Pratt Center for Community Development

Planning, Building, & Educating for Change.

Energy Efficiency Calculator

Heating Degree Days: HDD is an index of fuel consumption that tracks how far the day's mean temperature fell below 65°. HDD may be calculated for a day, week, month, season or year. For example, in NYC, the average annual number of heating degree days is about 4,800. (In Barrow, Alaska, the annual HDD is about 20,000). Don't be confused by the term; HDD does not measure the number of days that we use heat. You can find current HDD data on the weather page of the New York Times. You can also find current and past HDD data for NYC and NYS on NYSERDA's website external link icon.

How am I doing? Calculate your building's energy use

As building managers and owners, you know that your electric and heating bills are going up; but, the size of the checks you write do not tell you how efficiently your building is operating. As a first step in considering how to reduce energy costs, it is useful to analyze your energy bills to quantify energy use.

Monthly energy bills are affected by price and weather as well as by the physical conditions of the building and the human factor --the way the building is lived in and managed. Therefore, the standard measure of a building's energy efficiency, BTUs (energy use) per square foot per heating degree day (HDD), evaluates energy use rather than expense, and adjusts for weather by using an index called Heating Degree Days (see text box). The equation also relates fuel use to building size, measured in square feet.

According to several studies, the least efficient buildings use about seven times more energy than the most efficient buildings. (Andrew Padian, Energy Usage: How Efficient are Your Buildings? (or the 7 to 1 solution) Energy Efficiency, September 2004.) The least efficient buildings use in the range of 35 BTUs/sq foot/HDD while the most efficient NYC buildings use about four to five BTUs/sq foot/HDD.

Where does your building fall? By entering one year of fuel bill information, and the size of your building in square feet, this calculator will generate the BTUs/Square Foot/HDD for your building. We show the steps to guide you through the process. It is preferable to use bills from July 1 to June 30 so that your heating season matches the HDD data.

Thanks to Henry Gifford and Chris Benedict, R.A.