Opportunities for do-it-yourself energy projects extend deeper into your home than just weather-stripping the front doorway. You can even do your own energy savings calculations—to a point, that is. While hardly substituting for professional audits and the work scopes they generate, new web-based home energy modeling tools that would have required supercomputers in the old days (such as the Home Energy Saver, http://hes.lbl.gov) can empower homeowners to identify potential saving opportunities. And they’re free.

For the same reason that most people can’t diagnose car trouble as well as their mechanic can, you should ultimately seek the advice of a home energy pro for most energy retrofits. For example, professional energy auditors use tools like blower doors to accurately estimate the air leakage of a home—among other things, this is a critically important input for home energy software tools. That said, starting with a DIY energy audit provides some reference points and provide a sense of how much money you might save.

Think about what you want to learn from a tool. How much energy you can save from a particular technology upgrade?  From a behavioral change? Payback times for energy upgrades?  Environmental benefits (e.g., greenhouse-gas emissions)? Look for a tool that enables you to do the kind of analysis that’s important to you.

Once you locate the right tool, take the time to use it well. Even if a given web-based audit tool is technically accurate in the way it models things, the results can be skewed and misleading if the proper input values aren’t provided by the user (that’s you). Here are just a few things to watch for as you hone your modeling skills:

  • Measure and input your floor area accurately. Over or underestimating your floor area will skew the program’s results.
  • Most of these tools rely heavily on “default” values, i.e., estimates for things such as your appliance and equipment efficiencies, insulation levels, or even home size. Be sure to replace as many default values as possible with values that more accurately describe your particular home.  In some case, this may require “zeroing-out” assumed devices such as defaulted plug loads that are not actually present in your home.
  • Since weather conditions vary geographically, be sure to specify a “weather location” that resembles yours as closely as possible. Some tools have hundreds of locations to choose from, while others are more simplified and thus more subject to mis-estimation. Given variations in microclimates, keep in mind that the closest weather location to your home may not be the best one.
  • It’s important to accurately characterize your home’s envelope.  Do all you can to verify the exact type of insulation used.  Perhaps “derate” the full nominal R-value if you believe it has settled or was poorly installed.  Window heat gain and losses vary widely depending on the exact type of glazing as well as the window frame material.  Look for a tool that lets you characterize all these variables.
  • Look for a tool that lets you characterize your ductwork or boiler piping (as applicable). This should include the insulation levels, air leakage (for ducts), as well as where the distribution is located. If located in an unconditioned space, energy losses will of course be greater and you want the model to be able to account for that factor.
  • Look for a tool that lets you be very specific about lighting and plug loads. Take the time to describe your lighting accurately, and ensure that the tool “knows about” the various miscellaneous appliances and gadgets in your home.
  • If the tool you’re using provides energy bill savings and/or payback times for energy upgrades, be sure that you enter your actual energy prices (even down to the exact tariff structure, if the tool supports that) as well as the actual local upgrade costs.

Once you’ve accurately described the physical features of the home be sure to then accurately describe how you use the home. Among these considerations, look for a tool that allows you to specify different thermostat settings for different times of the year and even different times of the day. Specify the correct temperature setting for your water heater.  If you zone-heat or -cool your home, adjust your thermostat assumptions to approximate this.  Realistically characterize your actual use of lighting and miscellaneous plug loads in terms of hours per day by fixture or device.  If you use wood heating, be sure to account for that. Depending on how flexible the software is, this may require a “workaround” such as specifying less than 100% of your floor area being served by the primary heating system. The potential energy savings depend very much on these kinds of operational assumptions.

Truth have it, you will find yourself in situations where you have to guess at certain inputs. Elusive questions might include the amount of air leakage from the home and from the ducts, exact efficiency of the heating system or air-conditioner, energy use of the fridge, etc. Do your best. Perhaps even test the sensitivity of your results to a range of guesses to learn how important it is to nail these down. This is one place where a professional auditor can improve the energy-use estimates by using diagnostic equipment such as blower doors, duct-blasters, combustion-gas analyzers, and data-loggers.

When all is said and done, remember that these tools estimate energy use and savings under average weather conditions for a given location. If last summer was 10% hotter than average, then your actual energy use will likely be higher than the model predicts. This is not a failing of the model, but rather a fact of life that users have to adjust for.

Computer energy modeling is not easy. The energy flows in a home represent a complex territory, and software is at best a good map—even for the most highly skilled users.  When it comes to evaluating energy savings, it can help to focus on the percentage savings as opposed to the absolute savings.

Taking the time to run your home through one or more of the available web-based tools can really pay off.  Doing so will give you a stronger basis for discussions with the home energy professionals you may later ask to make upgrade recommendations.

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