Energy Storage, On-Site Power, Renewables, Solar

In California, clean energy “overbuild” doesn’t have to be a dirty word

Even among states and nations of the world setting ambitious renewable energy targets, California is an outlier.

Driven by dramatic investments in solar power and distributed energy built to take advantage of the state’s abundant sunshine, Gov. Jerry Brown enacted legislation last year that would make California carbon-free by 2045, putting one of the world’s largest economies on a path to a full clean energy portfolio.

At the same time, vehicle electrification is on the rise, with passenger cars and new investments in zero-emission fleet and EV infrastructure reshaping mobility in a way that aligns with the aspirations – of both the state and its people – to dramatically reduce their carbon footprint.

But these changes carry huge implications for California’s grid: How are utilities planning for increased load and charging? What steps are they taking to harmonize this need with grid operations? Is long duration energy storage the answer? Or should utilities “overbuild” solar projects, and invest in the transmission and distribution necessary to connect consumers with the power they – and their vehicles – need?

We recently sat down with group of California utility leaders to better understand the challenges of meeting the state’s zero-carbon goal, and the impact of EV charging on that goal. The conversation centered on the idea of “curtailing” – the idea of overbuilding solar energy projects to meet demand during seasons of low solar resource (e.g. winter) and effectively reducing their generation during times of excess (e.g. spring) to more closely align with demand.

Due to the declining cost of solar, overbuilding generation by 2X to increase output during hours of low generation, becomes a more cost-effective alternative to seasonal storage, which would require hefty investments in new storage technologies. This renewables overbuild, however, means much of the renewable energy during high production seasons will need to be curtailed.


Solar curtailment has long held the stigma that it effectively wastes renewable energy.  

The strictest definition of curtailment is the reduction of output of a renewable generator below what it could have otherwise produced. Oversupply conditions trigger production reductions that result in local or system wide curtailment.  During oversupply conditions, generation from baseload resources like nuclear, geothermal and hydro, are turned down to minimum production levels that still allow units to respond to evening ramps and system contingencies.

If over generation conditions remain when these resources are at minimum production, wind and solar generation are curtailed, essentially wasting the energy that the renewable generator could otherwise produce.

 Thus, what if we didn’t have to waste that curtailed energy? The magnitude of the “curtailed” energy from overbuilding by 2X is so large that it can theoretically supply all of the electricity needed for 100 percent of on-road vehicles in California if they converted to electric (or hydrogen).

If overbuilding becomes the strategy to meet SB100, there’s a huge opportunity to facilitate the transition to electric vehicles, including fuel cell vehicles, by “fueling” vehicles at a relatively low cost with otherwise “curtailed” energy.  In our analysis, there would need to be some additional energy storage still, though far less than deploying massive seasonal storage, as well as upgrades to transmission and distribution systems and significant additional chargers to serve the EV load relative to no EV load.  

By our internal estimates, the cost from all of the additional infrastructure required to meet full electrification amortizes to a few hundred dollars per on-road vehicle per year, over the next 30 years.  

Now, the devils in the details for such a scheme. In addition to curtailment of renewable energy, ramping to meet peak load is also a great concern. The “duck curve” entered the renewable lexicon in recent years when it became clear that solar will be oversupplying the system in the middle of the day when the sun is brightest–this being further exacerbated as larger amounts of renewable energy is adopted to meet California’s 100 percent Zero Carbon goal.

The greater the solar penetration, the larger the ramping capacity required from energy storage and other generators in the afternoon hours as the sun goes down and the load begins to ramp up as people get home from work.  This ramping becomes even worse if everyone who drove an EV goes home and plugs in at the same time.  In fact, utilities across the country are already experiencing this phenomenon associated with “unmanaged” charging, despite relatively low penetrations of EVs.

Thus, utilities are already exploring ways to “manage” the EV charging through workplace charging during the middle of the day, Smart Charging, and new electric rates to shape behavior and charging patterns.

This is where managed EV charging enters the picture. If EVs can be managed to charge coincident to renewable energy production (during daytime hours) or to a “flatter”, less peaky profile, fewer energy storage systems may be needed. High-penetration EV can take advantage of the electricity being generated during the middle of the day through workplace and public charging. Managed charging, in which a utility incentivizes charging up or down to meet the needs of the grid, could be an effective grid resource and give utilities and customers a beneficial alternative to unmanaged charging. Finally, vehicle-to-grid technologies can also offset the need for so much energy storage systems by charging during the day and supplying homes at night.  Treating EVs as a resource, rather than just load, can mean $100 billion or more in avoided costs associated with long-duration energy storage.

The projected growth of EVs is motivating new planning and solutions among utilities, which must consider where they can reliably predict, and control load related to increased charging stations, some of them high-power. Without adequate planning and investment, charging stations could tax the grid and bring about unplanned distribution grid investments to deal with transformer upgrades, brownouts or costly repairs.

That challenge is acute in regions like California, where zero-emissions aspirations have evolved into codified mandates. Traditional business models of utilities as the sole power generator are being upended by the emergence of end-customer and private company-owned renewable technology generation, like solar and wind, as well as other distributed energy resources (DER) and energy management solutions.

But with proper planning and the right kind of investments, these disruptive technologies can be turned into new streams of revenue while helping government meet its emission-reduction goals.

About the authors: Liz Waldren is an Electrical Engineer in Black & Veatch’s Renewable Energy business and leads multiple projects and initiatives related to the deployment and grid integration of renewable energy and energy storage resources. In recent years, she has supported electric utility and developer clients in the design and implementation of renewable and energy storage systems and performed grid integration and interconnection studies for utility-scale renewables and energy storage.

Mon-Fen Hong is a Renewable Energy Strategic Planning Services Manager for Black & Veatch, with more than 18 years in the power and renewable energy industry, leading innovative services to meet client needs in cutting edge renewable energy and battery storage developments. Hong is a recognized subject matter expert on renewable and distributed energy as well as power markets and utility tariffs.