An Estimate of Natural Gas Methane Emissions from California Homes

We estimate postmeter methane (CH4) emissions from California’s residential natural gas (NG) system using measurements and analysis from a sample of homes and appliances. Quiescent whole-house emissions (i.e., pipe leaks and pilot lights) were measured using a mass balance method in 75 California homes, while CH4 to CO2 emission ratios were measured for steady operation of individual combustion appliances and, separately, for transient operation of three tankless water heaters. Measured quiescent whole-house emissions are typically 4/day, though they exhibit long-tailed gamma distributions containing values >10 g CH4/day. Most operating appliances yield undetectable CH4 to CO2 enhancements in steady operation (4, with pilot lights contributing ∼30%. Emissions from steady operation of appliances and their pilots are 13.3 (6.6–37.1) Gg CH4/yr, an order of magnitude larger than current inventory estimates, with transients likely increasing appliance emissions further. Together, emissions from residential NG are 35.7 (21.7–64.0) Gg CH4/yr, equivalent to ∼15% of California’s NG CH4 emissions, suggesting leak repair, improvement of combustion appliances, and adoption of nonfossil energy heating sources can help California meet its 2050 climate goals.