Natural gas is a common source of energy for heating and cooking, but understanding how it is measured and billed can be confusing for many customers. The process involves two different types of units: one for the physical volume of the gas that flows through the meter, and another for the amount of heat energy the gas actually contains. Consumers pay for the useful energy they receive, not simply the space the gas occupies. Therefore, the volume of gas measured at your home must be translated into a standardized energy unit to ensure fair pricing.
Defining the Units of Measurement
Natural gas is first measured by volume using the cubic foot (CF), which is the unit displayed on your home’s gas meter. A cubic foot is the amount of gas needed to fill a container with a volume of one cubic foot under a set of standard conditions for temperature and pressure. Utility companies often use the term CCF, which stands for one hundred cubic feet, to represent larger quantities of this volume measurement on a bill.
The therm is the measurement unit used for billing because it quantifies the heat energy content of the gas. One therm is defined as 100,000 British Thermal Units (BTUs), where a BTU is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. By using the therm, utility companies can standardize the cost of natural gas based on the actual heating power delivered.
The Standard Conversion Rate
Natural gas volume is converted to energy content using an established industry standard for the heating value of the fuel. On average, one cubic foot of natural gas contains about 1,037 British Thermal Units (BTUs). Since one therm is equivalent to 100,000 BTUs, you can determine how many therms are in a cubic foot by dividing the average BTU content of the gas by 100,000. This calculation shows that one cubic foot (CF) of natural gas is approximately equal to 0.01037 therms. Therefore, 100 cubic feet (CCF), a common billing unit, will convert to about 1.037 therms.
Factors Influencing Natural Gas Energy Content
The conversion rate is not a fixed number because the heat content of natural gas is subject to physical and chemical variations. Natural gas is primarily composed of methane, but it also contains varying amounts of other hydrocarbons like ethane, propane, and butane, all of which have different heating values. The concentration of these heavier, energy-rich components directly impacts the total BTUs available in a given volume of gas. For instance, a higher concentration of propane means a higher BTU content and thus a higher therm conversion factor.
The presence of non-combustible impurities, such as nitrogen and carbon dioxide, can also lower the gas’s overall heating value. Additionally, the volume of gas measured by the meter is affected by temperature and pressure, an effect that is more pronounced at higher elevations. Because gas expands and contracts, a cubic foot of gas measured at a higher altitude contains fewer molecules, and therefore less energy, than a cubic foot measured at sea level.
How Utility Companies Use the Therm for Billing
Utility companies employ a precise, three-step process to translate the volume of gas measured by your meter into the therms listed on your bill. The first step involves the physical meter reading, which records the total volume of gas consumed in cubic feet (CF) or hundred cubic feet (CCF).
Step 2: Determining the BTU Factor
Next, the utility company determines the specific energy content, or BTU factor, of the gas delivered to your location during that billing cycle. This factor is calculated by routinely sampling the gas flowing through the distribution system to measure its exact chemical composition and heating value. This results in a unique, specific number, often around 1.02 to 1.03, which accounts for the local gas quality and environmental conditions.
Step 3: Calculating Therms
Finally, the volume reading is multiplied by this specific BTU factor, and the result is divided by 100,000 to complete the conversion into therms for billing. For example, if a customer used 100 CCF of gas with a BTU factor of 1.03, the usage would be billed as 103,000 BTUs, which equals 1.03 therms.