5.4 Residential & Commercial Natural Gas Leakage
| Category ID | Description | EIC |
|---|---|---|
| 868 | Natural Gas Distribution - Other Fuel Use | 33031801100000 |
| 2535 | Domestic - Natural Gas - Space Heating | 49999500100000 |
| 2536 | Domestic - Natural Gas - Water Heating | 49999500100000 |
| 2537 | Domestic - Natural Gas - Cooking | 49999500100000 |
| 2539 | Commercial - Natural Gas | 49999500100000 |
| 2541 | Domestic - Natural Gas - Other | 49999500100000 |
Introduction
Natural gas consists of mostly methane (CH4) with small amounts of other hydrocarbons such as ethane, propane, and butane. The emissions reflected in this methodology document include methane emissions from the leakage of natural gas both post-consumer meter and across the distribution network for the Residential and Commercial sectors. The following categories account for emissions from the leakage of natural gas (NG) in the Residential and Commercial sectors:
- Category 2535 – Post-Meter Leakage for Residential Space Heating
- Category 2536– Post-Meter Leakage for Residential Water Heating
- Category 2537 – Post-Meter Leakage for Residential Cooking
- Category 2541 – Post-Meter Leakage for Residential Other Appliances
- Category 2539 - Post-Meter Leakage for Commercial Applications
- Category 868 – Pipeline Leakage during Distribution of Natural Gas
The Residential Other Appliances category accounts for post-meter natural gas leakage emissions from appliances such as pool water heaters, spa and hot tub heaters, clothes dryers, and barbeque grills.
Category 2539 accounts for post-meter natural gas leakage emissions from non-permitted commercial equipment.
Category 868 covers emissions associated with leaks in the pipeline network during the transmission of natural gas, upstream of residential and commercial consumer meters. Components such as valves, flanges, pumps, compressors, and pressure relief valves (PRVs) in the utility-operated transmission system are potential leak sources.
Methodology
These categories are considered area sources as they account for emissions from combustion devices that are not permitted by the Air District and hence are not systematically or annually reported. The data used to estimate emissions for these sources must be extracted from either the California Air Resources Board (CARB) or California Energy Commission (CEC) databases. The methodology used to calculate emissions for the reported base years for these source categories is as follows:
Base Year(s) Emissions county,pollutant =
Activity Data × Emission Factorpollutant × Control Factorpollutant × Fractioncounty × Fractionin District × GWP pollutant
Where:
- Base Year: is a year for which activity / throughput data is reported by CEC and/or CARB, and available.
- Activity Data: is the throughput or activity data for applicable reported base years. This data may be determined in one or two ways:
- Apportioning Larger-Scale Data: Throughput data from a larger domain, such as state or national level, is scaled using the proportion of a representative metric in the regional domain relative to the larger domain. For example, the ratio of a county’s population to the state population can be used as a scaling factor to determine the county throughput from state-level throughput.
- Using Local Sources: Alternatively, data from a locally published and verifiable source may be used, such as the county-level natural gas usage data provided by the CEC.
- Emission Factorpollutant: is a factor that allocates an amount of emissions, in mass, of a particular pollutant by unit of activity data. For example, tons of CO2 per gallon of gasoline burned or pounds of N2O per million standard cubic feet (MMSCF) of natural gas combusted. This factor generally comes from a published literature source such as USEPA AP-42 (USEPA, 1998) or CARB’s Mandatory Reporting Requirement (MRR) for Greenhouse Gases (CARB, 2019).
- Control Factorpollutant : is a fractional ratio (between 0 and 1) that captures the estimated reduction in emissions as a result of Air District rules and regulations.
- Fractioncounty : is the fraction of total regional emissions (between 0 and 1) estimated to be allocated to a particular county. It is typically derived from regional socioeconomic metrics and/or actual county-level throughput data.
- Fractionin District : The Air District jurisdiction covers only a portion of Solano and Sonoma County. For this reason, additional allocation must be made for these counties to determine the proportion of the county’s emissions occurring within the Air District’s jurisdiction.
- GWPpollutant is the Global Warming Potential of a particular greenhouse gas (GHG) pollutant. The current version of the GHG emissions inventory incorporates the global warming potential (GWP) reported in the Fifth Assessment report of the Intergovernmental Panel for Climate Change (IPCC, 2014). The GWPs for the three principal GHGs are 1 for carbon dioxide (CO2), 34 for methane (CH4), and 298 for nitrous oxide (N2O), when calculated on a 100-year basis with climate-carbon feedback included.
Once base year emissions are determined, historical backcasting and forecasting of emissions relative to the base year emissions are estimated using growth profiles as follows:
Current Year Emissionscounty = Base Year(s) Emissioncounty x Growth Factor
Where:
- Growth Factor: is a scaling factor that is used to derive historical emissions estimates for years for which activity data and/or emissions are not available, and to forecast emissions for future years, using surrogates that are assumed to be representative of activity and/or emissions trends.
More details on the inputs and variables used above are provided in the following subsections:
Activity Data
Residential Natural Gas Leakage
Activity data for residential natural gas leakage categories is reported by the CEC (CEC, 2023a) as natural gas (NG) consumption usage for each Bay Area County. The usage represents total natural gas consumed in the Residential sector in energy units of million therms (MMTherms). The usage per county, once converted to units of MMSCF, is then proportioned into each of the residential source categories by multiplying the county usage by the estimated fraction of natural gas consumed in each category. The fraction is based on CEC’s 2019 Residential Appliance Saturation Study (RASS; CEC, 2020a) (see table below). This approach is applied to reported base years data from 2011 to 2022, for which natural gas consumption usage data are available from the CEC.
Space Heating | Water Heating | Cooking | Other |
Category 2535 | Category 2536 | Category 2537 | Category 2541 |
39.31% | 54.90% | 3.30% | 2.49% |
The leakage rate of 0.50% for residential appliances (CEC, 2018) is applied to natural gas usage to calculate the amount lost to leakage.
Commercial Natural Gas Leakage
The reported base years for source category 2539 are the years 2011 – 2019. The datasets used to determine throughputs for the reported base years for the Commercial sector are listed below:
- CEC Natural Gas Usage in Therms by County and by Residential and Non-Residential sectors, years 1990 – 2022 (CEC, 2023a),
- CEC Natural Gas Usage in Therms by County and by Economic sector, years 1990 – 2022 (CEC, 2023b), and,
- Reported natural gas throughputs by permitted industrial and commercial facilities to the Air District for years 2011-2019.
For this category, facilities subject to the registration requirements in Regulation 9, Rule 7 (BAAQMD, 2011) reported their annual throughput to the Air District from 2011 to 2019. In 2011, the Air District’s registration program for small commercial boilers and process heaters came into full effect, but prior to 2011 the throughput for these devices was not reported to the Air District. Therefore, the year 2011 is considered to be a representative year for historical backcasting, as it is the first year where throughput from these registered boiler/heaters were recorded.
In a similar manner, the year 2019 is chosen to be a representative year for estimating future trends as it is the last year where natural gas throughput data was collected from these registered sources.
Prior to collecting direct natural gas usage from registered facilities (in the 1990s and 2000s), the CEC Natural Gas Usage by Residential and Non-Residential sectors was used to estimate emissions. To remain consistent with historical calculations, the same data set is used but the Air District now uses a refined approach for proportioning natural gas usage to the non-permitted area source in the Industrial and Commercial sectors using an additional CEC Natural Gas Usage by Economic sector data set. After determining the amount of natural gas used for the Commercial sector for the SFBA using the two datasets, the natural gas usage of commercial point sources, based on the Air District’s registered dataset (category 307_com), are subtracted from CEC’s natural gas usage for the Commercial sector to avoid double counting.
The leakage rate of 0.28% for commercial appliances (CEC, 2020) is applied to natural gas usage to calculate the amount lost to leakage.
Pipeline Distribution Leakage
For category 868, throughput is calculated by summing the throughputs of all residential and commercial natural gas usage using the methods described above. A leakage rate of 0.2% for pipeline leakage (ACS, 2015) is applied to total residential and commercial natural gas usage to calculate the amount lost to leakage during pipeline distribution.
County Distribution / Fractions
County distribution is determined by using base year data for 2022 for Residential and Commercial sector natural gas usage from the CEC.
| ID | Description | ALA | CC | MAR | NAP | SF | SM | SNC | SOL | SON |
|---|---|---|---|---|---|---|---|---|---|---|
| 2535 | Domestic - Natural Gas - Space Heating | 0.21 | 0.16 | 0.05 | 0.02 | 0.13 | 0.11 | 0.23 | 0.04 | 0.05 |
| 2536 | Domestic - Natural Gas - Water Heating | 0.21 | 0.16 | 0.05 | 0.02 | 0.13 | 0.11 | 0.23 | 0.04 | 0.05 |
| 2537 | Domestic - Natural Gas - Cooking | 0.21 | 0.16 | 0.05 | 0.02 | 0.13 | 0.11 | 0.23 | 0.04 | 0.05 |
| 2539 | Commercial - Natural Gas | 0.22 | 0.07 | 0.04 | 0.03 | 0.10 | 0.18 | 0.26 | 0.03 | 0.06 |
| 2541 | Domestic - Natural Gas - Other | 0.21 | 0.16 | 0.05 | 0.02 | 0.13 | 0.11 | 0.23 | 0.04 | 0.05 |
| 868 | Natural Gas Distribution - Other Fuel Use | 0.13 | 0.51 | 0.01 | 0.01 | 0.05 | 0.04 | 0.18 | 0.05 | 0.02 |
BAAQMD Jurisdiction Fraction
The BAAQMD jurisdiction only accounts for a part of Solano and Sonoma counties. The remaining area is covered by other Air Districts. For these categories, the percentage of Solano and Sonoma County populations within the Air District boundary (or jurisdiction) are estimated using the Association of Bay Area Government’s (ABAG) Plan Bay Area 2050 dataset (ABAG, 2021). The dataset summarizes population by Travel Analysis Zones (TAZs). The population for each TAZ within Air District’s jurisdictional boundaries is summed and divided by the total county-wide population to estimate the percentage of population within the SFBA for Solano and Sonoma counties. These proportions are shown below:
County | % of Population within the District’s jurisdiction |
Solano | 0.70 |
Sonoma | 0.86 |
Emission Factors
The natural gas leakage methane emission factor is based on the methane content in natural gas and the density of methane (in units of lb/standard cubic feet (scf)). Emission factors are calculated using the values shown below:
Category(ies) | Characteristic | Value | Source |
All | % Methane | 97.3% | PG&E, 2023. Concentration varies but typically between 95% – 98%; historically the Air District has used 97.3%. |
All | Methane Density | 0.044 lb/scf | Engineering Toolbox, 2023 |
The calculation of the natural gas leakage emission factor using million standard cubic feet (MMSCF) as the basis follows as:
CH4 emission factor = 0.973 × 0.044 lb/scf × 106 scf/MMSCF = 42,812 lb/MMSCF
Local Controls
Natural gas usage for residential use and small commercial applications is highly impacted by the implementation and enforcement of the Air District’s Regulation 9, Rule 4 (BAAQMD, 2023b) which limits emissions of nitrogen oxides from natural gas-fired furnaces, and Regulation 9, Rule 6 (BAAQMD, 2023a), which limits emissions of nitrogen oxides from natural gas-fired water heaters and boilers in future years.
In 2029, Regulation 9, Rule 4 effectively stops the sale of residential natural gas space heaters and in 2027, Regulation 9, Rule 6 effectively stops the sale of residential and small commercial natural gas water heaters. As more of these appliances fail and need replacement, the consumption of natural gas and corresponding post-meter leakage for these end-uses is expected to decrease. This rate of replacement is encompassed in a composite equipment turnover rate which is shown below for each category.
Category(ies) | Estimate Equipment Lifespan | Equivalent Turnover Rate | Source |
2535 | 18 years | 0.056 | BAAQMD, 2023 |
2536 | 13 years | 0.077 | BAAQMD, 2023 |
2539 | 25 years | 0.04 | BAAQMD, 2007 |
868 | 17 years | 0.06 | Weighted Average of Above Lifespans |
Historical Emissions / Backcast
For historical backcast of natural gas throughput, the CEC natural gas usage by county for the Residential and Commercial sectors for 1990 to 2011 is used to produce a backcast profile. This profile is normalized to the year 2011 and applied to the year 2011 throughput obtained from the latest CEC publication (CEC, 2023a) to determine throughput for years 1990-2011.
Future Projections / Growth Factor
The CEC publishes an Integrated Energy Policy Report (IERP) every 2 years in which they forecast natural gas usage by an energy provider that accounts for achievable fuel substitution (AAFS) and achievable energy efficiency (AAEE). For all forecasting, a scenario based on a combination of AAFS and AAEE factors that best represents business-as-usual scenario is chosen. Business-as-usual is defined as existing conditions that include impacts from state and federal regulation adopted as of year 2022. To portray business-as-usual forecasting of emissions, the CEC recommends the use of Scenario 3 Programmatic AAFS and AAEE for the Pacific Gas & Electricity (PG&E) regional service area (CEC, 2023c). This forecast does not include the impacts of the following zero-emission appliance standards (ZEAS):
- CARB’s Zero-Emission Appliance Standard: as of this publication of the inventory, this standard has not been finalized
- Air District Regulation 9, Rule 6 (BAAQMD, 2023a), whose future impact is already included in the applicable control curve.
Reductions under Air District Regulation 9, Rule 6 are accounted for only once as a part of the control factor thereby eliminating any double counting errors in the forecast by relying on CEC data which does not include Air District rules in its projections. The use of this projection curve also avoids the inclusion of non-enforceable goals set by CARB.
Sample Calculations
An example calculation for year 2022 for San Francisco County, CH4 emissions for category 2535 (Residential Space Heating) is shown below:
Step 1 | Convert 2022 natural gas usage for Residential sector, obtained from the CEC for San Francisco County, from MMTherms (million therms) to MMSCF (million standard cubic feet) | 128.02 MMTherms × 100,000 MMBtu/MMTherms ÷ 0.001026 MMBtu/scf × 1E-06 MMSCF/scf = 12,477 MMSCF |
Step 2 | Calculate the fraction of natural gas lost due to leakage | 12,477 MMSCF × 0.0050 = 62.4 MMSCF |
Step 3 | Apply the fraction of natural gas usage proportioned for space heating | 62.4 MMSCF × 0.3931 = 24.5 MMSCF |
Step 4 | Natural gas leakage methane emission factor | 42,812 lb/MMSCF |
Step 5 | Convert emissions to million metric tons of CO2 equivalents (MMTCO2eq/year) by applying the GWP of 34 for methane | 42,812 lb/MMSCF × 24.5 MMSCF × 1/2000 ton/lb × 0.907185 MT/ton × 34 × 10-6 MMT/MT = 0.016 MMTCO2eq |
Assessment of Methodology
The general methodology for determining emissions for these categories has not changed. However, the determination of natural gas throughput allocated to each category has been significantly upon by the inclusion of post-meter leakage and an updated split between end-use operations by residential category that greatly improves the accuracy of resulting emissions.
Year | Revision | Reference |
2022 |
|
|
2015 |
|
|
Emissions
The table below summarizes greenhouse gas emissions for the base year 2022 in metric tons of CO2 equivalents (MTCO2eq).
| ID | Description | CH4 | Total |
|---|---|---|---|
| 868 | Natural Gas Distribution - Other Fuel Use | 455103.3 | 455103.3 |
| 2536 | Domestic - Natural Gas - Water Heating | 180438.7 | 180438.7 |
| 2535 | Domestic - Natural Gas - Space Heating | 129213.5 | 129213.5 |
| 2539 | Commercial - Natural Gas | 64616.5 | 64616.5 |
| 2537 | Domestic - Natural Gas - Cooking | 10849.3 | 10849.3 |
| 2541 | Domestic - Natural Gas - Other | 8180.0 | 8180.0 |
Summary of Base Year 2022 Emissions
Although the leakage rates seem small, the GHG emissions resulting from Residential leakage categories are around 0.5 MMTCO2eq for year 2022. As the usage of natural gas in the Residential sector is higher than that in the Commercial sector, and the leakage rate the highest overall, emissions from the residential post-meter leakage categories accounts for most of these emissions.
Contribution of Residential & Commercial Natural Gas Leakage Emissions by Sector| Subsector | Sector | Subsector GHG Emissions (MMTCO2eq) | Sector GHG Emissions (MMTCO2eq) | % of Sector |
|---|---|---|---|---|
| Residential & Commercial Natural Gas Leakage | Commercial + Residential | 0.85 | 12.85 | 6.60% |
Contribution of Residential & Commercial Natural Gas Leakage Emissions to Regional Total
| Subsector | Subsector GHG Emissions (MMTCO2eq) | Regional Total GHG Emissions (MMTCO2eq) | % of Regional Total |
|---|---|---|---|
| Residential & Commercial Natural Gas Leakage | 0.85 | 65.68 | 1.29% |
Trends
The time series chart below shows the emission trends for the applicable categories.
Summary of Trends
Historical and future trends follow historical NG consumption data for the SFBA and predicted future NG consumption for the region. However, the baseline increase predicted from 2023 onward is impacted by the expected implementation of Regulation 9, Rule 6 and Regulation 9, Rule 4.
Uncertainties
It should be noted that in this inventory update, the non-residential NG throughput data received from CEC is apportioned to Commercial sector based on heater/boiler registration data collected under regulation 9-7 requirements. Since 2019 is the latest year for when registration data is available, future inventory updates will use more recent registration data to apportion the CEC throughput for the most recent current years (2020-2023).
For residential leakage, an average leakage factor was applied based on in-home measurements from a sample of 75 California residences (CEC, 2018). Similarly, for commercial leakage, data were derived from measurements at approximately 100 commercial buildings (CEC, 2020). While these studies provide valuable insight, the relatively small sample sizes limit their statistical robustness and generalizability, especially given the diversity of building types and gas appliances across the state.
For pipeline distribution leakage, the leakage factor was derived from field measurements of natural gas distribution systems in the Boston metropolitan region (ACS, 2015). Within this study, only a subset of pipeline types and ages were sampled, limiting the representativeness of the findings for California’s distribution infrastructure.
Despite these limitations, direct measurement studies consistently show that top-down measurements of natural gas leakage—obtained using atmospheric monitoring or tracer release methods—yield leakage rates approximately two to three times higher than bottom-up inventory estimates, which are based on equipment counts and emission factors.
Contact
Author: Ariana Husain
Reviewer: Abhinav Guha
Last Update: 08/11/2025
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