EC Express: Use vs. Weather
Posted by Matthew Heinz on 06 June 2012 11:20 AM
Use vs. Weather
The Use vs. Weather charts in EnergyCAP indicate if, and how much, weather sensitivity exists for each meter, each year, and each summer/winter season. EnergyCAP calculates weather sensitivity by comparing building energy use with known weather data for the time period represented by the utility bills associated with that building.
IMPORTANT NOTE: During organization setup, each building should be assigned a correct postal code (Buildings & Meters > Building Properties) because EnergyCAP uses the building location to identify the nearest weather station in order to retrieve the degree day weather data for meters assigned to the building.
The EnergyCAP single linear regression approach to calculating weather sensitivity can be visualized as a plot of the utility bills for each calendar year, month-by-month, on a grid with consumption on the vertical axis and weather (as expressed using the “degree days” concept) on the horizontal axis. EnergyCAP does this internally via statistical analysis, but it is possible to obtain the same results by plotting the monthly consumption data in a spreadsheet.
The degree days are totaled for each day in each bill’s billing period.
Example 1 - Not Weather Sensitive
Consider this raw plotted data from an electric meter in the winter.
Each point represents an electric bill received in a winter month. The "best fit" line is found by statistical single linear regression analysis, and is called the "regression line."
For the charted data above, it is easy to see that the regression line would be horizontal. The meter uses about the same amount of electricity every month. This suggests that the meter is not weather-sensitive in winter, because the cold weather apparently has no effect on usage.
A line slope of zero means that the weather factor (amount of weather-related usage, expressed as KWH per degree day) would also be zero.
Example 2 – Highly Weather Sensitive
Consider this data from a gas meter in winter. This graph is very different. As the regression line makes clear, the usage increases as the weather gets colder, and decreases as the weather gets milder. Clearly, this meter is weather sensitive.
At the point of zero heating degrees (mild weather), the usage is zero. This means that the meter has no base load. There is no non-weather sensitive usage. All of the energy used is related to the weather.
Example 3 – Partially Weather Sensitive
This winter utility data from an electric meter shows a consistent slope which rises to the right, so the meter is weather sensitive. In the cold months such as December, January, and February, the weather load is greater, while in the mild months the weather load is smaller.
Note, however, that unlike the previous example, the Y-intercept (the point at which the regression line meets the vertical axis) has a value greater than zero —in this case, about 5,000 KWH. The Y intercept reveals the base load—the non-weather-related portion of the total usage. This meter uses 5,000 KWH even if there is mild weather (zero heating need degrees) plus some additional KWH depending on the weather.
The slope of the line is the weather factor in usage per degree day. The calendarization and normalization processors multiply the weather factor times the total degree days to determine how much weather sensitive energy to allocate to each month. The weather sensitive usage is then subtracted from the total usage. The remainder is the non-weather sensitive usage. This data is then day-prorated to each month of the billing period.
NOTE: The weather model used for each calendar year is derived from utility bill data for the previous year. If previous year data is incomplete, no weather adjustments are calculated.