Sustainability & Risk / Energy

The Weather Model

Note: The following topic is a reference for understanding how the application calculates the weather model. The weather model provides the data that is critical to effectively evaluating your energy consumption and costs. For information on the tasks you perform to get started using the Energy application, see Energy: Application Overview.

The Energy application creates a weather model that normalizes your energy use so that you can compare historical data and current use on an even basis. This is done by normalizing for weather. The application creates a baseline of energy use and compares it to energy consumption for subsequent years. The baseline is a statistical best fit of consumption data in the specified baseline period normalized for weather conditions.

You specify the Energy Baseline Period Start for each building when you enter building information using the Background Data - Facilities / Define Locations task.

With weather normalization, you can compare how much energy you would have used this year to how much energy you did use based upon the baseline period usage patterns, and this year's conditions (such as weather and number of days.) This assumes that there are no significant changes in building consumption patterns.

With weather normalization, savings are calculated as:

Savings = (How much energy we would have used this year) - (This year's energy use).

Or, using other terms:

Savings = (Baseline energy consumption) - (Actual energy consumption)

Prerequisite: To calculate how much energy would have been used, at least a full year's worth of energy bills for the baseline energy period to which you want to compare future use is needed. You also need to run the Energy Manager/Update Calculations task to get the weather data needed to calculate cooling and heating degree days and balance points.

Note: The regression model routinely leave off the last 6~7 months of weather model data. This is because the calculations need to compare a series of source values before returning to compute the weather model projection for a given time period. There’s a check in the code that stops the regression analysis once the series gets shorter than 8 records. This is done to preserve the fidelity of the regression analysis.

Calculations

The update for energy calculations is usually set up to run as a scheduled workflow rule (e.g., on a nightly basis). When the application updates the weather model calculations, either automatically or via the Update Calculations task, the application does the following:

  1. Calculates the cooling and heating balance points for the building. See Calculating the Cooling Balance Point and Calculating the Heating Balance Point. Alternately, you can manually enter the cooling and heating balance points for a building in the Background Data - Facilities / Define Locations task.
  2. The application uses the balance points to calculate the cooling and heating degree days that measure the building's demand for energy to heat and cool the building. See Calculating Degree Days.
  3. The application normalizes baseline period bills and degree days to adjust for different bill period lengths. This is done by dividing by the number of days to remove any discrepancies associated with different bill period lengths.
  4. The application performs a simple least squares fit linear regression analysis of the utility data versus the weather data to determine the best fit line through the data. This is the line that comes closest to all the bills. This line establishes the relationship between usage and weather. The best fit line represents the baseline period of utility bills. See How the Application Establishes Baseline Energy Use

  5. Using the best fit line equation, the application plugs in the number of days and the number of cooling degree days (or heating degree days for a heating scenario) from the current billing period into the best fit line equation to compare energy consumption for that billing period to the baseline period. See How the application determines baseline energy use.

Once you have run the Update Calculations task to create the weather model, you can review the Usage with Weather Model report to assess your energy consumption and costs.

If you have turned on debug mode, when calculations are run, the code logs the result of any anomalous conditions to the log file. For a description of the error messages, see Error Messages.