The study presents the development of a method for the long term monitoring of methane emissions from pressure relief valves (PRV1) of biogas storages, which has been verified during test series at two PRVs of two agricultural biogas plants located in Germany. The determined methane emission factors are 0.12 g CH4 kWh(el)(1) (0.06% CH4-loss, within 106 days, 161 triggering events, winter season) from biogas plant A and 6.80/7.44 g CH4 kWh(el)(1) (3.60/3.88% CH4-loss, within 66 days, 452 triggering events, summer season) from biogas plant B. Besides the operational state of the biogas plant (e.g. malfunction of the combined heat and power unit), the mode of operation of the biogas flare, which can be manually or automatically operated as well as the atmospheric conditions (e.g. drop of the atmospheric pressure) can also affect the biogas emission from PRVs. (C) 2016 Elsevier Ltd. All rights reserved.