A Load vs. Cold analysis is a process used to develop design day demand using actual measured daily volume and the corresponding degree days. From the data a linear regression can be completed to obtain the equation of the line of best fit in the form of y = mx + b. From this equation, the design day demand can be calculated by substituting the design degree day for the variable x. A confidence interval is commonly included to reduce the risk of exceeding the design volume on design day.
Natural gas utilities are expected to provide a firm level of service to customers on an extreme cold weather day called the Design Day. The design day is measured in heating degree days (HDD). In the gas industry, ambient air temperature is translated to heating degree days (HDD).
The volume of gas required to heat a building is directly proportional to the increase in HDD's. In other words, as the temperature decreases (becomes colder) the HDD's increase and the volume of gas required increases.
Gas utilities must choose the design day HDD to design its transmission and distribution systems for. There are two basic choices in choosing HDD's 1) Coldest Observed Temperature or 2) Probability of Occurrence.
Daily demand can be obtained by collecting daily measurement data. The data can be obtained from custody transfer measurement, area measurement and customer billing records.
Customers billing records are obtained from two different methods 1) monthly read meters and 2) Real time meters.
Monthly read meters are the typical residential and small commercial type customers and are referred to in this paper as General Service customers. Real Time meters are provided for Contract Rate customers and include electronic measurement which can provide actual hourly or daily measurement.