Why a window AC needs to be so much bigger than the heat load
A window AC's BTU rating is measured cooling an 80°F room. When a temperature controller holds it down to the mid-30s, the cold coil holds far less heat and frosts up, so real capacity drops sharply. That is why the sizing charts recommend an 18,000 BTU unit for a box whose heat load is only a few thousand BTU per hour. This calculator starts from that chart and scales it up or down by how much harder your box works than the chart's ideal one.
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What changes the size the most
- The job. Kegs, cases, wort, produce, and hanging game are different product loads. Frozen storage is not this kit.
- Door openings. Every opening dumps warm, humid air into the box. A busy tap box can easily double the load of a storage cooler.
- Sun. A dark trailer roof in direct sun runs far hotter than the air. Shade the AC side and the roof if you can.
- Insulation. Going from R-25 to R-13 roughly doubles the heat coming through the walls.
- Warm product. Loading warm kegs or a warm restock adds a one-time load. Chilling product before it goes in helps more than a bigger AC.
Assumptions
This is an estimate for planning a DIY build, not an engineering design. Heat load is walls, air from the door, warm product, and lights. Nameplate BTU is that load divided by about 14% at the mid-30s (a window AC's rating is measured at 80°F; the coil frosts when you pull it down to cooler temps). That factor is set so an 8×8×8 R-25 box still lands on the familiar 18,000 BTU recommendation. Boxes smaller than a walk-in, a target at or above outdoor air, or a freezer job no longer invent an AC size. Check your AC maker's guidance and your controller's compatibility list before buying. Job list: sheet 04.