How do I properly size an air conditioner for a very sunny room?

Question

I'm trying to size a window air conditioner for a room, but having trouble deciding if the usual guidelines are a good fit.

The room in question is about 230 sq. feet, and does not have full 8 foot ceilings, as part of the room is formed by the gable roof. Based on this I would pick a 6000 BTU unit. However, the room is also roofline with black asphalt shingles, east/west/southern exposure, the long dimension of the space being southern exposure with 2 south-facing skylights. There's no natural shading, and the room is also above an uncooled garage space. I typically see a 10% fudge factor recommended for "sunny rooms", but don't have a clear idea of what constitutes a "sunny room". Would this heat load significantly exceed that factor, and by how much?

Answer 1

This page from Energy Star gives some good basic guidelines.

• If the room is heavily shaded, reduce capacity by 10 percent.
• If the room is very sunny, increase capacity by 10 percent.
• If more than two people regularly occupy the room, add 600 BTUs for each additional person.
• If the unit is used in a kitchen, increase capacity by 4,000 BTUs.
• Consider where you install the unit. If you are mounting an air conditioner near the corner of a room, look for a unit that can send the airflow in the right direction.

I'd say your room fits the "very sunny" definition.