Wind and solar have one awkward habit: they make electricity when the weather says so, not when we want it. Storage moves that energy to the moment it is needed. Conservation and efficiency go the other way and shrink the amount we need in the first place. You need both.
📘 What you need to know
Wind and solar are intermittent — they only generate when conditions are right, so supply can miss demand.
Storage systems absorb surplus energy when production is high and release it when demand exceeds supply.
The four you must know: batteries, pumped hydroelectricity storage (PHS), fuel cells and thermal storage.
Peak-shaving is levelling out short bursts of very high demand using stored energy.
Energy conservation = changing behaviour to use less. Energy efficiency = using technology that needs less energy for the same task.
Both cut emissions, cut bills and reduce a country’s need to import energy.
Why storage is needed at all
A power grid has to balance every second. The electricity going in has to match the electricity coming out. With coal or gas you simply burn more fuel when demand rises. With wind and solar, you cannot ask the sky for more.
Wind power only produces electricity when the wind is blowing.
Solar power only generates during daylight, and less on cloudy days.
So there are moments when supply does not meet demand, and other moments when there is far more electricity than anyone needs. Storage fixes both ends of that problem at once: it soaks up the surplus and hands it back later.
Think of storage as a bank account for electricity. Wind and solar pay in whenever they can. The grid withdraws whenever it needs to. Without the account, every payment would have to be spent the instant it arrived, and anything extra would be thrown away.
Four ways to store energy
Batteries
Batteries store electricity as chemical energy and release it on demand. They are everywhere: electric vehicles, home solar systems, grid-scale installations. Example: Tesla Powerwall batteries store energy from household solar panels and supply the home during outages or high-demand periods.
Pumped hydroelectricity storage (PHS)
PHS is the biggest storage method we have. When there is surplus electricity, it is used to pump water uphill into a higher reservoir. When demand rises, that water is released back down through turbines, generating electricity again. Example: Dinorwig Power Station in Wales is one of the largest PHS systems in the world and is used to balance UK electricity supply.
PHS is a storage system, not an energy source. It gives back less than you put in, but it gives it back exactly when you need it.
PHS — ADVANTAGES
Large capacity: stores huge amounts of surplus electricity from windy or very sunny periods
Reliable: responds quickly to sudden demand increases (peak-shaving)
Long lifespan: plants run for decades with low maintenance, which helps their sustainability
PHS — DISADVANTAGES
Geographic limits: needs mountains, valleys and large reservoirs, so few sites work
Environmental impact: dams and reservoirs damage ecosystems and disturb wildlife
Economic cost: very high initial cost to build
Fuel cells
A fuel cell turns stored chemical energy, usually hydrogen, directly into electricity. No burning, no turbine. They are used in transport, such as hydrogen-powered vehicles, and as backup power. Example: Japan is investing heavily in hydrogen fuel cells for vehicles and buildings as part of its energy transition.
Thermal storage
Thermal storage keeps energy as heat, which can be released later to generate electricity or heat buildings. It pairs naturally with solar power plants: store the excess heat by day, use it when the sunlight drops. Example: the Crescent Dunes Solar Energy Project in the USA uses molten salt to store solar energy as heat, then generates electricity after sunset.
Method
Energy is stored as
Typical scale
Named example
Batteries
Chemical energy
Home and vehicle, up to grid-scale
Tesla Powerwall
Pumped hydro
Gravitational potential energy
Very large, national grid
Dinorwig, Wales
Fuel cells
Chemical energy in hydrogen
Vehicles and backup power
Hydrogen vehicles in Japan
Thermal storage
Heat
Attached to solar power plants
Crescent Dunes, USA
Peak-shaving: flattening the spikes
Demand is not steady through the day. It dips overnight, rises in the morning, and spikes hard in the early evening when people come home, cook and turn the heating on. On a cold winter evening that spike can be enormous.
Peak-shaving means using stored energy to level out those periods of very high demand so supply still meets it. The alternative is either a blackout, or firing up extra power stations that sit idle the rest of the time.
The shaded gap is energy that came out of a store rather than out of a power station.
Conservation and efficiency
These two words get swapped around constantly, and examiners keep testing them. Learn the split once and it stops being a problem.
The two-word ruleConservation = behaviour • Efficiency = technology
ENERGY CONSERVATION
Turning off lights when they are not in use
Wearing warmer clothing or opening a window instead of using heating or air conditioning
Walking, cycling or taking public transport instead of driving
ENERGY EFFICIENCY
Low-energy LED lighting in homes and buildings
Appliances with high efficiency ratings
Electric vehicles and other fuel-efficient transport
Better insulation and double-glazed windows to keep heat inside
They work together. Efficient technology is what makes conservation easy. Fitting LED bulbs is efficiency; remembering to switch them off is conservation. A good answer mentions that link.
Why it matters
Conservation and efficiency cut energy demand and waste, so less has to be generated in the first place.
Countries become less dependent on imported energy, which lowers costs and improves energy security.
Less energy generated means lower carbon emissions, which helps tackle climate change.
Four worked examples of saving energy
Smart lighting systems
LED bulbs combined with motion sensors cut electricity use twice over: the bulbs need less power, and the sensors make sure lights are only on when someone is there. This matters most in public spaces and large buildings where lights used to burn all night. Effectiveness: LEDs use up to 80% less energy than traditional bulbs.
Passive solar building design
A passive solar building uses natural sunlight to heat itself, so less artificial heating is needed. Large windows face the sun, and the building materials store heat and release it slowly. Effectiveness: works well in regions with consistent sunshine, cutting bills and making homes more efficient.
Designing goods to be recycled
The circular economy aims to cut waste by designing products that are easy to reuse, repair or recycle. Longer-lasting products made from recyclable materials mean less energy is spent making new ones. Effectiveness: reduces the energy demand of industry, not just of households.
Commercial shipping with sails
Shipping is hard to electrify, so the industry has gone back to an old idea. Modern cargo ships fit large automated sails, known as rotor sails or kite sails, which capture wind energy and cut fuel use. Effectiveness: wind-assisted propulsion can cut fuel consumption by 10 to 30% depending on wind conditions.
EXAM-STYLE
Explain how pumped hydroelectricity storage helps a country use more renewable energy. [3]
Step 1 — the problem
Wind and solar are intermittent, so at times they generate more electricity than the grid needs and at other times not enough.
Step 2 — storing it
Surplus renewable electricity is used to pump water into an upper reservoir, storing it as gravitational potential energy instead of wasting it.
Step 3 — releasing it
At times of peak demand the water is released through turbines, generating electricity when renewables cannot.
3 marks: intermittency, store surplus, release at peakSay “gravitational potential energy” rather than “stores water”. The energy word is what is being marked.
EXAM-STYLE
Distinguish between energy conservation and energy efficiency, using one example of each. [2]
Conservation
Changing behaviour to use less energy overall, for example switching off lights in empty rooms.
Efficiency
Using technology or design that needs less energy for the same task, for example replacing bulbs with LEDs that use up to 80% less energy.
2 marks: behaviour vs technology, with an example each“Distinguish” wants a clear contrast. Use the words “behaviour” and “technology” explicitly.
💡 Exam tip
Storage is not a source. Never write that a battery or a PHS plant “generates” energy — it stores and returns it.
Learn one named example per storage type. Dinorwig and Tesla Powerwall are the two that come up most.
Use the word intermittent when explaining why storage is needed. It is the term the mark scheme uses.
Peak-shaving is a specific term. Define it as levelling out short periods of very high demand.
Give numbers where you have them: LEDs up to 80% less energy, sails 10 to 30% less fuel. Figures lift a grade.
For “evaluate a storage method”, cover capacity, cost, lifespan and where it can physically be built.
⚠ Common mix-up
Conservation and efficiency swapped. The classic error. Behaviour versus technology — check every time.
Calling PHS a renewable energy source. It is a storage system that returns electricity someone else generated.
Confusing pumped hydro with ordinary hydropower. Hydropower uses river flow once; PHS moves the same water up and down repeatedly.
Saying storage reduces total energy use. It shifts when energy is used. Conservation reduces how much.
Forgetting fuel cells store chemical energy. Hydrogen is the store, not the source.
Missing the environmental downside of PHS. Reservoirs still flood land and disturb wildlife.
Up next: What Energy Security Means — the first of the HL sections, and the topic that ties energy to politics.
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