Direct sun does not heat the air around an enclosure; it heats the box itself. On a bright midday, a stainless wall-mount box in full sun can run 10-20 C (18-36 F) hotter than the ambient air. A dark-painted one runs hotter still. If your drive or panel PC is rated to 45 C ambient, that margin may already be spent before it dissipates a single watt. This page gives you the arithmetic that predicts the rise, and where to spend money to reduce it.
The internal heat load calculation is a separate problem, covered step by step in the enclosure heat calculation guide. Here we deal only with the input the sun adds on top of it.
What the sun actually delivers
Peak solar irradiance at sea level on a clear day is about 1000 W per square metre of surface perpendicular to the sun. That is the number behind every first-order estimate. A real box never presents a flat plate square to the sun, so a practical rule is:
- Full sun on a vertical face near midday: 600-900 W per square metre.
- A flat top in summer at mid-latitudes: 900-1000 W per square metre.
- Morning and afternoon on a vertical face: 300-600 W per square metre.
- Overcast or shaded: a tenth or less, usually not worth modelling.
Only the sunlit surfaces absorb this. Multiply the irradiance by the surface area facing the sun and by the finish's solar absorptivity to get the absorbed watts.
Absorptivity: why the finish matters
The same box in the same sun changes temperature by tens of degrees with its finish:
| Finish | Solar absorptivity (typical) |
|---|---|
| White or light grey powder coat | 0.25-0.4 |
| Brushed 304 or 316L stainless | 0.4-0.5 |
| Medium grey or beige paint | 0.5-0.7 |
| Dark grey, black, unweathered paint | 0.8-0.9 |
These are order-of-magnitude values for a first pass; paint pigments vary and stainless finishes vary with brushing. Weathered and dirty surfaces absorb more than new ones. Remember that when a box that was fine on commissioning runs hot two years later.
Converting watts into a temperature rise
A sealed enclosure in steady state sheds heat from every surface by convection and radiation. A metal box outdoors loses roughly 5-10 W per square metre per degree Celsius of surface-to-air difference; wind improves the convection. Combine that with the absorbed solar watts and you get a usable estimate:
The divisor of 6 W per square metre per degree is a conservative mid-point for light wind. In still air use 5; in a good breeze use 8-10.
A worked example
Take a 24 x 20 x 8 in (600 x 500 x 200 mm) wall-mount box, brushed stainless, in full sun:
- Exterior area: 2 x (0.6 x 0.5) + 2 x (0.6 x 0.2) + 2 x (0.5 x 0.2) = 1.04 square metres.
- Sunlit area: top and one side, roughly 0.5 x 0.6 plus 0.6 x 0.2 = 0.42 square metres, irradiated at about 800 W per square metre.
- Absorbed: 800 x 0.42 x 0.45 (brushed stainless) = about 150 W.
- Rise: 150 / (6 x 1.04) = about 24 C (43 F) on the sunlit faces, nearer 12-15 C (22-27 F) once the whole box averages out.
The same box powder-coated dark grey at absorptivity 0.85 absorbs about 285 W instead of 150 W. The average rise roughly doubles. Finish alone is worth 10 C (18 F) or more on a real box.
The sunlit faces run hotter than the average, and that matters: the component closest to the sun-facing wall sees the worst case. Place heat-sensitive parts, batteries and displays away from the west and south walls.
Choosing the design ambient
The rise adds to the air temperature, so pick the ambient honestly. Use the site's design maximum, not the average. Weather services publish 10- and 20-year extreme maxima, and a number near the 10-year value is a sensible design ambient for equipment that must survive summers unattended. A box that is fine on a 28 C (82 F) day may be 20 degrees past a component limit on the day that beats the record.
Suppose the equipment's limit is 45 C ambient and the site design maximum is 38 C (100 F). Sun-exposed stainless in the example above has already consumed the margin. That is the decision the calculation exists to make.
Cutting the solar load before buying cooling
Work down this list; the first two items are usually enough:
- Shade it. A simple sunshield, roof extension or a south-facing wall with an awning removes most of the 800-1000 W per square metre input. It is sheet metal, not machinery, and it costs far less than active cooling.
- Choose the finish. Light powder coat or brushed stainless instead of a dark coat cuts absorbed watts by half or more. Specify the colour on the order, not as an afterthought.
- Reorient the box. A top is the worst surface; it faces the noon sun nearly square-on. On wall-mount boxes, keep the top shaded or sloped.
- Move heat-sensitive parts. Batteries, power supplies and HMI electronics belong on the shaded side. If a screen is part of the design, see the HMI enclosure planning notes for door-mounted arrangements that keep the rating intact.
Then size the cooling for the total load
Once the solar watts are added to the internal losses, you have the total heat the enclosure must reject at the design ambient. Passive cooling through the walls may still be enough. If it is not, compare the options in the enclosure cooling: fan vs air conditioner vs heat exchanger guide. A sealed box that must keep its rating rules out a plain fan, so the choice comes down to heat exchangers or closed-loop cooling, sized for the combined load.
Remember the rating does not care about temperature. A sealed stainless box rated NEMA 4X / IP66 by the manufacturer, not UL listed, keeps out hose-directed water at any temperature; cooling must happen through its walls or through accessories specified for that rating. Radiation also works the other way at night: a low-emissivity stainless box sheds less heat by radiation. That is one of the mechanisms behind the condensation problems described in the condensation in a sealed enclosure article.
Verify with a measurement
Any calculation here is a screening tool; treat the first installation as the test. Stick a surface thermocouple or infrared reading on the sunlit face and a thermocouple on a component. Log them through the hottest day of the year and compare with the estimate. If the measured rise beats your number, the box has margin; if it exceeds the component rating, you have the evidence to justify a shield, a lighter finish or a cooler before the next failure.
What to specify
When sun drives the design, put these on the order: material and finish chosen for low absorptivity, mounting orientation on the drawing, and any shield or slope built in. We build custom stainless enclosures to your dimensions and cutouts from one piece, delivered in 10 business days from drawing approval, so a shaded, light-finished box is a drawing decision, not a rework. Start from custom stainless enclosures and the stainless wall-mount and HMI lines, and cross-check the siting basics in the outdoor electrical enclosures guide. For boxes that must keep electronics alive through cold nights as well as hot days, the unheated outdoor enclosure warming article covers the other half of the year.
Questions
How many degrees does direct sun add to an outdoor enclosure?
For a mid-size stainless box in full sun at midday, expect roughly 10-20 C (18-36 F) above ambient air on the sunlit face, less averaged over the whole surface. A dark-painted box in the same sun can run 25-30 C (45-54 F) above ambient. The only number you should trust for the final design is a measurement on the real box.
Does a stainless enclosure stay cooler than a painted one in the sun?
Usually yes. Brushed 304 or 316L stainless has a solar absorptivity around 0.4-0.5, so it absorbs roughly half the incident sun. A light powder coat absorbs around 0.3-0.4, a dark coat 0.8-0.9. Stainless also has low emissivity, which slows radiative cooling at night, but during the day the low absorption wins and the surface runs cooler.
Should I shade the enclosure or spec a bigger cooler?
A sunshield is often the cheaper fix: a simple shield that keeps direct sun off the walls can remove most of the 1000 W per square metre input. If the box must sit in the open, add the solar watts to the internal heat load and size the cooling for the total. Compare fan, heat exchanger and air conditioner options once you have that number.
What ambient temperature should I use for the calculation?
Use the design high for the site, not the average summer day. Weather services publish extreme-value statistics for most locations; a 10-year or 20-year maximum is a sensible design ambient. Add the solar rise to that, then check every component's maximum rated ambient against the result, with a small margin for the days that beat the statistics.
