HOME ENERGY CALCULATOR
David Swaim, Atlanta
You (and your computer) could become pretty popular when word gets out that you can analyze the benefits of home improvements on fuel bills. This program is in Microsoft (Apple, PET, OSI, etc.) and Atari BASIC.
Lately there has been a great deal of interest in saving energy in the home. Nobody needs to be reminded that fuel costs are rising. We all want to reduce our energy bills. The way to do this is simple: reduce household energy consumption. There are a number of ways this can be done.
The cheapest way is to change habits. An example would be setting the thermostat back to a lower temperature and wearing heavier clothes. If you're not too keen on that, the next alternative is to improve the ability of the house to protect you from the elements. Insulation could be added to the walls, floors, attic, and heat ducts. Weather-stripping could be applied to windows and doors. Storm windows and doors could be added.
Improvements such as these reduce the amount of heat that the house will lose to the outside. But which of the above items would save us the most money? Which one would cost the least to implement? Or, better yet, which will give the greatest savings for the least amount of cost? It's this last question we really want to answer.
The best measure of the cost effectiveness of an energy saving improvement is the payback period. That is simply the amount of time (in years) it takes for the savings in energy costs to add up to the total cost of installing the improvement. Obviously, the item with the shortest payback period is the best candidate for implementation. To determine the payback period, we must know two things: how much it will cost to make the improvement, and how much it will save us on utility bills for a year (a heating season). Obtaining the improvement cost requires consulting a contractor or, if we plan to do it ourselves, a building supply store.
Finding out how much the Improvement will save us in heating costs over a season is not quite as easy to determine. One way would be to keep records of our heating bills for one season, make the improvement, and then keep records of our heating bills for the next heating season. There are two drawbacks to this method.
First, the severity of the weather will vary from one year to the next. If the first year is severe and the second is mild, our heating bills would be less even if we made no improvements. This problem can be corrected by adjusting the heating costs using weather data for the two years.
The second and biggest drawback to this method is that you can't find out if an improvement is cost effective until after you have installed it. If it turns out not to be cost effective, it is too late to decide not to implement it!
What we need is a way of predicting savings. If we know the weather and the heat loss characteristics of the house, we can estimate the heating cost. By calculating the heating costs based on heat loss characteristics of the house both before and after the improvements, we can obtain the estimated savings due to the improvements. This is what the program here does.
To gather the data needed by the program, you will need to make some measurements and observe insulation levels in your house. The first thing the program calculates is the heat loss of the house. Heat loss of a house depends on three things: the thermal resistance, known as the R-value, of the structure; the total area of the structure exposed to the elements; and the temperature difference between the inside and outside of the house. So we simply need the area, R-value, and the difference in temperature.
The only problem is that different parts of the house have different R-values. Windows will have a lower R-value than walls, for example. In general, you can divide the external area of the house into five categories: windows, doors, walls, ceiling, and floor. The program requests information on each of these five categories in turn.
For windows it requests height, width, number of windows (it calculates total window area from these items), and type of frame and number of layers of glass. The number of types and/or sizes of windows is requested first. Most houses will have several sizes of windows, and there may be storm windows on some and not on others. The program allows for up to ten different types and/or sizes of windows. If you need more, change the dimension of S in statement 180.
Only one size and type of door is allowed. If you have sliding glass doors, you should consider them another type of window. You need to get the height, width, and number of doors. Remember: these are exterior doors only.
Information needed for the walls consists of type of construction and R-value of the insulation in the wall. If you enter a negative number for the R-value of the wall insulation, the program will give you a list of typical R-values for wall insulation. To get the area of the wall, the program asks for the ceiling height, total perimeter of the house, and the number of stories in the house. The program will calculate the gross wall area from this data and subtract the total window and door area to obtain the proper wall area.
One Hand Calculation
The only time you have to calculate area yourself is for ceiling and floor. For the ceiling, you will be asked for the number of inches of insulation in the attic and the type of insulating material. For the floor, the type of foundation is requested.
In addition to the heat losses mentioned so far, there are two others. The first of these is infiltration of outside air through cracks in windows and doors. The program asks if the windows and doors are weather-stripped. It uses this information and the total length of the cracks around windows and doors to calculate infiltration. The other heat loss is in the heat ducts from the furnace to the heat registers. The program asks if your heat ducts are insulated and where they are located. This concludes the input needed for calculating the total heat loss of the house. At this point the heat losses are displayed, and you are asked if you wish to make improvements to the house.
If the answer is "Y", you will be asked if you wish to improve each item. You can make improvements to one item or to any number of items. As you probably noticed, the first question you are asked is what the outside design temperature is. The outside design temperature for my area (Atlanta, Georgia) is 23 degrees. The outside design temperatures for other areas are tabulated in Table 1. For a more complete list, consult one of the references listed at the end of this article.
Actually, you do not need to put any specific temperature in here as long as it is less than 75 degrees, the inside design temperature used by the program. The program will still give you valid results for savings and payback. However, using the correct outside design temperature gives you the advantage of seeing what the furnace size would be for your house with and without the improvements. In fact, heating engineers use the same basic method as this program does to size furnaces for houses.
When the program finishes calculating the heat loss of the house after improvements, it is ready to do the cost analysis. First you are asked for the type of heating fuel you use: electricity, fuel oil, or natural gas. Next you must input the cost per fuel unit of the heating fuel.
Table 1: Winter Design Temperatures
|LITTLE ROCK AR||23|
|DES MOINES IA||-3|
|BATON ROUGE LA||30|
|ST. PAUL MN||-10|
|JEFFERSON CITY MO||6|
|CARSON CITY NV||7|
|SANTA FE NM||11|
|OKLAHOMA CITY OK||15|
|SALT LAKE CITY UT||9|
Note that this unit cost is in dollars, so if natural gas in your area is 35 cents per therm, you should input .35 dollars per therm.
Using this data and the heating degree days, the program calculates the total energy needed to heat the house for the entire heating season. The degree days and name of the city are on line 7010. You should change this line to reflect your own location. Some sample degree days for different cities are listed in Table 2, and a more complete list can be found in any of the references. The last thing you must input is the total cost of the improvements you made. From this data the program calculates the payback period in years.
Table 2: Yearly Heating Degree Days
|LITTLE ROCK AR||3219|
|ST. LOUIS MO||4484|
|OKLAHOMA CITY OK||3725|
|RAPID CITY SD||7345|
|SALT LAKE CITY UT||6052|
I got pretty popular in my neighborhood when word got out that my home computer could calculate how cost effective it would be to add insulation. I have also learned a great deal about my own home from running this program. Much of what I concluded was what I expected, but some conclusions surprised me. The program can definitely help home owners in assessing home energy improvements; it can also enable a home owner to spot dishonest "energy-saving" schemes pretty quickly.
1. ASHRAE Handbook 1981 Fundamentals. Atlanta, Georgia: American Society of Heating, Refrigerating and Air-conditioning Engineers, Incorporated, 1981.
2. Other Homes and Garbage, Jim Leckie, Gil Masters, Harry Whitehouse, and Lilly Young. San Francisco, California: Sierra Club Books, 1975.
3. Refrigeration and Air-Conditioning, Air-Conditioning and Refrigeration Institute. Englewood Cliffs, New Jersey: Prentice-Hall, 1979.