Why is this number useful? “Heating degree days” is related to the average outside air temperature for the billing period.   It makes sense that the colder it is, the more energy your building uses to keep you warm.   A particularly mild winter?   You use less energy for heating.   If you are trying to understand whether changes in energy use come from changes in outside air temperature, your operations or equipment changes, or some combination, you need to know about outside air temperature.   And of course, as an Energy Steward you want to assess whether you are making progress in using energy more intelligently, no matter what the outside air temperature!

Do you want the science, with some formulas, that connects heating a building and temperature, expressed in degree days?   Take a look at David Mackay’s explanation in technical Chapter E of Sustainable Energy: Without the Hot Air, the book described in this recent post.

Even without the science, you can see how degree days can help us understand building performance.  Let’s start with the arithmetic for “heating degree days”:

1. Pick a reference temperature (the typical temperature is 65 degrees F in the U.S.  We’ll use this number).

2. For each day in a time period, take the difference between 65 and the average daily temperature.

Example:  On March 14 in Madison, WI, the average daily temperature was 53 degrees.  So the heating degree days associated with March 14 is 65-53 = 12 heating degree days.

What if the temperature for a day on average is above 65?  For a simple heating system in a simple building, you won’t need to heat the building much.   So people agree that the heating degree day should be zero any time the average daily temperature is 65 or above.

Do you want the heating degree days for the 33 days of a utility bill?  You need to know the average daily temperature for each day, compared to 65 degrees, and then add those differences up to get a total for the month.

Many utilities save you the trouble of doing this arithmetic by giving you the total heating degree days on the bill.   This is a nice service, since many utility bills don’t match exact calendar month periods and the number of days in the billing cycle may change, month to month.

With heating degree days and energy use, what’s the connection?  In many buildings, there is often a surprising, simple relationship.

Here’s an example for a 134,000 square foot middle school in southern Wisconsin.  We used data from the utility from January 2010 through April 2012.  Wow!  We see roughly a straight line relationship between therms and HDD!

So what?   The simple relationship means that you can predict energy use—for example, for a time period with 1000 HDD, we predict the building will use a bit more than 6000 therms, perhaps about 6300 therms.   Of course, the prediction depends on the future being about the same as the past, in terms of hours of operation, the patterns of equipment use, and the mechanical details of the heating system.  Let’s call this past set of conditions the “baseline conditions.”   If you change the school to use energy more efficiently, you can compare the predicted use given baseline conditions to the actual use and our comparison has accounted for changes in outside air temperature.    If the actual value is less than the predicted value by more than the scatter of the baseline points about the line, that’s a pretty good sign that, accounting for weather, energy use is less.

Extensions

1. If you know how to get an equation from a plot using a spreadsheet or other tool, you can plug in different HDD values to get different predicted therms.   You can fit relationships more complicated than a straight line, too.   The equation that matches the straight line shown in the middle school example is Therms = 2.69 x Heating Degree Days.
2. Buildings also use energy to cool them off, so you shouldn’t be surprised that people have invented “cooling degree days.”     You still need a reference temperature but now the arithmetic focuses on temperatures above the reference temperature.    Example:  On August 20 in Madison, WI, the average daily temperature was 77 degrees.  What are the cooling degree days based on a 65 deg F reference associated with August 20?  12: because 77 – 65 = 12
3. ENERGY STAR uses degree day information but looks at temperature totals 12 months at a time in reporting ENERGY STAR ratings and weather adjusted energy intensity numbers.   We like monthly data to get a basis for simple monthly predictions.
4. If you have daily meter readings, you can use the same approach:  now plot daily energy use vs daily HDD (or equivalently, the daily outside air temperature.)   You can get daily temperature from a building control system equipped with outside air sensor or from a nearby weather station  www.weather.gov/climate/  is the portal to access reports from NOAA weather stations.  There are also database services that for a modest fee will provide you tables with climate information for thousands of locations around the country.
5. Engineers use the same ideas as I’ve described above to model energy use in buildings.   They may start with engineering formulas (again, check out the MacKay book, chapter E) or use more factors than HDD to predict energy use.   In fact, ENERGY STAR calculates weather adjusted energy use by the same approach we sketched.
6. In Energy Stewards®, you have the option to enter heating degree days and cooling degree days when entering meter records.   These values are stored and available for analysis when you download records as a CSV file on your detailed energy use page.

The author has made his book available free for download from his website, www.withouthotair.com.

Mackay believes that understanding our energy requires numbers and he presents the energy numbers in an accessible and inviting way.  Indeed, “…the aim of this book is to help you figure out the numbers and do the arithmetic so that you can evaluate policies; and to lay a factual understanding so that you can see which proposals add up.”[emphasis in the original text], p. 16.

If you don’t already know about the book, I encourage you to go get it and read it so you can build your own understanding of future energy resources and use.

Let us know what you think of the book, too!

UPDATE, May 1:  I have finished all but a couple of technical chapters.   MacKay is a master at synthesizing and presenting numbers and science related to sustainable energy on a country and planet scale.  He organized his book to sequester the more challenging science and math in those technical chapters, which should be accessible to high school students and others willing to apply themselves a bit.

The 1-100 scale is a yardstick developed from a surveyed set of facilities of a specific type.

Each 1-100 yardstick is FIXED and does not vary by the number of buildings tracked in Portfolio Manager.

Most* of the 15 types of facilities for which there is a rating use the Commercial Building Energy Consumption Survey (CBECS) managed by the federal Energy Information Agency.  The most recent CBECS survey used by ENERGY STAR is based on 2003 data.

Usually, CBECS is conducted every four years but a technical glitch caused the 2007 survey to be set aside; the 2011 survey got caught in budget cuts.   The good news is that there is a survey planned that will use 2012 data, to be conducted in early 2013.   The data will then be available to ENERGY STAR next year to redo the older yardsticks.

For each building type, ENERGY STAR creates a separate yardstick.   EPA staff and consultants look at energy use for each building.  They also look at a set of factors that influence energy use like weather, size and additional building attributes, e.g. hours of operation and numbers of PCs for office buildings.  The technical wizards at EPA take all this information and create a statistical model that expresses the energy in terms of the factors.  Next, they rank the buildings in the survey from lowest energy use to highest.    Finally, they map those ranks into a 1-100 scale in a sensible way.  For example, the 50 point rating value on the scale maps to the middle (median) of the set of ranked buildings.

If you’re interested in the technical details, this link describes the general method.   The ENERGY STAR site also has a separate document with statistical details for each building type; the documents are hard to find.   The easiest way we know is to enter “Technical methodology” in the ENERGY STAR page search box.   You will then find a series a pdf files with address of the form http://www.energystar.gov/ia/business/evaluate_performance/XXX, where XXX will be a distinct pdf file name for each of the 15 building types.

As the yardsticks age, they become less useful for making statements about the absolute performance of a building, relative to all buildings of its type.  Building standards, equipment, and practices do change in important ways over 10+ years.

Older yardsticks still are useful for tracking relative progress over time and comparing batches of buildings because the yardsticks themselves don’t vary.

So, keep using the ENERGY STAR ratings in your energy management program and look for several revised yardsticks sometime late in 2013!

(*A few of the scales use special surveys and have newer yardsticks.  Scales for hospitals, senior care facilities, data centers and waste-water treatment plants were developed over the last couple of years  through partnerships between EPA and interested third parties like the American Society of Healthcare Engineering (ASHE).)

First English participates in the Energy Stewards Initiative of Lutherans Restoring Creation — a national grassroots program dedicated to encouraging ELCA congregations to care for creation. Twelve ELCA congregations are currently participating in the initiative. The pilot program is using the Energy Stewards platform to effectively network participants from across the country.

David Rhoads, a leader of the program, tells Living Lutheran that a goal of Lutherans Restoring Creation is to incorporate the care for creation into the full life of a congregation — from worship to taking responsibility for buildings and grounds, from lifestyle of members at home and work to education about energy efficiency and more. David is director of Lutherans Restoring Creation and professor of New Testament at the Lutheran School of Theology at Chicago.

“The ELCA has a strong interest in charting the progress of congregations in reducing the use and cost of energy for their buildings,” says David, adding that a primary goal of the initiative is to help congregations reduce, on average, at least 5 percent of the energy used per year.

“Many congregations are suffering to maintain older and inefficient buildings,” he says. In his experience, “congregations have a desire to receive practical advice on these matters and want to be able to monitor and report progress in energy reduction.”

The program is half-way through its first year. It includes a series of six webinars.

Let’s take a look at the electric and gas use charts from an office building in Madison, WI.

These graphs show a very strong seasonal pattern (for electricity–high in summer, low in winter; for gas, high in winter, low in summer).  It looks like the seasonal peaks (the summer highs for electricity use and the winter highs for natural gas use) are coming down over three years.   That’s a good thing.  Still,  the monthly values jump around a little bit, even if you keep the seasonal pattern in mind.   Is there really improvement in energy use?

A skeptic might wonder whether the apparent improvements stem from milder weather or a decrease in hours of operation.

ENERGY STAR® ratings, graphed month by month, provide an answer. Energy Stewards communicates with ENERGY STAR and captures your building’s ENERGY STAR® ratings.   Energy Stewards then graphs the ratings, month by month, as well as the 75 level cut point to achieve an ENERGY STAR award.   Here’s a graph of ratings of the same office building:

This graph tells a simple story:  steady improvement in energy performance over the past three years; the building is very close to the 75 rating, marked by the green line.

Remember that each month’s ENERGY STAR rating totals up all the energy used by the building over the previous year, and then adjusts for weather and attributes specific to the building type.  For example, an office building needs to track size in square feet, the number of employees, the number of hours of operation and the number of personal computers in use.  If any of those attributes change during the year, ENERGY STAR takes those changes into account.

Of course, if you or your group administrator don’t keep your building’s information up to date in ENERGY STAR Portfolio Manager, the ENERGY STAR ratings won’t give you the appropriate message.

So, keep an eye on ENERGY STAR ratings over time displayed on the Detailed Energy Use page. And keep your building attributes current.  You’ll have a solid basis for judging energy performance.

WHAT IF YOUR BUILDING IS NOT RATED?  As of 31 March 2012, ENERGY STAR provides ratings for 15 types of facilities.  If your building is not rated, Energy Stewards plots monthly weather-adjusted energy intensity.  This number is a simplified version of an ENERGY STAR rating–it only takes into account weather and square feet of the building. Like the rating, the weather-adjusted energy intensity is a rolling 12 month total.  Here’s an example of this measure for a building that is mostly manufacturing and repair shop:

The impression again is of a big improvement in energy use.  Now, it could be that the facility had a drop-off in business in 2010 and 2011, thus accounting for the decrease.  We asked that question–and in this case, their business did not drop off, they really did decrease energy use.

REMINDER ABOUT ENERGY STEWARDS GRAPH SYMBOLS  What are those green triangles on the graph?  Remember that the triangles represent actions taken by the building users and facilities staff to reduce energy use.   If the actions are effective, we expect to see actions line up in time with reduced energy use, like we see in our example.  When you are working in Energy Stewards and the graphs are “live”, you can click on each triangle to see the list of actions associated with the month aligned with the triangle’s downward facing vertex.

Energy Stewards® provides the web platform to knit the work together of the Champions and bring the calculations from ENERGY STAR®’s Portfolio Manager to a wide audience of users.

We’re proud to be a part of the MPower program, which has raised awareness and had a measurable environmental impact here in south central Wisconsin.  We’ve already started working with the 2012 class of 16 champions, who will get their formal introduction to Energy Stewards in three weeks.

It was testament to the cumulative impact of a dedicated group of businesses, taking action to make a difference to their bottom lines and in our community.

There were 17 members of the class representing a wide range of participants, including a semi-pro baseball team (Madison Mallards), a 4,400-square-foot retail store (The North Face), a 283,500-square-foot insurance office building (American Family), and a 233,000-square-foot manufacturing plant (Uniek, Inc.).

Together they have completed (0r are completing) 81 projects that surfaced  in the year-long program, coming in under the headings of employee engagement, energy efficiency, community outreach and transportation.

Their total estimated annual savings: 878,774 kWh of electricity, 14,783 therms of natural gas, 516,000 gallons of water, 11,436 gallons of gasoline — accounting for some 2.4 million pounds of CO2 that won’t be pumped into the atmosphere on their behalf.

This was the third class of the Mpower program. The Class of 2012 — another 17 businesses — attended the celebration and got plenty of good ideas to shorten their learning curve in their journey in the year ahead.

You can learn more about the program at: http://mpoweringmadison.com/mchampions

Congratulations to all. And thank you for your leadership!

The Zooniverse web-site claims over 599,000 participants are taking part in science projects world-wide as of 3/13/2012.

I learned about one of Zooniverse’s project, transcribing weather data from Royal Navy log books, in the February 2012 issue of Significance, a joint publication of the American Statistical Association and Great Britain’s Royal Statistical Society.

Royal Navy ships sailed all around the world over many decades.   The standard procedure:  every four hours, every day, sailors took and recorded weather and location data in log books.   This set of records provides an extensive baseline of temperature records that can be used for climate-modeling and climate change assessment.

The “Old Weather” project is transcribing log records from the beginning of the 20th century.

Here’s a sample log book page, ready for transcription:

Are Navy log books simpler to decode than utility bills?

There are still schools and public buildings that aren’t yet tracking their data in ENERGY STAR®’s Portfolio Manager (and then they could use Energy Stewards®, too!)   The initial hurdle, even in the 21st century, is getting energy record data into ENERGY STAR.

What would it take to “crowd-source” entering energy data into an appropriate database?

It’s getting easier every day, as shown by our colleagues at Zooniverse.   We may not have to wait until utilities have to make billing records ready to port over to ENERGY STAR.  We may be able to tap the power of willing volunteers.

Click here to view the embedded video.

In Energy Stewards, there is one other place where you can look at estimates of CO2 associated with your building’s use of electricity and natural gas.

On your building’s home page, scroll to the bottom to see the thumbnail graphs of electricity and natural gas (if your building uses natural gas.)

You can choose to look at energy use for your building in energy units, in dollars, or in lbs CO2, all normalized by the area of your building.  Using the same choices of units, you can also compare your building in thumbnail graphs for electricity and natural gas.

For the thumbnail graphs, Energy Stewards also uses the eGRID numbers to estimate the CO2 associated with electricity generation. We use a state-based number based on your building’s address rather than the more precise match of building zip code to eGRID region used by ENERGY STAR.

For natural gas, we use 11.7 lbs CO2 per therm, which aligns with the CO2 calculation for natural gas used by ENERGY STAR, converting metric to ordinary units.

Again, as we said in the previous post, these CO2 estimates do not account for fuel extraction but only combustion of natural gas directly or for the combustion of the fuel to provide electricity to the grid.   The numbers we present should be seen as representing “at least this much CO2.”

(And as we noted last time, since natural gas is mostly methane (CH4) and methane per gram has 20 times the greenhouse gas impact as a gram of CO2, leaks from natural gas extraction are an important factor in total greenhouse gas accounting.  In fact, the leaks associated especially with hydraulic fracturing (“fracking”) make some analysts believe that the recent discoveries of lots of natural gas in the U.S. and other places is a net negative with respect to climate impact rather than a bridge to a less carbon intense future.)

If you have questions about carbon numbers in your building’s use of energy, let us know–we’ll work through the arithmetic with you!