The Energy Institute’s most recent Statistical Review of World Energy found that renewables’ share of total electricity generation rose from 29% to 30% in 2023. Excluding hydropower, renewable generation grew by 13% to a record high of 4,748 TWh, driven almost entirely by wind and solar, which accounted for 74% of all net additional electricity generated. The International Energy Agency (IEA) projects that by 2030, renewables will generate 46% of the world’s electricity, with wind and solar leading the way. But, of course, every night the sun goes down and the wind does not always blow.

Mitigations to the variability of renewables include diversifying generation sources, adding energy storage, expanding and upgrading transmission and distribution networks, implementing flexible demand, and innovating grid operations. Successful large-scale grids worldwide have already achieved renewable energy penetration rates of up to 71%, leading to greater reliability and lower electricity prices, according to a 2024 report from the Pembina Institute. However, failure to implement integration measures could jeopardise up to 15% of global solar and wind generation by 2030, resulting in a 20% smaller reduction in carbon emissions from the power sector, warns the IEA.  

How intermittent is intermittent? 
Solar generation is naturally limited to daylight hours. It peaks at midday and drops to zero at night. Clouds and fog can significantly impact output, creating short-term fluctuations. Seasonal variations also play a role. For example, the daily average light energy incident on a horizontal surface in summer in London is six to seven times higher than in winter. Regions closer to the equator experience more stable solar production due to less seasonal variation. Solar output can be predicted more reliably in areas closer to the equator with less seasonal variation, and generally less cloud and fog.  

Wind power is less predictable than solar, fluctuating based on local weather conditions. However, aggregating multiple wind farms across large areas helps overcome rapid changes and maintain grid stability. Studies show that regional wind output rarely fluctuates by more than ±5% of installed capacity thanks to the geographical spread of turbines. Even during storms, where turbines must shut down for safety, the transition is gradual and predictable and modern turbines have storm control features to manage power reduction. For example, during a severe Danish storm in 2005, wind power declined from 2,000 MW to 200 MW over six hours.

These variations become more significant for grid management when wind power reaches 5–10% of annual electricity demand. Grid operators must accurately predict fluctuations and develop strategies to counteract supply swings. Fortunately, solutions such as improved forecasting, energy storage, diversification, and stronger grid interconnections are proving effective in mitigating variability.

Wind and solar complementarity  
Although wind and solar power rely on different natural resources, their patterns of generation typically complement each other. Wind speeds typically pick up at night when solar panels are inactive, and wind generation is strongest in winter, while solar production is highest in summer. This seasonal and daily complementarity reduces power fluctuations, enhancing grid reliability. While the extent of this complementarity varies by location, it’s a universal phenomenon that exists to some degree worldwide.

And it is valuable. Integrating wind farms with solar plants into hybrid energy systems increases renewable energy use in power grids whilst reducing the need for energy storage and backup systems. Co-locating wind and solar plants also facilitate shared grid connections and development costs, making projects cheaper and quicker to become operational. Studies indicate that co-locating wind and solar can increase electricity output by up to 100% compared to using either technology alone. This hybrid wind and solar approach is particularly valuable for microgrids, where it can significantly reduce battery storage needs. Though hybrid systems still need some battery backup for windless nights, they require far less storage than single-source systems.

Many countries have successfully integrated high levels of variable renewable energy generation into their grids and have managed this challenge for years. By late 2021, Denmark surpassed 50% and Germany, Ireland, Spain and the UK had all integrated above 25% variable renewable energy, meaning that during certain periods of the year variable renewables have supplied up to almost all generation, according to an IEA report.

The 2024 Pembina study examined six diverse electricity grids – California, Texas, Ireland, Germany, South Australia and Denmark – where wind and solar penetration ranged from 32% to 71%. Despite significant differences in climate and population size, each grid successfully integrated renewables through hybrid systems, energy storage expansion and inter-regional transmission lines (interties). The study’s authors found that a combination of hybrid wind and solar grids, increasing energy storage capacity to improve grid flexibility, and connecting jurisdictions with interties made it possible for the six grids to integrate high levels of non-dispatchable energy. For example, California added over 11,000 MW of clean energy and expanded battery storage to nearly 10,000 MW between 2022 and 2024. Without increasing fossil fuel generation, these measures enabled the state to withstand record-breaking demand during last year’s summer heatwave whilst exporting surplus electricity.  

Notably, higher renewable penetration often leads to lower electricity prices, as most power markets set prices based on the most expensive dispatchable source – typically natural gas. This encourages further renewable adoption.

The role of interties  
Strong regional grid connections are essential for handling high renewable penetration. These links allow power to flow between regions, moving excess wind or solar energy from high-generation, low-demand areas to those in need.

In recognition of their importance, the EU has set an interconnection target of 15% by 2030, requiring each member country to export at least 15% of its electricity capacity to neighbouring states.

Long queues for project approval are a major issue with interties. In 2023, IEA analysis identified over 1,500 GW of renewables stuck in connection queues globally. Interest in developing renewable energy has grown thanks to policy support and competitive technology prices, leading more projects to enter connection queues, which has in some cases led to higher project lead times. 

Policy changes aim to help overcome this issue. For instance, Australia plans to build 127 GW of large-scale wind and solar power by 2050. To overcome current problems with grid connections that delay projects and hurt their finances, the country is establishing Renewable Energy Zones (REZs). These zones are designed for clusters of renewable projects with coordinated planning of both power generation and transmission infrastructure. The REZs aim to speed up connections, share costs between projects, and boost system reliability.  

Storage – another key solution 
Energy storage is critical for managing renewable variability. Different storage technologies apply for various needs. They range from lithium-ion batteries for short-term balancing, providing fast-response grid stability, to pumped hydro storage of large amounts of energy for days or weeks, to thermal storage (such as molten salt, hot water tanks) to potentially green hydrogen for long-term seasonal storage.

Battery costs are declining rapidly, with IRENA predicting a 50–60% price drop by 2030, and installed lithium-ion systems potentially cost less than £160/kWh. These price drops, driven by improved manufacturing and materials use, will come alongside better battery performance and longevity. As storage becomes more affordable, grids will increasingly rely on it to cushion renewable fluctuations.

• Further reading: ‘New technology options for long-duration energy storage’. The increasing incorporation of local renewable generation capacity into electricity grids has led to the development of new energy storage technologies.
• Find out why we need a whole-system strategy for green electricity.