Wind and solar power plants rely on intermittent sources of energy that vary with the weather. We cannot control the weather, so we cannot control the output from wind and solar power plants. As a result, even though we can sometimes get a significant amount of energy from them, wind and solar alone cannot guarantee us energy all the time, especially during extreme weather events (e.g. extended cold or heat waves) – when we need them the most. When only modest amounts of wind and solar are on the grid, intermittency is manageable, as the dispatchable sources (fossil, nuclear and to a lesser extent, hydro) can supply energy when wind and solar are unable to. However, with high penetration of wind and solar, the bulk of the capacity of the grid would be from intermittent sources with little dispatchable energy support. This would drastically increase the risks of grid outages and insufficient supply. Adding excess wind and solar capacity alone will not solve the problem, as it is only a matter of time when the weather pattern will prevent these new wind turbines and solar panels from producing meaningful amounts of energy, for example: very low or very high wind speeds outside of the wind turbines’ operating range will result in minimal generation from wind farms ; at night and during times of heavy cloud cover, the output of solar farms is negligible as well. And there are times of year when low or high winds occur at night or during times of extensive cloud cover.
Intermittency management strategies such as industrial scale battery storage have been proposed to facilitate 100% wind and solar. While this solution to decarbonization is theoretically possible, it is not practical or cost effective. According to the Clean Air Task Force, if the California Internal System Operator (CAISO) were to supply its entire 2018 demand with 50% wind and 50% solar, surplus energy generated during the peak summer season would need to be stored for use during the winter, when both wind and solar generation is low. To store all of this energy would require a battery system with 35,946,633 MWh of storage capacity; this is approximately 14% of California’s annual electricity demand (Figure 1) .
To gain further perspective of the impacts of intermittency, if one were to attempt to capture all of the surplus energy generated under this scenario during the periods of highest wind and solar peak, the Clean Air Task Force estimates that “you would need a storage system equivalent in instantaneous capacity larger than the generating capacity of the entire U.S. electric grid” .
Figure 1: California energy surplus and deficit in CATF 2018 50% Wind, 50% Solar scenario 
Wind and solar have the potential to generate a lot of energy at times when conditions are just right. However, these conditions do not reliably occur, so generation will seldom be at that magnitude. But the battery system must capture all of the surplus wind and solar energy produced, so it will need to be able to match that maximum capacity (rate of energy transfer; in this case, battery charging), even if it is only for short spurts. These requirements for storage size and charge/discharge capacity result in the need for a massive battery system. Manufacturing such a large system will be very expensive, and will have significant environmental impacts due to the necessary mining of huge quantities of raw materials, notably lithium and cobalt.
Replacing existing fossil fuel power plants and constructing wind and solar farms will also incur significant costs. Examining the cost impact of wind, solar, and batteries for California, the Clean Air Task Force predicts energy supply costs will increase exponentially as wind and solar penetration approaches 100% when battery systems are installed to support them: $57/MWh at 60%, $389/MWh at 80%, and $1,402/MWh at 100% wind, solar, and batteries  (Figure 2). By way of comparison, it costs about $30/MWh from coal.
Figure 2: The majority of battery storage capacity would be used for seasonal storage, very little (about 1%) of capacity would actually output electricity (and generate revenue) every day. Most of the battery capacity is only discharged a few times per year, but the capital (construction) cost of the battery system remains the same. Seasonal storage results in the batteries’ capital cost being covered by much less energy output compared to daily-use batteries, causing the cost per MWh output from seasonal storage batteries to be very high. The predicted MWh cost of a mixed energy system including firm (reliable) carbon-free sources is shown to the right of pure wind and solar 
Even with the assumption that capital costs will decrease by ~85% from their 2018 rates (~$500/kWh to $80/kWh), the capital cost of building a storage system capable of supporting 100% wind and solar for the California’s system is $2.9 trillion . This is greater than California’s 2019 real GDP of $2.8 trillion . Such high costs can deter the public from believing we can achieve carbon-neutral energy generation and will ultimate prove difficult to implement into policy.
Furthermore, even with the use of mitigation strategies, the intermittency of wind and solar presents insurmountable challenges at high penetration. According to a meta-study of 40 deep grid-decarbonization studies, assuming the U.S. were to upgrade to a sufficient amount of nationwide high-speed transmission lines, have a widespread deployment of battery storage, and have sufficient demand flexibility (through controllable electrical vehicle charging), an 100% wind and solar energy supply would still need to curtail the equivalent of 40% of annual demand, meaning 40% of the annual demand would be wasted .
It is important that we recognize potential drawbacks of each energy technology in our efforts to sustainably meet the growing energy demand of our global society. We must find energy solutions that can mitigate our greenhouse gas emissions while supplying all people with reliable, clean energy with minimal impact on the environment.
While it is not practical to depend on wind and solar for 100% of our electricity, that doesn’t mean wind and solar cannot play an important role in our sustainable energy future, along with proper amounts of other low-carbon or carbon-negative technologies.
1 – “How Do Wind Turbines Survive Severe Storms?” Energy.gov, Department of Energy, www.energy.gov/eere/articles/how-do-wind-turbines-survive-severe-storms#:~:text=The%20cut%2Din%20speed%20(typically,its%20maximum%2C%20or%20rated%20power.
2 – Cohen, Armond. “Re: SB 100 Joint Agency Report: Charting a Path to a 100% Clean Energy Future, Docket No. 19-SB-100.” Clean Air Task Force, 19 Sept. 2019, www.catf.us/wp-content/uploads/2020/01/CATF-Comments-SB100-Letter-1.pdf.
3 – “U.S. Federal State of California – Real GDP 2000-2019.” Statista, Statista, 20 Jan. 2021, www.statista.com/statistics/187834/gdp-of-the-us-federal-state-of-california-since-1997/#:~:text=In%202019%2C%20the%20real%20GDP,was%202.79%20trillion%20U.S.%20dollars.
4 – Jenkins, Jesse D, et al. “Getting to Zero Carbon Emissions in the Electric Power Sector.” Joule, vol. 2, no. 12, 19 Dec. 2018, pp. 2498–2510., doi:https://doi.org/10.1016/j.joule.2018.11.013