A reliable supply of electricity is often thought to depend on having a large proportion of fossil fuels in the energy mix. However, a clean and reliable power sector can be achieved by ramping up the deployment of renewables and providing a range of flexible energy sources, which will also increase energy security in countries that import fossil fuels.

This explainer examines the reliability of a renewables-dominated system using the example of Great Britain. Once the largest source of greenhouse gas emissions in Britain, the electricity sector has achieved a remarkable reduction in greenhouse gas emissions since the early 1990s, initially by making greater use of natural gas in place of coal and latterly, particularly over the last 10 years, through the growth of wind and solar energy. Delivering a fully decarbonised power sector will be vital to tackle climate change and must be achieved in such a way to guarantee a reliable supply of electricity.

How can flexibility of supply be achieved in a decarbonised power system?

For the power system to operate in a stable manner, the rate of production of electricity must closely match the rate of use. In Britain the system operator is responsible for ensuring that supply and demand match on this basis. Historically, doing so has depended on the ability to store fuel – which in Britain has primarily meant fossil fuels – until needed to meet demand for electricity. When demand increases, additional power generation is switched on; when demand falls – overnight, at weekends and over the summer – any generation plant that is not required can have its output reduced or be shut down. To cover the risk that demand is higher than normal or some generation plant is unavailable due to technical problems, a certain minimum total generation capacity is maintained, providing a capacity margin over the peak demand.

As we move to a power system with a greater share of renewables – in 2023, 28% of Britain’s electricity production was from wind and 5% from solar, up from 10% for wind and solar combined in 2013 – the variability of energy from renewables must be considered within system planning and operation. Residual demand for electricity – the difference between demand at a particular time and the power generated at that time from wind and solar – can change quite quickly, requiring what is known as system ‘flexibility’ to ensure supply and demand match on a second-by-second basis.  Achieving system flexibility to ensure supply meets demand is possible through changing demand for energy, modulating the output of generators or managing flows of energy across interconnections with other countries. System operators are learning how to make use of flexibility without depending on fossil fuels, such as through smart charging of electric vehicles (which can be managed to match times of high renewable energy availability) or use of ‘grid scale’ batteries.

How can periods of limited and excess renewable power generation be managed?

A low-carbon electricity system requires low-carbon sources of energy to meet demand when wind speeds and solar irradiation are low, potentially over days or weeks of a relative ‘wind drought’. Conversely, the large total capacity of wind and solar generation expected in the system in the future will also result in periods when the available power from renewables will exceed demand, providing the opportunity to either export the excess power or store it for future use. Many studies are now showing that production, storage and use of hydrogen provide a viable means of dealing with these two scenarios – in effect, moving energy between periods of surplus renewable power, when hydrogen might be produced via electrolysis, and periods of low renewable output, when stored hydrogen could be used to generate electricity.

There are also other options for ‘schedulable’ low-carbon sources of power to complement renewables, e.g. nuclear power and natural gas with carbon capture and storage (CCS). Interconnections with other countries will also play an important role, especially when the natural resources fit well with each other. For example, Norway can make use of Britain’s surplus wind power instead of using its hydropower reservoirs, allowing the water to be stored and the power it generates to be used at times of low wind speeds for its domestic consumption plus for export to Britain.

All of this means that fossil fuels are not required for a reliable energy system as clean alternatives can help balance the system on short as well as longer time scales. Furthermore, boosting both renewables and replenishable energy storage capacity as part of our power system will reduce dependence on fossil fuel imports, strengthening domestic energy security and resilience.

Authors and contacts
This background briefing was written by Keith Bell, holder of the Scottish Power Chair in Future Power Systems, University of Strathclyde and edited by Georgina Kyriacou (London School of Economics) and Caterina Brandmayr (Imperial College London). 

It was produced as part of a UK-focused ‘myth-busting’ project between the LSE and Imperial College London Grantham Institutes. The series of ten explainers will be published as a single volume in spring 2024. The project is designed to deepen understanding of climate change action among current and prospective decision makers, the policy community and the public in the UK in the run-up to the 2024 general election.

Media enquiries: grantham.media@imperial.ac.uk  
Policy enquiries: c.brandmayr@imperial.ac.uk 

Read other essays in this series:

How cost-effective is a renewables-dominated electricity system in comparison to one based on fossil fuels?
How the transition to net zero will affect the UK economy (LSE)
What do times of economic hardship mean for the UK’s transition to net zero? (LSE)
What does more North Sea oil and gas mean for UK energy supply and net zero? (LSE)
How will climate policy impact the British public and what factors underpin support for climate action? (LSE)