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.
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.