Wouldn’t it be nice if you knew how much your energy bills would be before they arrived? While you might not be able to control outside temperatures, you can use daily information about the weather to estimate your expected energy usage each month. Cooling degree days (CDD) provide you with the ability to make a comparison of this year’s temperatures with those of previous years in a way that allows you to predict increases and decreases in costs for home comfort control.
What Are Cooling Degree Days?
CDD is a unit of measure that is computed based on the average of the high and low temperatures on a given day. Sixty-five is subtracted from this average to determine the CDD for the day. A similar process is used to predict the demand for heating energy needed in cold months by subtracting the average of a day’s high and low temperatures from 65. The calculation for the heating season is referred to as heating degree days or HDD.
How to Estimate Energy Usage and Costs with Cooling Degree Days
Your newspaper or weather person may keep you informed about how the year’s temperatures relate to those of previous years. You may make simple observations about how much cooler or warmer a summer season feels. Using websites that provide cooling degree data, you can make a measurable comparison of the issue based on your city or zip code. Some agencies provide monthly information and helpful graphs for previous years. Others allow a comparison on a monthly, weekly or daily basis.
As you explore the options, you can consider those reports that are easiest for you to access and interpret. A list of statistics allows you to make numeric comparisons. A graph, on the other hand, provides feedback at a glance about the current year in comparison to recent years’ temperature variations. Using the data for a given period in two different years, you can compute the percentage of difference by dividing the larger value by the lesser value. However, some CDD sites conveniently provide these comparisons for you. You can estimate a similar percentage change in your air conditioning energy costs. You can also make comparisons from month to month, anticipating billing changes as summer temperatures climb later in the season.
Cautions in Using CDDs to Estimate Bills
While an overview of periodic weather data can be helpful in predicting your utility costs, you do need to remember that cooling energy only represents a portion of those summer bills. Approximately 50 percent of your annual energy costs are attributed to heating and cooling activity. Summer bills may include a greater percentage of HVAC costs, whereas fall bills may include little or no cooling energy usage. It may be helpful to evaluate your electricity bills for months with no CDDs to establish an estimate of your non-HVAC usage and costs. Subtract that baseline from bills for months with high CDD records to obtain a more accurate indication of how the unit of measure relates to increases or decreases in your costs.
It’s also important to note that there are several factors aside from temperatures that can affect your home’s energy usage during the summer months. Some of these include:
- Personal comfort preferences – while 65 degrees is the baseline most used for computing CDDs, you may not require cooling energy at this level. You may aim for a higher or lower baseline temperature in your home, an issue which may affect your usage levels and make percentage changes less accurate for obtaining exact energy usage amounts. While you may observe a CDD of 300 or greater for April, for example, you may not need to use your air conditioning equipment for the month.
- Changes in household activity – the needs in an active household may vary from those in a home with just a couple of residents. Children leaving for camp, time away for vacation or significant visitor activity can lead to increases or decreases in the cooling energy used regardless of outside temperatures.
- Structural changes – expansion of your home may increase cooling energy requirements, making it difficult to accurately predict billing. Improving your home’s energy efficiency through renovations of windows, the roof or your HVAC system can lead to lower utility costs, meaning that CDDs may provide a high estimate of your expected bills.
- Billing periods – for many homeowners, billing of utilities doesn’t fall at the exact beginning or end of the month. A mid-month meter reading means that your CDDs for the billing period are different than monthly CDDs given through weather sites.
- Lack of history in your home – if you are new to your home, your utility costs will be less predictable in the first couple of years. You may be able to predict one month based on a previous month’s billing. However, you will need to establish a history in the home before you can make year-by-year comparisons and estimates.
What if My Estimates and Bills Are Drastically Different?
A spike in your utility costs that is inconsistent with the CDD changes may reflect an equipment problem. Your energy performance can decline if your air conditioning system is not well-maintained. An inspection and tune-up may provide valuable insight about potential problems. The professionals at Griffith Energy Services can assist with routine maintenance service to optimize your equipment or to evaluate energy performance problems. Call our office to schedule an appointment.
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Written by Kevin Spain
