Steam deliverability of the original 20 wells, supplying steam to PG&E Unit 13 power plant, has undergone annual exponential decline rates of 11.5%, 5%, 9%, 28.6% and 22% during 1980–81, 1983–84, 1985–86, 1987–88 and 1988–89 respectively. High decline rates after 1984 were influenced by the regional effects associated with the installation of new capacity in the adjoining areas totalling 383 MW (net) in 1983–84 and 347 MW (net) in 1985–86. Unit 13 production declined harmonically for the first 6.6 years, nearly exponentially for the next year and hyper-bolically to harmonically thereafter. The near exponential decline trend resulted from the combined effect of local and regional pressure drawdowns and an accelerated makeup well drilling program. Analysis of the decline rate versus generating capacity suggests that a sustainable 15% annual exponential decline rate (2 MW per month) is achievable if the capacity is held at about 170 MW (net) in the Unit 13–16 wellfield and 830 MW (net) in The East Geysers. This estimate excludes the positive effects of injection seen in late 1989 and 1990.