How to Calculate Yearly and Rolling Totals and Rolling Averages
There are various ways in which a permit may limit the emissions from an operation. Although some limits may appear similar, they are calculated much differently. The following examples explain the differences between permit limits based on calendar years, rolling totals, and rolling averages.
The chart below provides an example of a painting operation, which began in March 1998. This operation has a permit that provides limitations on the amount of paint it can use. The monthly paint usage for 1998, 1999, and 2000 are indicated in the left column.
Example 1: “Yearly Total” – The permit limits the operation to no more than 100 gallons of paint use per calendar year.
SOLUTION: Add the usage for each month of the year. In 1998, this adds to 74 total gallons; in 1999, this process equals 79 total gallons; and in 2000, this process equals 20 total gallons.
Example 2: “Rolling Total” - The permit limits the operation to no more than 100 gallons of paint use per any consecutive 12-month period or 12-month rolling period.
SOLUTION: March 1998 through February 1999 is a 12-month period; the total for the operation during that period would be 88 gallons as noted in the 12-Month Total columns. At the end of March 1999, add the paint use number for March 1999 (9 gallons) to the existing 12-month total (88 gallons) and then subtract the March 1998 value (7 gallons) for a new 12-month total of 90 gallons as indicated for March 1999 in the Rolling 12-Month Total column. Do this for each successive month.
Example 3: “Rolling Average” – The permit limits the operation to no more than an average of 8 gallons of paint use per any month on a yearly or 12-month average or 12-month rolling average.
SOLUTION: Calculate the rolling totals as in example 2 above and divide the result by 12 to obtain the monthly averages as shown in the column on the right.
Produced by: Nebraska Department of Environment and Energy, P.O. Box 98922, Lincoln, NE 68509-8922;
phone (402) 471-2186. To view this, and other information related to our agency, visit our web site at http://dee.ne.gov