Degree Days

Introduction

Degree Days are the basis for a long-accepted method used to create a ‘normalized’ or common denominator in energy usage. It is not a perfect system and there are many applications where it doesn’t work at all. However, for basic heating and cooling applications, using Degree Day (DD) formulas can produce reasonable results.

In the past, Degree Days were calculated by measuring and recording the ambient temperature every hour at the top of each hour. Although some weather stations (such as at large airports) may still use this method, more often DD are calculated based on the daily high and low temperatures. In locations of extreme weather conditions, this is not as accurate.

Another system for measuring DD is called the ‘Johnson Degree Days’. The JDD takes into consideration the affects of solar radiation and wind speed by measuring the temperature inside of a black-colored container exposed to both direct sunshine and wind. Because a standard DD does not take into consideration the direct affects of the sun and wind, JDDs can be more accurate for applications that are sensitive to these factors, such as greenhouses and other kinds of commercial buildings with large expanses of glass or located in high wind areas.

Yet another common term is the ‘Modified Degree Day’ (aka: Adjusted DD). The MDD is a DD that has been factored by an arbitrary percentage. Because standard Degree Days do not take into consideration solar and internal heat gains, using a DD only can lead to over-estimating the heating cost. MDD are typically 30% to 40% lower than DD. Some Auditors will use MDD to easily estimate heating costs; a more accurate method would be to do BOTH a heat loss AND heat gain analysis and estimate the net heating costs by subtracting the heat gain from the heat loss numbers. (See Heat Loss Calculations for more details)

Heating Degree Days

Heating Degree Days (HDD) measures the average number of degrees below 65 degrees the temperature was for a given period. HDD are calculated on a daily (24 hour) basis, but can be reported for Daily, Monthly, Seasonal or some other period of time. Obviously at lower temperatures more fuel will be needed to heat the facility. During the months from approximately October through April the heating bill will be dependent on how cold the weather is. IF, other things are equal, fuel consumption will be dependent on the number of Heating Degree Days.

Graphing fuel usage and Degree Days on a spreadsheet will likely produce similarly shaped curves. Calculating BTU use per Degree Day will produce valuable data that can be used to estimate heating costs based on Degree Days. For many facilities there is not an exact linear relationship between BTU usage and Degree Days. Often the Use per DD will be LOWER at colder temperatures. There are probably several reasons for this, including:

at warmer temps – when some heat is needed but not much and not in all locations, some over-heating takes place; some people may open windows, etc.
heating equipment short-cycles and is less efficient at low load factors
humidity is higher during shoulder months, which can make it feel colder outdoors at a given temperature than at that same temperature in the middle of the winter when the air is dryer; this can cause people to turn-up the thermostat
the highest number of DD occur at night, when many buildings are unoccupied and the heating system is on set-back

Calculating Heating Degree Days

Using the Daily Average Method to calculated DD would use the following method:

Daily High Temperature = 35F
Daily Low Temperature = 18F
35 + 18 = 53 / 2 = 26.5F
65F Base – 26.5F = 38.5 (rounded to 39) DD for that day

The better hourly method would be:

0100 = 18F
0200 = 19F
0300 = 18F
0400 = 20F, etc., through Hour 2400.
Take the Sum of the 24 Hourly Reads and divide by 24 = Weighted Average Temperature
65F Base – Weighted Average Temperature = DD for that day

Cooling Degree Days

Cooling Degree Days (CDD) measures the average number of degrees above 65 degrees the temperature was for a given period. CDD are calculated on a daily (24 hour) basis, but can be reported for Daily, Monthly, Seasonal or some other period of time. For plants with air conditioning or a “temp air system” (that is a system that decreases temperature and humidity but does not provide full air conditioning), this is one factor that can be used to estimate chiller and air conditioner load. The primary impact will be increased electrical use during the summer months. Even industrial plants with just air conditioning in offices, and roof exhaust fans will see a summer load increase though it will be smaller than a plant with full air conditioning or an air tempering system.

Many office spaces actually require cooling when the outdoor temperature is above 45F, or some temperate much cooler than 65F. Other facilities with high heat gains, such as data centers, will being cooling year round. CDDs do not take into consideration humidity and direct solar gain, which can easily account for more than 50% of the cooling costs. For these reasons and many others, the use of Cooling Degree Days is much less common than Heating Degree Days.

Calculating Cooling Degree Days

The method used to calculate Cooling Degree Days is the same as with HDD.

Full Load Hours – Cooling Hours

A more common factor used in estimating cooling costs is Full Load Hours (FLH), also called Cooling Hours. FLH assumes that during the course of the cooling season, factoring all of the part-load conditions together is the same as a certain number of full-on run hours. FLH are used in VERY simple cooling calculations that ASSUME that IF the cooling system was properly sized, multiplying the ‘tons of cooling system size’ or the ‘hourly cooling system operating cost’ by the FLH will result in a estimated ‘Ton-Hours’ or ‘Annual Cost’ to operate the cooling system.

For Example: A small restaurant has 30 tons of cooling; if it is located in Michigan with 700 FLH, it could be estimated to use 30 x 700 = 21,000 ton-hours of cooling. If a ton-hour costs $0.15, then the seasonal cooling cost estimate is 21,000 x $0.15 = $3,150.

Take that same restaurant and place it further south in an area with the same peak demand (same 30 tons properly sized) but a longer season of say, 1,200 FLH. The same formula shows: 30 x 1,200 = 36,000 ton-hours x $0.15/ton-hour = $5,400 per season.

More on Cooling Factors

It can be more difficult to see a direct relationship between Cooling Degree Days and cooling costs than with heating. There are many other factors that affect electrical usage; however, if the cooling load is substantial when compared to other electrical equipment, it should be possible to see the cooling impact when graphing the electric bills against the CDDs; especially if cooling is used seasonally, and not in a year-round or near-year-round application.

Cooling is heavily influenced by solar gain, internal heat gains from people and equipment, and relative humidity. More than 50% of the cooling load can be from these sources. Therefore, it is rare that reliable cooling costs can be estimated from Cooling Degree Days or Full Load Hours alone. Careful consideration must also be given to the specific application and building design.

Sources for Degree Day Information

There are several sources for Degree Day information. Pre-internet, books were published by the government and various services charged a fee for a monthly subscription service. Today, it’s a simple matter of finding a source on-line that publishes data for the needed location. Many local television weather stations provide local data that may be more current that what is available on the government web sites.

One of the best government sites is:

National Climatic Data Center, NOAA: www.ncdc.noaa.gov/oa/ncdc.html