Calculating the Energy Savings From Insulation Improvements

Calculating energy savings from heating or insulation improvements are complicated by the fact that heating energy consumption varies with the weather. Unfortunately you can't just take your energy consumption from before you added insulation (or improved your heating in some other way), and compare it with your energy consumption from after, because the weather before and after will no doubt have been different, and will no doubt have affected your energy consumption one way or the other.

The amount of energy required to heat a building (e.g. your house) depends on how cold it is. So it'll typically take a lot more energy to heat a house in the middle of winter than it will to heat that house in the middle of summer.

Fortunately, however, there is a simple way to account for these variations in the weather, so that you can fairly compare the heating energy consumption from one period with that from another. The secret lies in a simple form of data called heating degree days.

In a nutshell: the amount of energy required to heat a building for any given period is proportional to the heating degree days (or HDD) over that period. If your building is in a location that had 5000 HDD in 2007, and 6000 HDD in 2008, you should expect that, all else being equal, the energy consumption in 2008 will have been about 20% greater than the energy consumption in 2007.

Using degree days to calculate the energy savings that have resulted from an insulation or other heating improvement is fairly simple. I've broken it down into the following steps:

1. Get together your energy bills, and calculate your total heating energy consumption for a period before you fitted the insulation, and a period after you fitted the insulation. Ideally you'll be able to work out the kWh that you used for 12 months before and 12 months after. (12-month periods are good because they help to cancel out certain inaccuracies introduced by seasonal variations in energy consumption that degree days can't automatically remove).
2. Get the total heating degree days (e.g. from this HDD site) for each period of energy consumption that you've identified. You can simply sum degree days together, so, if, for example, you have a period of energy consumption that covers March 2007 to Feb 2008, you can get the monthly degree days for each month in question and sum them together to get the total. If your dates aren't so regular, you can simply get the daily degree days for your location, and then sum the appropriate HDD figures together to get totals for each relevant period.
3. Now divide your before kWh by your before HDD, and divide your after kWh by your after HDD, to get the before and after kWh per HDD.
4. Simply compare the kWh per HDD figures to work out the percentage savings that you've made.

Important points to note:

• This simple approach will only work well if you have energy consumption figures that represent your heating energy consumption only. If you have electric heating, it'll work fine if that's metered separately, but it won't work nearly as well if your kWh figures include all your other electrical consumption too. (Hot water energy consumption is usually relatively small compared to heating energy consumption, so it's probably not worth worrying too much if that is on the same metered supply.)
• Any figures you calculate will be approximate. Degree-day-based calculations can only ever be approximate - it's not an exact science.

About the author

Martin Bromley is a professional in the energy-saving industry, specifically in managing energy consumption. Through this work, he has developed a good knowledge of various energy-data-analysis techniques, including the use of degree day data for calculations such as that described in this article.