a box is K, surface area and temperature difference multiplied, and it comes out in watts. The result gives an order of magnitude and a starting point for plate count and duration.

27 March 2026Updated 28 September 2026

Short answer

Heat entering a box is found in watts as Q = K × S × ΔT: K is the heat transfer coefficient of the insulation (W/m²K), S the surface area of the box, ΔT the difference between the outside and the internal target. In an IR-class box K is at most 0.40, and a sum made with that upper limit shows the worse case.

The heat entering a box is the product of three values: Q = K × S × ΔT. K is the heat transfer coefficient of the insulation (W/m²K), S the surface of the box (m²), ΔT the difference between the outside and the internal target. The result comes out in watts.

In ATP, S is the geometric mean of the inside and outside surface areas of the box. The K from the box's report is used; the class limit is the upper value: 0.70 for a normally insulated (IN) box and 0.40 W/m²K for a heavily insulated (IR) one.

Input	Example
K (IR upper limit)	0.40 W/m²K
S	5 m²
ΔT (30 °C outside, 5 °C inside)	25 K
Heat gain	0.40 × 5 × 25 = 50 W

50 W is 180 kJ per hour (W × 3600 s). The sum does not see door openings, the thermal mass of the load, a surface heating in the sun or wind; it gives an order of magnitude, and plate count and duration are confirmed by measurement. Use the hottest day for the outside temperature, because an average does not hold at noon.