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South Carolina / Rates and Tariffs

Weather Normalization

How a weather normalization adjustment can affect your natural gas bill.
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Questions & Answers About WNA

WNA is part of the bill calculation that adjusts Dominion Energy South Carolina’s natural gas rate (excluding purchased gas costs) to normal weather conditions for a given billing period. WNA is only applied to the November through April billing periods. WNA adjusts the rate downward in colder than normal weather and upward in warmer than normal weather.
Since winter weather patterns can vary significantly from year-to-year, WNA was implemented to offset the effects of unusually cold or warm weather for the Company and its customers. WNA allows the Company to recover the fixed costs (such as system maintenance, safety regulations, billing systems and 24/7 customer service) that are associated with operating and maintaining a safe, reliable natural gas distribution system without implementing permanent, year-round rate adjustments due to abnormal weather.
If the WNA did not exist during periods when the winter heating season weather is warmer-than-normal, the alternative would be a general gas rate increase applied year-round. However, the Public Service Commission approved the gas WNA in 1991 after examining this alternative of a general rate increase. The general rate increase would result in a year-round charge, whereas WNA can actually result in a reduction to customers' bills during winter heating season months where weather is colder-than-normal and customer usage is higher.
Weather normalization adjustments can occur during the billing months of November - April. The WNA does not apply in the non-winter season of May - October.
Normal weather for the Dominion Energy South Carolina service territory is determined by the official 30-year average published by the National Oceanic and Atmospheric Administration, or NOAA, based on 30-year average temperatures for our service area.
In 1991, Dominion Energy South Carolina received approval from the Public Service Commission of South Carolina to implement the gas WNA. This is not just a Dominion Energy practice. Similar adjustments are used by natural gas utilities across the country.
No, WNA is based on weather variance not customer usage. WNA was implemented to recover fixed costs such as operating expenses, materials, maintenance costs, etc. It is not applied to the cost of purchased gas. WNA revenues are used to help insure that the gas distribution system is reliable and safe.

When weather is colder than normal, WNA adjusts the rate per therm downward and when the weather is warmer than normal, WNA adjust the rate per therm upward. Heating Degree Days (HDDs) are a measure of how cold it is on a given day.

Dominion Energy South Carolina uses a system-wide value of HDDs based on multiple weather stations. HDDs are determined by comparing the mean (the average of the high and low) outdoor temperature to a base temperature of 65°F (threshold at which heat would typically be necessary), by weather station. The HDD values are then averaged. The colder the average temperature, the higher number of HDDs and generally, the higher amount of natural gas needed to heat your home. Actual HDDs are calculated and compared to normal HDDs produced by NOAA to determine if the weather was colder or warmer than normal.

Here is an example of how it works. Let’s say the high low temperatures for a particular day are as follows:

  • High Temperature = 55° & Low Temperature = 33°
  • The temperature mean for that day : (55 + 33) / 2 = 44°
  • Since 44° < 65°, then 65 - 44 = 21 Heating Degree Days