1:5:10:365 EcoTip Blog

December 11, 2008

:346 Energy Used per Degree Day

Take the 1:5:10:365 challenge: Do one thing – for 5 to 10 minutes – 365 days a year to make our home and planet environment better.

1:5:10:346 EcoTip: To see if the energy saving measures you have installed are actually saving you money it is necessary to calculate the kilowatt hours per degree day or therms per degree day for the years you want to compare. A lower number means greater savings.

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 Additional Information:

Suggested Review: :343, :344, :345 

Your monthly utility bill should show the Kwh and therms used each month as well as a comparison with the previous year. Most people think they can compare these numbers directly to track their energy savings – but it doesn’t work that way. If your area has a colder month this year than last year – you may not see the savings on your bill even though your home is more energy efficient. Here’s how you can check to see how the monthly energy use compares:

  1. Determine if you furnace is gas or electric. If its gas you will be comparing therms. If it is electric you will be comparing Kwh. Keep in mind that there may be other appliances that use energy that may affect your estimates. For example your hot water heater and stove may also be gas – so, unless your furnace is metered separately you will be getting an answer that combines all three.
  2. Follow the instructions in 1:5:10:365 Tip :345 to generate your degree days your home for the months you want to compare.
  3. Collect the Energy Use data from you utility bill.
  4. Divide the energy used for the month last year by the Heating Degree Days for last year. Then do the same with the data for this year.
  5. Compare the two figures to see how much your energy savings are when the differences in weather between the years are taken out of the picture.

 Here’s an example of the energy comparison section of a bill.

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It appears that all your efforts at energy savings were wasted – you used exactly the same amount of energy. But you can only tell by checking the HDD.

  • Last year you used 100 therms and the degree days were 25: 100/25 = 4
  • This year you used 100 therms and the degree days were 33: 100/33 = 3

The calculation using Heating Degree Days shows your energy savings actually increased -you are using 25% less energy to heat the same space. Of course this assumes your cooking and hot water use remained the same. If you started saving on you hot water use, then its savings would be blended in with the heat savings. The method isn’t exact – but at least it gives you a better idea of what is happening.

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December 9, 2008

:344 Calculating Daily HDD

Take the 1:5:10:365 challenge: Do one thing – for 5 to 10 minutes – 365 days a year to make our home and planet environment better.

1:5:10:344 EcoTip: Heating degree days are a calculated number that is used to help quantify those periods of time where the outside temperature is cold enough for heating to be necessary for the building to be comfortable. The bigger the number – the more energy that will need to be consumed to warm the building.

The calculation can be complicated since temperatures can vary quite dramatically throughout a day. A simplified way to estimate Heating Degree Days is to determine the average temperature for the day (add the high and low temperature together then divide that number by two). If the average for the day is 65 degrees F – or higher your home won’t require heating and there are no heating degree days that day. If the number is less than 65 degrees F, then subtract it from 65. The answer will give you an approximation of that day’s heating degree day number. 

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 Additional Information:

Suggested Review: :343

As an example, if the high temperature for the day is 55 and the low temperature is 35, then the average temperature is 45. Subtracting 45 from 65 would result in 20 HDD for that day. Adding the HDD for each day in the month would give you the HDD for that month.

As I said before – this is oversimplified. Tomorrow’s tip will be a web-site that can be used to calculate this for you much more accurately.

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