Degree days and energy usage.

As my wife and I have been planning to build our house, we’ve been doing a lot of research into energy usage. We will construct a passive-solar house, meaning that we intend to capture the energy of the sun to heat and cool the structure. This is how houses have been built for centuries. It’s only in the past fifty years that people started building houses without regard for their surroundings, failing to take the simple step of letting sunlight into their homes to provide warmth in the winter. One of the things that I wanted to figure out was how much of our existing energy usage, in our rental house, was for heating and cooling. That would help me to determine how much we might stand to save by building with proper insulation, heating with the sun, cooling with proper shading, etc.

The first thing that I did was figure out how much need that we have for heating and cooling. The measure of this is the “degree day.” That’s simply the measure of how far the temperature deviates from 65° each day. So if the temperature averages 70° in a 24-hour period, that would be calculated as five degree days. If it’s 0° for a 24-hour period, that’s 65 degree days. If the temperature sits at a daily average of 0° for a month, that’s 1,950 degree days (65 * 30). Using the degree day data provided by the NWS’ Climate Prediction Service, I was able to determine how many heating degree days (when the temperature was below 65°) and cooling degree days (when the temperature was above 65°) there had been in the previous year. That yielded a graph like such:


In this period, December had by far the most degree days, and September the fewest. Clearly heating is a far bigger need than cooling, since the overwhelming majority of degree days are from temperatures below 65°

The next thing that I did was combine a year’s worth of home energy usage — derived from our Dominion bills — with this data. In this graph, degree days are now represented by the line graph, while the bar chart represents kilowatt hours.


It’s easy to see that the two of these co-vary. No regression analysis is necessary to see that our power bill fluctuates in tandem with degree days. Our power bill is paying for heating and air conditioning; every other use of power pales in comparison.

I was able to use these figures to determine what our power bills are likely to run us in a properly-constructed house, one with a BTU/ft2/kWh rating closer to 1.0 than the whopping 9.2 that — using these numbers — I estimate for our current dwelling. By calculating how much we’d save with such an efficient envelope (remembering that mortgage payments are tax-deductible, while energy costs aren’t, but mortgage payments include interest) gives me the amount of money that we can afford to put into making our house more energy efficient without actually spending any more. Turns out that’s a whole lot of money.

Consider comparing degree-day data for your area to your last year’s energy usage. You may find, as I did, that you’re spending a lot of money for the privilege of not wearing a sweater around the house. In fact, I’m wearing flannel-lined jeans, wool socks, slippers, and a sweater as I type this. Live and learn.

Published by Waldo Jaquith

Waldo Jaquith (JAKE-with) is an open government technologist who lives near Char­lottes­­ville, VA, USA. more »

13 replies on “Degree days and energy usage.”

  1. That’s pretty interesting. My children complain quite a bit about how cold the house is in the winter.

    On a sort of related note, two years ago we had to replace a heat pump that was about 16 years old. I immediately noticed a drop of 50% in our electricity bill in the months when heating or cooling were necessary.

  2. Waldo,

    It’s exciting that you are building a passive solar house. We built one for my mom, though we made a lot mistakes.

    One of the mistakes is that we didn’t make the eves long enough

    So we are getting unwanted solar gain in the summer (though I plan to fix this with some lattice!)

    Another mistake is that in a vain attempt to save money,my mother went with a dark shingle roof.

    Big, Big mistake!

    Make sure you do ether a white shingle roof or a Energy Star recycled steel roof.

    You can reduce your unwanted summer heat gain significantly by using ether of those options: cuts your cooling bill by 1/3 in some instances.

    Another mistake is that we didn’t use Energy Star light fixtures; even though their are over 20 CFL’s in the house it would have been nice to have gone the extra mile.

    But, all the and the appliances are Energy Star–by the way, ES appliances usually pay for their extra cost in about 6-12 months.

    The final mistake (though there was no real alternative considering the design of the house), the final mistake was utilizing a concrete floor in the front half of the house.

    This was done for mass to capture the solar gain, but also to house a supper efficient radiant floor system powered by a on-demand gas powered hot water heater.

    Making concrete uses a lot of energy. The world cement industry accounts for about 5-10% of all man-made CO2 emissions.

    We could have used Fly Ash concrete, a recycled material, one of the byproducts of burning coal. We just didn’t know about it, though.

    Anyway, on the bright side you do not have to run the heat at all during the day . . . sometimes, and even in bellow 40 degrees temps, you actually have to open the windows because it gets too hot!

    For your hard wood needs you should check out APPALACHIAN SUSTAINABLE DEVELOPMENT:

    They are awesome, they do not clear cut and use a solar powered kiln, we used them for the wood flooring (beautiful, and cheap!)

  3. 65 F is a little cool for most folks. Is there a specific reason you used it, or could you have used 70 F to calculate degree days?

  4. It’s not a yardstick of my intention — the degree day is an international standard, and is widely employed for agricultural and energy management purposes alike. How the magic number of 65°F was arrived at, I can’t say. I suspect that it comes from, in part, the recognition that houses absorb heat from the sun and retain heat (unlike the outdoors), and 65°F may be sufficient to make it more like 70°F inside. I just made that up — I have no idea if it’s true — but you get the idea. :)

  5. Don’t forget hot water…15 percent of all energy in a house is used for domestic hot water. Solar hot water systems are the most effective and simple of solar systems.

    Think you would enjoy this from Ed Mazria:

    …the maximum amount of global warming the planet can tolerate is two degrees Celsius. If we continue on our current path, we’ll achieve that rise by about 2050, and we’d reach a rise of three degrees Celsius by 2070. With a temperature rise that high, the consequences are catastrophic-the polar ice caps would melt, warmer ocean temperatures would result in severe weather patterns, and we could lose 25 percent of the species on the planet.

    The 2030 Challenge: Carbon-neutral building by 2030.

    That the fossil fuel reduction standard for all new buildings be increased to:

    60% in 2010
    70% in 2015
    80% in 2020
    90% in 2025
    Carbon-neutral by 2030 (using no fossil fuel GHG emitting energy to operate).

  6. Sounds like a great project–keep everyone posted on the details, including mistakes (Jon Sheridan’s comments on the mistake they made are quire useful). And when you’re done, let us know how your energy usage looks.

    I hope you won’t use electricity to heat your new home–not too efficient. Natural gas is relatively low on the carbon scale, and if your home is as well designed as I bet it’ll be, you won’t need much (use it for cooking and clothes dryer–more efficient than electricity for any kind of heating).

    Two years ago we replaced an attached greenhouse with an enclosed sunroom. In the process, we got rid of our electric greenhouse heater, used in winter months just to heat the little room to about 58 degrees. Little did I know, it was using 10,000 kwh per year! We practically paid for the addition in reduced payments to Dominion.

    Now, with various conservation steps, including replacing most lighting with CFL’s and use of a few solar electric panels, we’ve cut our electricity usage to just a few hundred kwh’s per month. (Our gas bill is another story). I just wish we could start over with passive design to begin with.

    Even if you don’t install solar electric panels now (and I don’t recommend it, economically, at present) at least plan for the future possibility in siting your house–give it a clear southern exposure (you probably will anyway to maximize passive heating).

    Good luck with the project.

  7. This is such a cool project! Have you checked into chilled beams yet? A few envirobloggers have raved about them. Popular in Europe and reportedly very cost-effective. Chilled beams combine a radiant cooling system with conventional overhead ventilation, which reduces energy usage, improves comfort levels, and reduces the architectural impact of ductwork and other mechanical systems. (Yep, almost verbatim from the enviropundits–if you haven’t checked them out already, you can find a lot of info there.) According to one architect-commenter, they are most effective for an open-floor plan, so a lot will depend on the design you pick.

  8. Early on, we had to resign ourselves to the understanding that we cannot do everything perfectly. You could go nuts thinking through all of the ways in which to reduce your ecological footprint.

    SIPs will save energy, but since there’s no local SIP manufacturer nearby, that means we’ll have to have them shipped. But what of the energy required for that transportation? Does the offset of the energy savings from using SIPs from the shipping energy expenditure mean that they don’t make sense anymore? Does that mean that I should prefer wood harvested in Virginia…even if it wasn’t harvested sustainably?

    Life’s too short for all that. We’ll do our best to make good choices for ourselves, our land, and our future — and welcome lots of advice so that we can do that — but there’s no need to drive ourselves to distraction. :)

  9. Heh, nice send up to enviromental angst, Waldo. It’s an old debate, just a different product. E.g., polyester vs. rayon vs. cotton. Most polyester is from plastics and, as a petroleum product, should be shunned except that plastics are now recyclable… and hmm–shouldn’t we be for recycling? Rayon is at least made from wood pulp, which is both a renewable and a recyclable source, but it takes a heckuva lot of petrol to convert pulp to that end product. Then there’s all-natural cotton, which, as a cash crop, provides us with lots of hidden herbicides and pesticides. Even when organically produced, it eats up the earth like tobacco, and so that’s not good either. Perhaps we should all go back to wearing oversized fig leaves. :-D

  10. Waldo, have you ever been to any of the Green Festivals? I went to the one in DC, and while there were a lot of nutjobs/conspiracy theorists, it was pretty cool. They had the usual books, food, random organic products of every type for sale, and a wide variety of free stuff, but they also had a bunch of seminars on building “green” homes or making your pre-existing home more environmentally friendly. The next one in DC is in October 2007 (and you should go if you can!) but they’ve got lots of information and links on the website that I linked to earlier. It seems like you totally already know what you’re doing, but the info might be interesting nonetheless.

  11. TLPatten brings ups an important debate and a good take on it . . . The problem is that people with a “purists” attitude usually drive this debate.

    This is something that deserves much more time and space to discus.

    Anyway SIPs are nice, I wouldn’t worry about the energy need to ship them (a lot of the components of your house are going to be shipped a long distance, for instance fancy windows),but they will save A LOT of energy if you use them in your structure.

    Though, just making 4×6 exterior walls, or even double 2×4 wall will get you nice R value as well.

  12. If we had the time, we could let market forces deliver SIP’s, Solar Systems, improved lighting, and other energy improvements to Lowes, Home Depot, and Joe Builder. But we don’t. So energy consumption of new construction should be regulated to a goal for carbon-neutral in 20 years. Like Jim Lovell says about going to the moon, “It wasn’t a miracle, we just wanted it”.

  13. Waldo,

    There are policy options that would free you from worry about ecological impacts — indeed, as you mentioned it is impossible to take it all into account.

    For example, look at the acid rain program that the EPA is running — SO2 and NOX cap and trade markets, where the price of environmental compliance is already included in the price of the electricity you consume. In this scenario, consumers best protect the environment by consuming as little electricity as possible, and, if they have electric choice, choosing the electricity that is the least expensive.

    The same could be done for carbon… and it is likely that such a policy will pass in the next few years. I think the two most likely contenders are either a cap and trade scenario or a carbon tax scenario.

    It will be interesting to see how things shake up with the Democratic congress.

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