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Since 2006, EIA has reported two measures of aggregate capacity, one based on demonstrated peak working gas storage, the other on working gas design capacity.

Demonstrated Peak Working Gas Capacity: This measure sums the highest storage inventory level of working gas observed in each facility over the 5-year range from May 2005 to April 2010, as reported by the operator on the Form EIA-191M, "Monthly Underground Gas Storage Report." This data-driven estimate reflects actual operator experience. However, the timing for peaks for different fields need not coincide. Also, actual available maximum capacity for any storage facility may exceed its reported maximum storage level over the last 5 years, and is virtually certain to do so in the case of newly commissioned or expanded facilities. Therefore, this measure provides a conservative indicator of capacity that may understate the amount that can actually be stored.

Working Gas Design Capacity: This measure estimates a natural gas facility's working gas capacity, as reported by the operator on the Form EIA-191A, "Annual Underground Gas Storage Report," and sums the result across all fields. It is a measure based on the physical characteristics of the reservoir, installed equipment, and operating procedures particular to the site that is often certified by Federal or State regulators. However, logistical, operational, and practical considerations may preclude attainment of the maximum design capacity of a storage field, so the sum of design capacities is likely to exceed actual available maximum storage capacity.