Material Insights · Understanding the Material World

Storing energy: from fuel stocks to batteries: Key terms, quiz and sources

Understanding Energy / Module 10: Storing energy: from fuel stocks to batteries Wrap-up

Storing energy: from fuel stocks to batteries: review

Key terms

Power capacity
How fast a store can charge or discharge, measured in kW, MW or GW.
Energy capacity
How much energy a store can hold, measured in kWh, MWh or GWh.
Duration
Energy capacity divided by power capacity: how many hours a store can run at full output.
Round-trip efficiency
The share of energy put into a store that comes back out.
Cycle life
The number of full charge and discharge cycles before a battery keeps only about 80% of its capacity.
Calendar ageing
Loss of battery capacity with time, even when the battery is not being used.
Augmentation
Adding battery modules during a project's life to make up for capacity fade.
Thermal runaway
A self-feeding rise in a cell's temperature that releases heat and flammable gas and can spread to neighbouring cells.
Battery management system
Electronics that monitor each cell's voltage and temperature and keep it within safe limits.
Lithium iron phosphate (LFP)
A cobalt-free, nickel-free lithium-ion chemistry that is cheap, stable and long-lived.
NMC
A nickel-manganese-cobalt lithium-ion chemistry with high energy density, used in long-range cars.
Sodium-ion battery
A battery that uses sodium instead of lithium, with lower energy density but abundant materials.
Pumped hydro storage
Storing electricity by pumping water uphill and releasing it through turbines when needed.
Compressed-air energy storage
Storing electricity by compressing air into caverns and expanding it through turbines later.
Sensible heat storage
Storing energy by raising the temperature of a material such as water, rock, sand or molten salt.
Latent heat storage
Storing energy in the melting or freezing of a material, such as ice or wax.
Carnot battery
A store that turns electricity into heat and later turns the heat back into electricity.
Levelised cost of storage (LCOS)
The total lifetime cost of a store divided by the energy it discharges, per MWh.
Strategic petroleum reserve
Government-held oil stocks kept for emergencies, measured in barrels or days of imports.

Try it in the table

  • Open "Fuel stocks and strategic reserves" and compare India's strategic oil reserve with its daily crude imports. How many days would it cover, and how does that compare with the IEA's 90-day rule?
  • In "Pumped hydro storage", note the typical duration and efficiency, then open "Lithium iron phosphate batteries" and compare. Which would you choose to shift solar output into the evening, and which to cover a two-day lull?
  • Compare "Sodium-ion batteries" with "Nickel-rich lithium-ion batteries". Which minerals does each avoid, and what does each give up in return?
  • Open "Sensible heat storage" and "Carnot batteries". Both store heat, so why is one efficient and the other not?
  • In "Hydrogen storage", follow the chain from electrolyser to turbine and multiply the efficiencies yourself. Then open "Ammonia, methanol and e-fuels" to see how converting hydrogen changes how it is stored.
  • Look at the "Lithium" and "Cobalt" commodity tiles to see where each is mined and refined, then check the "Cathode materials" chemical tile for who makes the battery powders.

Final quiz

Which pair correctly gives the units of power capacity and energy capacity?

Power is a rate (MW); energy is an amount (MWh). Their ratio is the duration in hours.

About how much of the electricity put into a modern grid lithium-ion battery comes back out?

Cell and inverter losses and cooling take roughly 12 to 15%; PNNL assumes 86% for current systems.

What was the average lithium-ion pack price across all uses in BloombergNEF's 2025 survey?

Prices fell 8% to USD 108 per kWh; stationary storage packs averaged USD 70.

Batteries now exceed pumped hydro in gigawatts. Why does pumped hydro still hold more stored energy?

Most batteries run for two to four hours; many pumped hydro plants run for six to twelve hours or more.

What does Lazard's 2025 figure of USD 115 to 254 per MWh for a four-hour US battery measure?

LCOS spreads capital, financing and operating costs over all the energy the battery delivers in its life.

Which battery chemistry avoids lithium altogether?

Sodium-ion uses sodium, which is abundant, and usually hard carbon instead of graphite.

Roughly how much electricity comes back from hydrogen stored for power to power?

Electrolysis, storage and reconversion each lose energy, leaving about a quarter to two fifths.

Why does Smil expect the energy transition to increase mining rather than end it?

Transition technologies are material-intensive, raising demand for lithium, nickel, copper, graphite and more.
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Sources for this module