Overbuilding solar at up to 4 times peak load yields a least-cost all-renewables grid


Solar capacity reaching up to 4.3 times peak load in sunny regions, and wind capacity of up to 2.1 times peak load in windy regions, would form the basis of a least-cost all-renewables resource mix in regions across the United States.

Those are the results of modeling by the energy firm Wartsila. The Finland-based firm used the Plexos resource planning model, together with technology cost projections for 2030 from Bloomberg New Energy Finance, to find the least-cost resource mix that would meet demand at all hours of the year.

Overbuilding enough renewable capacity to meet winter demand eliminates the need for seasonal storage. Overbuilding of solar implies substantial curtailment of solar generation during the summer, as shown in the image above, at the top of the hump.

“In all regions we have to overbuild solar and wind capacity, in many cases multiple times over the peak load,” explained Wartsila Power System Analyst Antti Räty. “This is purely the most economic option. Integrating a lot of flexibility into the systems allows us to take full potential of the installed renewables and keep the excess capacity to a minimum.”

Under the modeled resource plans, most Americans would get between 57% and 81% of their electricity from solar power, while wind power would provide up to 76% of electricity in windy regions.


Only four to ten days of multi-day storage capacity would be needed, depending on the region. That would require shifting only 3% to 7% of total generation to powering hydrolyzers, to produce hydrogen from water, and perhaps other technologies to produce a fuel from hydrogen. Fuel would be stored and later used to help meet electricity demand during stretches of the lowest solar and wind generation.

Substantial battery capacity would be needed in regions with the highest percentage of solar generation. In the Southeast, batteries would store up to 36% of annual generation for delivery after the sun goes down. In the windiest regions, battery capacity would be modest, shifting less than 10% of total generation.

A clean sheet

Wartsila’s modeling took a clean sheet approach, starting with only existing hydropower and geothermal units, and letting the model select the best mix of additional renewable and storage resources. Wartsila obtained data on hydropower and geothermal units from Lappeenranta University of Technology studies on 100% renewable energy systems.

If electricity demand were to increase without changing the load profile, the same proportions of each type of capacity would still be optimal, said Räty. The extent to which the load profile would change from electrification of transportation and heating was not evaluated. Electrified transportation may be able to provide flexibility to the system, Räty noted, as vehicle batteries may substitute for some grid batteries, and “smart charging could allocate more demand to times of high solar output.”

The modeling results will inform Wartsila’s consultative sales work, as “with our stakeholders and clients, we can together come up with solutions that will take into account what type of assets they should be investing in, and when,” said Saara Kujala, a Wartsila business development executive, in a company post.

Regional results

Wartsila’s modeling results are available from an interactive map on a web page titled “Our Vision: 100% Renewable Energy.” Clicking on a region of a map brings up top-level results for the region, and then scrolling down provides further information.

Here are the optimal resource mixes selected by Wartsila’s modeling for U.S. states and regions, ranked by the percentage of solar generation:

Wartsila’s interactive map links to 145 countries and regions globally. In each one, “pivoting towards 100% renewable energy is possible” says a Warsila post.

Related work

Wartsila’s modeling corroborates the “overbuild/curtail” modeling results from Clean Power Research Senior Researcher Marc Perez and others.

Last fall, Vibrant Clean Energy modeled a low-carbon grid for Colorado in which electrification of transportation and heating allowed high levels of solar and wind on the grid, reducing costs for consumers.

Last winter, Mark Jacobson of Stanford and co-authors modeled a U.S. grid with electrified transportation and heating, powered by 2,000 GW of solar and 2,300 GW of wind, plus 3,300 GW of batteries and a large amount of flexible load. The modeled system yielded cost savings for consumers, largely from electrification of transportation and heating.

A Science journal article last summer examined rapid solar deployment, describing how to reach “a future with ~10 terawatts of PV by 2030 and 30 to 70 terawatts by 2050, providing a majority of global energy.”