Methane emissions from natural gas appliances remain the least characterized portion of the fossil-fuel supply chain. Here we examine water heaters from 64 northern California homes to (1) quantify methane emissions from natural gas leaks and incomplete combustion while off, turning on or off, and in steady-state operation from 35 homes; and (2) characterize daily usage patterns over ∼1−2 months per water heater to estimate activity factors from 46 homes. Individual tankless water heaters emitted 2390 [95% CI: 2250, 2540] g CH 4 yr −1 on average, 0.93% [0.87%, 0.99%] of their natural gas consumed, primarily from on/off pulses. Storage water heaters emitted 1400 [1240, 1560] g CH 4 yr −1 on average, 0.39% [0.34%, 0.43%] of their natural gas consumption. Despite higher methane emissions, tankless water heaters generate 29% less CO 2 e 20 than storage water heaters because they use less energy to heat a unit of water. Scaling our measured emissions by the number of storage and tankless water heaters in the United States (56.8 and 1.2 million, respectively), water heaters overall emitted an estimated 82.3 [73.2, 91.5] Gg CH 4 yr −1 , 0.40% [0.35%, 0.44%] of all natural gas consumed by these appliances, comparable in percentage to the EPA's estimate of methane emissions from upstream natural gas production.

California hosts ∼124,000 abandoned and plugged (AP) oil and gas wells, ∼38,000 idle wells, and ∼63,000 active wells, whose methane (CH 4 ) emissions remain largely unquantified at levels below ∼2 kg CH 4 h −1 . We sampled 121 wells using two methods: a rapid mobile plume integration method (detection ∼0.5 g CH 4 h −1 ) and a more sensitive static flux chamber (detection ∼1 × 10 −6 g CH 4 h −1 ). We measured small but detectable methane emissions from 34 of 97 AP wells (mean emission: 0.286 g CH 4 h −1 ). In contrast, we found emissions from 11 of 17 idle wellswhich are not currently producing (mean: 35.4 g CH 4 h −1 )4 of 6 active wells (mean: 189.7 g CH 4 h −1 ), and one unplugged wellan open casing with no infrastructure present (10.9 g CH 4 h −1 ). Our results support previous findings that emissions from plugged wells are low but are more substantial from idle wells. In addition, our smaller sample of active wells suggests that their reported emissions are consistent with previous studies and deserve further attention. Due to limited access, we could not measure wells in most major active oil and gas fields in California; therefore, we recommend additional data collection from all types of wells but especially active and idle wells.