Solar panels produce most of their electricity in the middle of the day. People use most of their electricity in the evening. Nothing is wrong with either fact, but together they create the single biggest problem in renewable energy — and storage is how we solve it.
📘 What you need to know
Wind and solar are intermittent: they generate only when conditions are right, so supply may not match demand.
Energy storage saves surplus energy when production is high and releases it when demand exceeds supply.
The main storage methods are batteries, pumped hydroelectricity storage (PHS), fuel cells and thermal storage.
Peak-shaving means levelling out periods of high demand using stored energy, avoiding blackouts and extra power stations.
Energy conservation is changing behaviour to use less energy.
Energy efficiency is using technology and design that need less energy for the same task.
Both cut demand and waste, reduce dependence on imported energy, and lower carbon emissions.
Examples worth learning: LED and motion-sensor lighting, passive solar design, designing goods to be recycled and wind-assisted shipping.
Why storage is needed at all
Some renewable sources produce energy intermittently, meaning they only generate power when conditions allow.
Wind: only produces electricity while the wind is blowing.
Solar: only generates during daylight, and much less under cloud.
So there will be times when supply does not meet demand — and, just as awkward, times when supply is far greater than demand and the extra electricity has nowhere to go. Storage systems fix both halves of that problem: they store the excess when production is high and release it when demand exceeds supply.
The gap between the two peaks is roughly six hours. Bridging it is what batteries, pumped hydro and thermal storage are all for.
How energy is stored
Batteries
Batteries store electricity as chemical energy, which is released when needed.
Uses: common in electric vehicles and home solar systems.
Example: home battery units such as the Tesla Powerwall store energy from rooftop solar panels and supply the house during outages or periods of high demand.
Strengths and limits: fast to respond and can be installed almost anywhere, but expensive per unit of energy stored, and they rely on mined materials with a limited lifespan.
Pumped hydroelectricity storage (PHS)
PHS is a battery made of water and a hill. When there is surplus electricity, it is used to pump water uphill to a higher reservoir. When demand rises, that water is released back down through turbines to a lower reservoir, generating electricity on the way.
Nothing is created here. Some energy is lost in pumping, but a store that can be switched on in seconds is worth that loss.
PHS advantages
PHS disadvantages
Large capacity — stores huge amounts of surplus electricity from periods of high renewable output
Geographic limits — needs mountains, valleys and space for large reservoirs
Reliable — responds quickly to sudden demand increases, which is peak-shaving
Environmental impact — dams and reservoirs damage ecosystems and disturb wildlife
Long lifespan — plants run for decades with low maintenance
Economic cost — very high initial cost to build
Fuel cells
Fuel cells convert stored chemical energy, often hydrogen, directly into electricity.
Uses: transport, such as hydrogen-powered vehicles, and backup power systems.
Example: Japan is investing in hydrogen fuel cells as part of its energy transition, particularly for powering vehicles and buildings.
Thermal storage
Thermal storage saves heat, which can later generate electricity or provide heating directly.
Uses: often paired with solar power plants, where surplus solar energy is stored as heat and converted to electricity when sunlight is low.
Example: the Crescent Dunes project in the United States uses molten salt to hold solar energy as heat, then generates electricity after sunset.
🧩
Four stores, four forms of energy
Batteries store it as chemical energy, pumped hydro as gravitational potential energy, fuel cells as chemical energy in hydrogen, and thermal storage as heat. If you can name the form, you can explain how the store works.
Peak-shaving
Storage systems are used for peak-shaving: levelling out periods of high demand so that supply can meet them. When electricity use spikes — a cold winter evening, for example — stored energy is released to cover the extra. That avoids blackouts and removes the need to fire up additional power stations just for a couple of hours.
Conservation and efficiency: two different things
The distinction examiners testConservation = changing behaviour to use less energy Efficiency = using technology and design that need less energy for the same task
Energy conservation (behaviour)
Energy efficiency (technology)
Turning off lights when they are not in use
Installing low-energy LED lighting
Wearing warmer clothing or using natural ventilation instead of heating and air conditioning
Buying energy-efficient appliances with high efficiency ratings
Walking, cycling or taking public transport instead of driving
Developing fuel-efficient transport, such as electric vehicles
Running machinery less often or at lower levels
Designing buildings to conserve heat through better insulation and double glazing
The two work together. Efficient technology makes conservation easier, and efficiency measures are how conservation targets are usually achieved in practice.
Why it matters
Both reduce energy demand and waste.
Lower demand makes a country less dependent on imported energy, cutting costs and improving energy security.
Using less energy means lower carbon emissions, which helps limit climate change.
Four examples worth learning
Strategy
How it works
How effective it is
Smart lighting
LED bulbs plus motion sensors, so lights are only on when a space is being used
LEDs use up to 80% less energy than traditional bulbs, and sensors cut waste in public buildings
Passive solar design
Large windows facing the sun and materials that store and release heat, so buildings warm themselves
Effective in regions with consistent sunlight; lowers bills and reduces the need for artificial heating
Designing goods to be recycled
The circular economy approach: longer-lasting products made from recyclable materials that can be reused or repaired
Cuts the energy needed to produce new materials, lowering industrial energy demand
Wind-assisted shipping
Large automated rotor sails or kite sails capture wind energy to help drive cargo ships
Reduces fuel use by 10 to 30% depending on wind conditions, cutting greenhouse gas emissions
Why shipping is a clever example. Cargo ships are almost impossible to run on batteries — the crossing is too long and the load too heavy. Sails do not replace the engine, they just reduce how hard it has to work. When a source cannot be swapped out, using less of it is the realistic answer, and that is efficiency doing the job renewables cannot.
Worked examples
WE 1
Explain why energy storage is needed for renewable energy
Explain why energy storage systems are important for countries using large amounts of wind and solar power. (3 marks)
Point 1: the problem
Wind and solar are intermittent, generating only when the wind blows or the sun shines, so supply does not always match demand.
Point 2: what storage does
Storage saves surplus energy when production is high and releases it when demand exceeds supply.
Point 3: the result
This gives a reliable supply and allows peak-shaving, so blackouts and extra power stations are avoided.
Storage moves energy through time, from when it is made to when it is wanteduse the word “intermittent” — it is the term the mark scheme is built around
WE 2
Evaluate pumped hydroelectricity storage
Outline how pumped hydroelectricity storage works and evaluate its use. (4 marks)
How it works
Surplus electricity pumps water to a higher reservoir; when demand rises the water is released back down through turbines, generating electricity.
For
Very large capacity, quick response for peak-shaving, and plants last decades with low maintenance.
Against: siting
It needs specific landforms — mountains, valleys and space for reservoirs — so it can only be built in certain places.
Against: impact and cost
Dams and reservoirs damage ecosystems and disturb wildlife, and initial construction costs are very high.
Excellent where the landscape allows it, impossible where it does notan evaluation needs a judgement; “it depends on geography” is a perfectly good one here
WE 3
Distinguish between energy conservation and energy efficiency
Distinguish between energy conservation and energy efficiency, giving one example of each. (3 marks)
Point 1: conservation
Conservation means changing behaviour to use less energy, for example turning off lights or cycling instead of driving.
Point 2: efficiency
Efficiency means using technology and design that need less energy for the same task, such as LED bulbs or double glazing.
Point 3: the link
They are not opposites — efficient technologies are often how conservation is achieved in practice.
Behaviour reduces the task; efficiency reduces the energy each task needs“distinguish” means write the contrast explicitly, not two separate definitions side by side
💡 Exam tips
Explain storage using the pair: store the surplus, release it at peak.
Name all four storage types and one example each: battery, PHS, fuel cell, thermal.
Learn peak-shaving as a term and be ready to define it.
For PHS, give a geographic limitation — it is the disadvantage examiners look for.
Keep conservation (behaviour) and efficiency (technology) clearly apart.
Quote effectiveness figures where you have them: LEDs up to 80% less, sails 10 to 30% less fuel.
⚠ Common mix-ups
Saying storage generates energy. It only moves energy from one time to another, and some is lost on the way.
Using conservation and efficiency as synonyms. This is the most commonly penalised error in the topic.
Assuming PHS can be built anywhere. It needs the landforms and a lot of space.
Forgetting storage has impacts too. Batteries need mined materials; reservoirs flood habitats.
Describing peak-shaving as reducing demand. It levels the peak by adding supply from a store.
Giving examples with no effectiveness. A strategy plus a figure or outcome scores far better.
That completes 7.2 Energy Sources: Uses & Management. The three pages run in one line: every source has a cost somewhere in its life, demand keeps rising and some industries cannot switch yet, so the practical answers are storing what we generate and needing less of it in the first place. Up next: What Counts as Waste, which turns to what happens at the other end of resource use — everything we throw away.
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