Frequently Asked Questions: Heating Values and Therm Factors

What are heating values and how are they determined?

Heating value is the number of British Thermal Units (BTUs) in one cubic foot of natural gas. Heating values vary depending upon the source of the gas. Each week, the actual heating value of gas in each BTU area is measured by sampling the gas and analyzing it with a gas chromatograph (except in collection areas, as explained below).

What are BTU areas?

BTU areas are pre-defined sections of Pacific Gas and Electric Company's gas pipeline system. Every gas meter is assigned to a specific BTU area. BTU areas are designed such that all the gas within each BTU area has the same heating value.

How do I know which BTU area to use?

For noncore customers, your BTU area can be found on the second page of your Pacific Gas and Electric Company detail of bill, in parenthesis following the therm factor used for billing.

How is heating value determined for customers located in collection areas?

Collection areas are a small subset of BTU areas. Some customers, usually those in rural areas served primarily by local gas wells, are located in collection areas. In collection areas, heating value is only sampled once every six months.

What are therm factors?

Therm factors are used to convert the volume of gas metered (in hundreds of cubic feet) to energy units (therms).

How do I calculate the therm factor for my specific billing period?

The therm factor applicable to a billing period is equal to the average of the weekly heating values within that billing period. To calculate the therm factor for a billing period, add the weekly heating values shown for any date that falls within the billing period and divide by that number of heating values, then divide that number by 1000.


Hv = weekly heating value

((Hv1 + Hv2 + Hv3 +Hv4) / Number of heating values) / 1000 = Therm Factor

Noncore gas accounts are billed on a calendar month basis. Most calendar months will have only four heating values; however, some months (April, July, October, and December in 2007) will have five heating values that must be averaged to obtain the applicable therm factor for a calendar month billing period.

The formula is the same except in this case there are 5 heating values:

((Hv1 + Hv2 + Hv3 +Hv4 + Hv5) / Number of heating values) / 1000 = Therm Factor

Therm Factor Calculation Example Using Sample Heating Values
3/5/07 2/26/07 2/19/07 2/12/07 2/5/07 1/29/07 1/22/07
1,017 1,017 1,017 1,018 1,017 1,019 1,018

To calculate the therm factor for the billing period 2/1 to 2/28 for BTU area code B02, we use the heating values from 2/26, 2/19, 2/12, and 2/5.

The therm factor calculation is: ((1,017 + 1,017 + 1,018 +1,019) / 4) / 1000 = 1.018

Where can I find more information?

If you have any further questions, please call your California Gas Transmission Account Services Representative.