When the wind drops: Wind power needs the wind to blow, and solar power needs the Sun. Their output is intermittent, but people need electricity all the time.
The points to remember
- Supply must match demand every second, or the grid fails.
- Solar stops at night and falls under cloud and in winter.
- Wind stops when the air is calm; tides come and go twice a day.
- So their output is variable: it depends on weather, season, time of day or tides.
- Energy must be stored when production is high, for when it is low.
- Sources that can be turned up at will: hydro with a reservoir, geothermal, biomass, fossil fuels.
Intermittent
- Solar: night, cloud, winter
- Wind: calm days
- Tidal: slack water twice a day
Can be turned up at will
- Hydropower with a reservoir
- Geothermal and biomass
- Fossil fuels and nuclear
Not 'overproduction': Storage is needed because supply is variable, not because renewables 'make too much'. Say: store at times of high production for times of low production.
Real example: in November 2024 a cold, cloudy, windless spell, a Dunkelflaute, cut Germany's wind and solar output to a trickle, and electricity prices jumped.
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Storage is how a grid moves energy from when it is plentiful to when it is needed.
The points to remember
- Demand has peaks: in many places the biggest is in the early evening.
- Solar output peaks at midday, when demand is not at its highest.
- Peak-shaving = levelling out the peaks: store surplus energy, release it at the peak.
- Without storage, the peak is met by fossil fuel stations kept on standby.
- Storage lets a grid use more wind and solar and waste less of it.
Midday
Solar output is high: charge the stores.
Evening
Solar falls, demand peaks: release the stored energy.
Result
The peak is 'shaved': fewer standby fossil fuel stations.
Remember it as: Store it when it is plentiful, use it when it is needed.
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Pumped hydroelectricity storage (PHS) is the world's largest kind of electricity storage.
The points to remember
- Two reservoirs at different heights.
- When there is surplus electricity, pump water uphill.
- At the peak, let it flow down through turbines to make electricity.
- It returns about three quarters of the energy used to pump it.
- For: huge, long-lasting, quick to start. Against: needs mountains and water; floods valleys; costly.
Surplus power
- Night or a windy, sunny spell: cheap electricity pumps water up.
Upper reservoir
- The water stores the energy, ready for hours.
Peak demand
- Water rushes down through turbines and makes electricity within seconds.
Real example: Dinorwig, built inside a mountain in Wales and opened in 1984, can go from standby to 1,728 MW within 16 seconds and stores about 9.1 GWh; it meets the surge when millions of kettles go on at the end of a big football match.
Large batteries are the fastest-growing kind of storage. They are built in months, not years.
The points to remember
- Lithium-ion batteries store electricity as chemical energy.
- For: respond in a fraction of a second; built anywhere, quickly; costs falling fast.
- Against: store only a few hours; need mined lithium, cobalt; fire risk; wear out.
- Big batteries also keep the grid's frequency steady when a power station trips.
Real example: Hornsdale Power Reserve in South Australia, built by Tesla next to a wind farm after a statewide blackout in 2016, saved electricity users over A$150 million in its first two years.
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Two more solutions store energy as a fuel or as heat, so it can be kept longer or used at solar power stations.
The points to remember
- Hydrogen and fuel cells: surplus power splits water (electrolysis); a fuel cell turns the hydrogen back into electricity, giving out only water.
- Hydrogen can be stored for weeks, but much of the energy is lost on the way.
- Thermal storage: surplus heat warms molten salt; later the heat makes steam for a turbine.
- Used at concentrated solar plants to keep generating after sunset.
Electrolysis
- Surplus electricity splits water
- Hydrogen is stored in tanks
Fuel cell
- Hydrogen and oxygen combine
- Electricity out; only water released
Molten salt
- Sunlight heats salt to hundreds of °C
- Heat makes steam after sunset
Real example: at Andasol in southern Spain, mirrors heat tanks of molten salt during the day; the stored heat keeps the turbines running for about 7.5 hours after sunset.
Each kind of storage suits a different job. A grid with lots of wind and solar uses several.
| Storage | How long it stores | Strength | Limit |
|---|---|---|---|
| Batteries | Seconds to a few hours | Very fast, built quickly | Mining, fire risk |
| Pumped hydro | Hours to a day or more | Huge, lasts decades | Needs mountains, floods valleys |
| Hydrogen and fuel cells | Days to weeks | Long-term storage | Loses much of the energy |
| Thermal (molten salt) | Hours | Cheap with solar heat | Only at solar thermal plants |
The points to remember
- Seconds to hours: batteries. Hours to days: pumped hydro. Days to weeks: hydrogen.
- No single store fits every need: grids combine them.
- Other solutions: interconnectors to neighbours, demand response (using power when it is plentiful), a mix of sources.
- Every store loses some energy and has its own environmental cost.
Why fossil fuel back-up is still common: Long calm, dark spells can last days, longer than batteries can cover. Until storage and interconnectors grow, many grids keep gas power stations on standby.
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How this comes up: Paper 2 Section A (May 2026): 'state why some renewable sources create the need for energy storage' [1]. Expect a follow-up on how one storage solution works.
Germany makes almost half of its electricity from wind and solar power.
(a) State why energy production from some renewable sources creates the need for energy storage systems.
(b) Outline how pumped hydroelectricity storage can help supply meet demand.
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