Outside weather conditions can have a dramatic impact on energy usage, especially in inclement climate zones. These impacts can make energy analyses difficult when trying to discern usage due to building operations/equipment versus extremes in outside weather conditions. Weather normalization addresses this problem.
Weather normalization is a calibrated adjustment to actual energy usage to remove the variability of weather. This process allows for an assessment of energy usage over time, regardless of the variable weather conditions year to year. It is a way to allow sites to evaluate past and present energy usage in a true “apples-to-apples” comparison by eliminating, or normalizing, the variable of weather.
Daily Regressions
Weather normalization begins with the gathering of actual energy usage and daily dry bulb temperatures from the nearest weather station. Weather stations are chosen automatically based on proximity, but they can be overridden in the Weather Normalization window.
Actual usage is broken out into daily intervals and plotted against daily temperatures readings. Incremental regressions are performed from 40° to 80° Fahrenheit, establishing a mathematical correlation between actual energy usage and outside air temperature. Regressions are performed for each applicable energy source type in both heating and cooling environments. The regression with the strongest correlation is chosen as the weather signature.
Weather Normalization Signatures
The weather signature identifies the balance point at which conditioning begins, the baseload that stays constant regardless of weather conditions, and the slope that represents energy usage as a linear function to outside weather conditions.
Balance Point Temperature
The Balance Point Temperature is the temperature at which heating begins in the heating signature and cooling begins in the cooling signature.
B3 does not assume a generic 65° heating or cooling balance point temperature for all sites across an entire region. A unique balance point temperature is determined for each site for each energy source type within B3.
The balance point temperature is NOT a building’s thermostat setting, though it is loosely related. Actual heating and cooling frequently lags behind thermostat settings due to factors such as insulation, daylighting, occupancy, etc.
Y-Intercept (Baseload)
The Y-intercept (Baseload) represents the energy usage that stays constant regardless of outside air conditions. For electric, these baseloads are typically plug loads, fans and lighting. For heating energy source types, baseloads are typically hot water heaters and cooking loads.
Outside Air Temperature
Average outside air temperature is utilized at the daily level to determine a weather normalization signature. Beyond that, B3 does not acknowledge average air temperatures at the monthly level due to temperature extremes that can occur within a given month. To better account for these fluctuations, B3 uses heating degree days and cooling degree days.
Heating Degree Days (HDD)
Heating Degree Days is a calculation of the number of days and number of degrees below the heating balance point temperature that a site operates in each month.
If a site’s heating balance point is determined to be 60°, all the days with an average daily temperature below 60° are gathered and summated with the number of degrees below.
Heating degree days can typically be a thousand or more per month in colder climates during the winter months. Heating degree days are typically 0 during the summer months.
Cooling Degree Days (CDD)
Cooling Degree Days is a calculation of the number of days and number of degrees above the cooling balance point temperature that a site operates in each month.
If a building’s cooling balance point is determined to be 80°, all the days with an average daily temperature above 80° are gathered and summated with the number of degrees above.
Cooling degree days can typically be a thousand or more per month in warmer climates during the summer months. Cooling degree days are typically 0 during the winter months.
Slope
The slope is the linear relationship of energy usage as it pertains to heating degree days/cooling degree days. The bigger the slope, the more dramatic effect that weather plays on energy usage.
Baseload-Only Signatures
If no heating or cooling pattern can be identified, it is considered a baseload-only signature. Weather normalization is not applied to baseload-only signatures.
Curve Quality (R2)
R-squared (R2) is a statistical measurement that represents the proportion of variance of a dependent variable that is explained by an independent variable or variables in a regression model.
In short, it is a measurement of the closeness of theoretical usage as compared to actual usage. B3 displays this measurement as a percent. A curve quality of 99% represents a high correlation of energy usage to outside weather conditions. A curve quality of 25% represents a low correlation between energy usage to outside weather conditions.
It is not uncommon for delivery energy source types such as fuel oil and propane to be unable to determine quality weather signatures due to the nature of delivery and usage.
Weather normalization is not applied to signatures with less than 50% curve quality.
Average Heating Degree Days & Average Cooling Degree Days
In addition to calculating actual HDD and actual CDD values for each month, B3 calculates an Average HDD and an Average CDD based on the last 20 years of daily temperature data. The Average HDD and Average CDD is crucial in applying weather normalization adjustments to actual usage data.
Applying Weather Normalization
The final step in the weather normalization process is applying the viable weather normalization signatures to actual usage to accommodate the differences in actual degree days compared to average degree days. That difference is applied to the non-baseload portion of usage.
Renewables
When dealing with electric renewable energy source types (PV, Wind), weather signatures are determined using the total of all electric energy source types. If a signature can be determined, heating and cooling adjustments are applied to the grid purchased electric usage only. Though PV and Wind can play a part in the overall signature, their generation is typically not directly impacted by heating or cooling degree days.
Weather normalization is not available in water mode.
Disclaimers
- The weather normalization signatures produced by B3 are only as good as the data that feeds it. Usage data is typically gathered on a monthly basis from utility bills and prone to entry errors.
- Usage data is analyzed at the daily level to determine weather normalization signatures. For data that is entered monthly, this involves calculating daily averages based on the monthly input.
Weather Data
B3 gathers daily dry bulb outside temperatures from over 8,900 high-quality weather stations across North America every day. This data is gathered from web services provided by the National Oceanic and Atmospheric Administration (NOAA).
High-quality weather stations are typically international and municipal airports.
If a weather station fails to report an average daily temperature for a given day, a reading is pulled from the nearest neighboring weather station within a 50-mile radius maximum. Pulled readings are accounted for in determining a weather station’s score. Weather station score is a calculation of the number of actual daily readings since January 1, 2000, divided by the total numbers of days since January 1, 2000.