Cooling Degree Days


Click Graphic

Recently in a conversation with Paul Shard (Distant Shores), the question of air conditioning operating cost as a function of season and location came up.

Back when I got paid for thinking thermodynamically (and was considering building a passive solar home), I became familiar with the concept of cooling degree days (CDD). These are used to compare regional requirements for air conditioning systems.

The curves above are the 2012 CDD data for the locations shown. The lower bound of each curve is based on temperatures. The upper bound is based on Heat Index when the local temperature exceeds 80°F. Each upper bound is driven by relative humidity, and none of these include any beneficial effects from breeze. Heat Index is a measure of heat stress and has as much impact on where people set their thermostats as temperature does (although it doesn’t affect water cooled A/C run time all that much once the chilled air is dried).

Click Graphic

Click Graphic

CDD are a simple concept: subtract 65 from the daily average in °F and sum the results across a period of time, in this case a calendar month. The “65” base comes from days gone by when electricity seemed to be almost free. This chart shows the benefit of shifting the base (think thermostat setting) upward in five degree steps.

Given roughly equal Caribbean rates for KWH ($0.60 US) boats on the lee side of Grenada will likely spend about 40% more total for AC than boats in the Eastern Abacos, should they chose to use it.

BrunswickTHCInterestingly, at least for us, last year Brunswick and Deale, Md weren’t that much different. Brunswick had a warmer fall which we could have used as we ran from the cold (and attendant condensation).

This year, Brunswick, for all the heat, has been cooler than Deale!

3 responses to “Cooling Degree Days

  1. Hey Chris that’s great! And it looks like setting the thermostat up a few degrees has a very large impact. If we did have AC I would be setting it at 75 for sure… Paul Shard

  2. Interesting. Do you use your AC when you are in The Bahamas?


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