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Most utility bills have start and end dates that do not correspond to an exact calendar month:

Exact calendar month:

Start date: 9/1/2011 End Date: 10/1/2011
Start date: 10/1/2011 End Date: 11/1/2011


Typical utility bill:

Start date: 9/12/2011 End date: 10/14/2011
Start date: 10/14/2011 End date: 11/13/2011


Calendarization is the process of splitting bills into the appropriate calendar month by estimating the cost and usage attributable to the portion of the bill that falls in each month. Without daily meter readings the splitting can’t be exact, but it is reasonably accurate.


Why calendarize? Calendarization provides better month-to-month and year-to-year comparisons for reporting purposes by more accurately allocating monthly usage from bi-monthly and quarterly bills, estimated bills, and bills with variable billing periods (28 days one month, 34 days the next). The calendarized data is therefore more representative of actual monthly energy use when used in fiscal reporting, budgeting, auditing, accruals and energy performance evaluation.


In UtilityManager, two methods of calendarization are used.

Simple average daily use and cost allocation:

When a meter is not weather sensitive, the bill is simply prorated into each month based on the number of billing period days. A bill from 9/12 to 10/14 has 32 days (the first day counts, the last day doesn’t because the next bill begins on 10/14). There are 19 days in September, so 19/32 of the usage and cost is allocated to September and 13/32 to October.

Weather (degree day) allocation:

When a meter is weather sensitive, the simple method described above is inadequate and provides less reliable results. A much more sophisticated method is used to prorate the weather sensitive portion of the energy bill based on the number of degree days in the appropriate billing period in each month, and to prorate the remaining non-weather sensitive portion based on the number of days. See the Use vs. Weather topic for more details.


Note that calendarizing does not appreciably affect annual totals. Since the calendarization process merely moves usage and cost data to the most appropriate calendar month for reporting and analysis purposes, the annual usage and cost totals should be almost identical to the actual data (the data shown on a billing period basis). There may be slight differences due to year-end roll-over, but that is typically less than 1%.


In UtilityManager, calendarized data is displayed in two places: calendarized reports and the Buildings & Meters – Calendarized Data tab.