From the air, the Mount Signal Solar facility looks like a strange agricultural development: regular rows covering mile after mile of the southern Imperial Valley, west of Calexico near the Mexican border. Up close, the rows resolve into more than three million thin-film solar panels tracking the arc of the sun on single-axis mounts, converting the intense desert radiation into electricity that flows north to San Diego and east to Southern California Edison's grid. At 794 megawatts peak capacity, Mount Signal Solar is among the largest photovoltaic power stations on Earth — built in three phases over nearly a decade on land the environmental groups supporting it described as low-productivity farmland.
The facility's history began with a different technology. The California Energy Commission approved a project called SES Solar Two on September 29, 2010 — a power station designed around Stirling engine technology, which converts heat from focused sunlight into mechanical power. AES Solar subsequently renamed the project Imperial Valley Solar before notifying the commission in June 2011 that it intended to abandon the design. The pivot came in February 2012, when AES Solar and 8minutenergy Renewables announced a joint plan to revive the project using photovoltaic technology instead of solar thermal. The name became Mount Signal Solar, after the prominent peak visible from the project site.
Construction on the first phase began in 2012, and Mount Signal 1 came online in 2014, providing 266 megawatts peak (206 megawatts AC) to San Diego Gas and Electric under a 25-year power purchase agreement. More than three million thin-film cadmium telluride photovoltaic modules from First Solar were deployed on 138 skids designed and manufactured by Elettronica Santerno. Upon completion, it was the world's largest solar project using single-axis trackers — mounting systems that rotate panels throughout the day to follow the sun's path, increasing energy capture compared to fixed installations. The first phase cost $365 million.
Phases two and three expanded the facility for Southern California Edison under separate power purchase agreements. Phase 3 — a 328 megawatt DC (254 megawatt AC) installation on 2,000 acres — came online in July 2018, using First Solar's Series 4 thin-film panels and Nextracker's NX Horizon smart solar tracker systems. Mount Signal 2, at 200 megawatts DC (154 megawatt AC) on approximately 1,260 acres, was commissioned in January 2020. Together, the three phases bring the facility to 794 megawatts peak, covering approximately 5,000 acres of the Imperial Valley floor.
The choice of cadmium telluride thin-film panels from First Solar for the initial phase reflects a deliberate technology selection. CdTe thin-film panels produce electricity differently from the crystalline silicon panels that dominate the residential solar market: a thin semiconductor layer is deposited on glass rather than cut from silicon ingots. First Solar, an American manufacturer, has been the dominant producer of CdTe thin-film panels, and utility-scale projects like Mount Signal have been their primary market. The combination of thin-film panels with single-axis tracking represented the leading edge of utility-scale solar design when construction began in 2012.
The Imperial Valley's solar resource is exceptional: high insolation, low cloud cover, and the flat terrain that makes large-scale ground-mounted installations practical. Mount Signal 1's reported capacity factor of 29.7% — meaning the plant produces roughly 30% of what it would if operating at full capacity continuously — reflects this resource quality. The annual net output of Mount Signal 1 alone averaged 537 gigawatt-hours between 2015 and 2017. The environmental case for the project rested partly on the land's prior use: low-productivity farmland that generated less economic and ecological value than the solar installation that replaced it, making the conversion relatively uncontroversial among the environmental groups that might otherwise have opposed a development of this scale.
Mount Signal Solar is located at approximately 32.67°N, 115.64°W, west of Calexico in the southern Imperial Valley near the US-Mexico border. The 5,000-acre facility is one of the most visually distinctive features in the region from the air — a vast grid of solar panels visible from cruising altitude as a dark-toned rectangle against the pale agricultural landscape. Mount Signal peak provides a visual landmark to the west. El Centro Regional Airport (KELN) and Calexico International Airport (KCXL) are the nearest facilities.