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Cost-effective approaches to maintaining resource adequacy in wind and solar electricity systems over decades of weather variability

Natasha D. Reich, Tyler H. Ruggles, Edgar Virgüez, Jacqueline A. Dowling, Ken Caldeira, and Nathan S. Lewis · Environmental Research: Energy · 2026

Key finding. For systems planned on a short weather record and then operated across decades of real variability, coordinated spending across both additional storage and additional generation improved resource adequacy more than spending the same amount on either category alone; wind-only additions reached adequacy standards whatever storage was available, while solar-only additions often could not without enough storage to cover the night.

What question did this research address?

Capacity expansion models pick generation and storage by optimizing against a limited number of historical weather years. The system they design then has to operate for decades, through weather the optimization never saw.

That leaves a gap between the capacity a short optimization specifies and the capacity long-run reliability actually requires. This paper asked how large the gap is, and — given a fixed budget to close it — what to spend it on.

What did we find?

The analysis uses a stylized single-node macro-scale energy model of the contiguous United States: plan the system on a limited weather period, then operate it across decades of weather-derived variability and measure how much extra capacity is needed to hold resource adequacy.

Splitting a fixed budget across both storage and generation outperformed concentrating it in one category. The two shortfalls a long record exposes — too little energy and too little capacity at the wrong moment — are not addressed by the same asset.

Where spending had to go to one category, wind was the one that worked. Additional wind generation alone could reach resource adequacy standards for the operational period regardless of which storage technologies were available.

Additional solar alone frequently could not, because solar contributes nothing overnight and so cannot substitute for storage in the hours where adequacy actually fails.

The implication for planning is that both wind generation and storage capacity need to be expanded beyond whatever a short-horizon optimization specifies — the short optimization systematically under-builds both.

The authors argue current adequacy standards need revisiting. A criterion such as NERC's one-hour-in-a-decade loss-of-load expectation is not framed around extreme events, and they suggest treating deep multi-day wind droughts the way flood planning treats a once-in-a-century flood.

Why does it matter?

It turns a known modelling limitation into a design rule. That short optimizations under-specify capacity was already understood; this says which assets to buy with a limited budget to fix it, which is the question a planner actually faces.

The asymmetry between wind and solar is the practical core. They are usually discussed together as variable renewables, but for closing a reliability gap they are not interchangeable — solar cannot cover the hours where adequacy fails, so it needs storage alongside it in a way wind does not.

Framing the binding case as a once-in-a-century wind drought rather than an hours-per-decade average changes what a reliability standard is even measuring, and standards written the old way may pass systems that would fail on a rare deep event.

Citation

Natasha D. Reich, Tyler H. Ruggles, Edgar Virgüez, Jacqueline A. Dowling, Ken Caldeira, and Nathan S. Lewis (2026). Cost-effective approaches to maintaining resource adequacy in wind and solar electricity systems over decades of weather variability. Environmental Research: Energy.

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