Calorific value (CV)
On this page you will find details of how the calorific value (CV) of gas is calculated. We use CV to determine the amount of energy transported by shippers and suppliers, which in turn is used to bill gas consumers.
Calorific value (CV) is a measure of heating power and is dependent upon the composition of the gas. The CV refers to the amount of energy released when a known volume of gas is completely combusted under specified conditions. The CV of gas, which is dry, gross and measured at standard conditions of temperature (15oC) and pressure (1013.25 millibars), is usually quoted in megajoules per cubic metre (MJ/m3). Gas passing through our pipeline system has a CV of 37.5 MJ/m3 to 43.0 MJ/m3.
Importance of CV
Knowledge of the CV of natural gas is an essential part of our day-to-day activities, as this information is used to determine the amount of energy we transport. CV information is provided daily to gas shippers and suppliers, which is then used to bill gas consumers. We also use this data to determine transportation charges for gas shippers and suppliers.
The CV of natural gas is measured continually, using process gas chromatographs. Process gas chromatographs separate natural gas into its constituent compounds (i.e. methane, ethane, carbon dioxide, etc.) and measures the amount of each in the gas. The physical characteristics of each component, as defined by ISO 6976, are programmed into the chromatograph and an overall CV is derived from the measured composition.
The determination of the CV of gas is carried out in accordance with international standards and the Gas (Calculation of Thermal Energy) Regulations, as amended in 1997. These regulations stipulate when and where the CV of gas is measured and the type of instrument to be used. The regulations are enforced by Ofgem, who also perform audit checks on the primary data.
The calorific value of natural gas is measured at reception terminals and other locations on our pipeline system. At present, the CV of gas is measured at over 110 different locations in the UK mainland.
Calculating thermal energy
The amount of energy consumed by a customer is calculated using the formula below. The United Kingdom mainland has been subdivided into thirteen charging areas. We provide a daily CV average for each charging area to the gas shippers and suppliers. It's calculated as follows.
The volumes of all inputs into the charging area are measured daily and a daily CV average determined for each input. The daily CV average for the charging area is then calculated by summing the product of the CV and volume for all the inputs and dividing by the total volume of gas entering the charging area.
CV = (38.2 x 6) + (40.2 x 1) + (39.6 x 3) (total energy) / (6 + 1 + 3) (total volume)
The maximum daily CV average for the charging area permitted by the regulations is equal to 1.0 MJ/m³ above the lowest measured daily CV average of the inputs into the charging area. All domestic customers and most industrial customers are billed on the basis of the daily CV averages for the charging area in which their premises are situated and are applied to the volume of gas consumed. Some very large consumers of gas (e.g. gas fired power stations) have CV measuring apparatus installed on the pipeline leading to their premises, allowing full accounting for the energy delivered.
In addition to the thirteen charging areas, there are a small number of communities in Scotland with isolated pipeline systems, including Stornoway which receives liquid petroleum gas (LPG), where the billing CV is based on a declared CV, which is set in advance. We ensure that the average CV of gas supplied to these consumers is never less than the declared value.
To find out which charging zone you belong to visit the Xoserve website and select ‘Postcode exit zone data’ under ‘Other’.