California’s geothermal sector in 1990 stood at an inflection point—benefiting from decades of public investment, shifting regulatory landscapes, and the persistent optimism that clean energy could, and would, scale. For analysts or policymakers looking back, tracking the growth of this sector means marrying technical data with business realities. ISIC 3510—electric power generation, transmission, and distribution—serves as a natural starting point, though the code is a catch-all, covering everything from coal to hydro, and, of course, geothermal.
The process begins by assembling a roster of all ISIC 3510-registered firms operating power plants in California around 1990. This population includes major utilities, independent power producers, and smaller municipal agencies. Not all of them were involved in geothermal, so the next step is to identify which among them were actually operating geothermal plants. Utility annual reports, California Energy Commission filings, and trade press from the period can be mined for lists of project owners and operators. The state’s largest field, The Geysers, provides a useful focal point—at the time, it was home to a constellation of operators, from industry giants to new market entrants.
Once the geothermal cohort is established, the task shifts to capacity additions. This involves collecting commissioning dates, installed megawatt ratings, and, if possible, project-level output data for each geothermal plant. Regulatory filings, technical conference proceedings, and, sometimes, environmental impact statements hold much of this information. A year-by-year breakdown shows which firms brought new capacity online and which simply maintained or retrofitted existing facilities.
But output data alone can mislead. Geothermal’s technical potential is determined by subsurface heat—the geothermal gradient. Publicly available geological maps, published by the US Geological Survey and academic groups, detail where gradients are highest and where energy extraction is most feasible. Overlaying the location of new (or expanded) geothermal projects onto these maps gives a sense of how well actual investment matched physical opportunity. Some projects clustered tightly in known hotspots, others took chances on less-proven fields, sometimes encouraged by policy or speculative optimism.
Integrating gradient maps with firm-level output is a multi-step affair. Each plant can be mapped to its local gradient, and output over time can be compared to the estimated potential of the underlying resource. Analysts can then look for patterns—did operators consistently build where gradients were strongest, or were other factors (land rights, water availability, incentives) equally decisive? Did underperforming plants correspond to marginal gradient areas, or to operational or financial challenges?
Another dimension is to consider the regulatory and incentive context. The late 1980s and early ’90s saw a range of state and federal incentives for renewable energy, from tax credits to mandated utility purchases under the Public Utility Regulatory Policies Act (PURPA). Project approval and commissioning timelines often reflect these policy rhythms, with flurries of new capacity following major incentive announcements. Careful attention to these policy shifts helps avoid misattributing capacity surges to resource quality alone.
The documentation of all steps—how firms were classified, how output and gradient data were matched, how policy events were integrated—is crucial. Not all records are complete; plant ownership shifted, and measurement standards evolved. The resulting analysis is rarely linear, but it does reveal the connections between geology, business strategy, and public policy.
Through the lens of ISIC 3510, and by layering technical and corporate data, it becomes possible to reconstruct the geography and timing of geothermal expansion in California at the start of the 1990s. The sector’s evolution was shaped by more than just subsurface heat—it was, as always, a product of regulatory choices, market entrants, and a degree of risk-taking by operators betting on the still-emerging promise of renewable power. The patterns are messy, the boundaries sometimes blurred, but the momentum is there, running just beneath the surface.