Long-duration energy storage has spent five years as a slide in an investor deck. On August 12 it became a dirt job.
Granite Construction was awarded an approximately $31 million Early Works Grading Package by Hydrostor for the Willow Rock Energy Storage Center, a planned 500 MW Advanced Compressed Air Energy Storage facility in Kern County, California. What Hydrostor is buying is not a battery installation. It’s mass excavation in the Mojave Desert, on a schedule.
The scope is earthwork and underground mining
Granite’s package covers early site infrastructure across an 89-acre site. The centerpiece is a nine-acre pad engineered to support the drilling and excavation of a cavern of roughly 1.3 million cubic yards, about 2,000 feet below the surface. The scope also includes offsite road improvements, onsite stormwater basins and laydown areas sized for the subsurface mining spoil.
That last item is the one contractors should notice. Where the spoil goes is a design problem, not an afterthought, and Hydrostor is planning laydown for it in the early works package rather than solving it during the mining. Granite says the grading phase began in July 2026 and concludes in March 2027. Hydrostor says the completed plant will power more than 400,000 homes for eight hours.
Every one of those figures comes from the developer or the contractor. None has been independently verified, and the total project cost and main construction award value aren’t public.
A different contractor profile than utility-scale storage has used
Grid-scale storage to date has been an electrical scope. Lithium-ion installations are containers on pads with switchgear and controls, and the work has gone to EPC firms and electrical subcontractors. Compressed air storage is not that. It’s a mining and heavy civil job with a power plant attached, and it opens a category of energy-transition work to firms that have watched the sector go to trades they don’t have.
Hydrostor is Toronto-based, so the developer isn’t a U.S. company, but the work and the contractor are. Granite is carrying a record backlog and just added a project type most of its competitors have no reference for.
Watch the sequencing, not the dollar value
$31 million is small for a 500 MW facility. The tell is that it was let separately and early, ahead of the main contract. Owners who split early works out are usually doing one of two things: derisking the geotechnical unknowns before pricing the main scope, or buying schedule while financing closes. Either way the early works contractor learns the site conditions first and prices the follow-on accordingly.
Advance packages ahead of a much larger main contract are standard practice in heavy civil for exactly this reason. The Army Corps ran the same play at Garrison Dam, letting a $23.8 million dredging and riprap contract years ahead of the $1.8 billion spillway rebuild.
Follow the main award. That’s where the money is, and Granite has bought itself the incumbent’s seat.
Source: Hydrostor. Award details via Granite Construction’s investor release.