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Storing energy: from fuel stocks to batteries: Why storage matters and how much the world holds

Understanding Energy / Module 10: Storing energy: from fuel stocks to batteries Part 1 of 4

Why storage matters and how much the world holds

Energy is produced and used on different timetables. The sun shines at noon while Indian demand peaks after dark; gas is produced all year but burned hardest in winter. Storage bridges those gaps. Measured by energy held, fuel stocks still dwarf every other store, but electricity storage is growing faster than any other part of the system.

1.1 Storage bridges time, as transport bridges distance

Every energy system has to match supply and demand over several timescales. Over seconds, the grid needs instant response when a power station trips. Over hours, solar output peaks at midday while Indian electricity demand peaks in the evening. Over days, a calm, cloudy spell can cut wind and solar output across a whole region. Over seasons, Europe burns far more gas in winter than in summer, and Indian reservoirs fill in the monsoon and are drawn down in the dry months.

Fossil fuels solved most of this by being easy to store. A coal yard, an oil tank or a gas field holds energy for months at very little cost, and a power plant converts it whenever needed. Electricity itself cannot be held as electricity in useful amounts, except for moments in capacitors. To store it, it must be turned into another form: chemical energy in a battery, height in a reservoir, pressure in a cavern, heat in a tank of water or molten salt, or a fuel such as hydrogen. Each conversion costs some energy on the way in and on the way out.

Two numbers describe any electricity store. Power capacity, in megawatts, is how fast it can charge or discharge. Energy capacity, in megawatt-hours, is how much it can hold. Divide energy by power and you get duration. A 100 MW battery holding 400 MWh can run for four hours at full output. Many headline figures give only the power, which flatters short-duration stores: a gigawatt of two-hour batteries is not the same thing as a gigawatt of ten-hour pumped hydro.

A third number, round-trip efficiency, is the share of the energy put in that comes back out. A fourth is cost, which has to be judged per unit of energy delivered over the store's life, not just per unit of capacity built. Both return in later comparisons.

Value chain The main families of energy storage
  • Energy storage
    • Fuel stocks months to years
      • Oil tanks and strategic reserves
      • Gas in fields, aquifers and salt caverns
      • Coal stockpiles
      • Biomass and biofuel stores
    • Electrochemical minutes to hours
      • Lithium iron phosphate (LFP)
      • Nickel-rich lithium-ion (NMC, NCA)
      • Sodium-ion
      • Flow and lead-acid batteries
    • Mechanical seconds to days
      • Pumped hydro
      • Compressed and liquid air
      • Flywheels and gravity
    • Thermal hours to seasons
      • Sensible heat: water, rock, sand, molten salt
      • Latent heat and ice
      • Carnot batteries
    • Chemical fuels weeks to seasons
      • Hydrogen in caverns and tanks
      • Ammonia, methanol and e-fuels
Source: Smil, Numbers Don't Lie: 71 Stories to Help Us Understand the Modern World (2020), via Smil Mastery Course, M7 'Grids, storage and the real cost of electricity'

1.2 Fuel stocks dwarf every electricity store

Measured by energy, the world's largest stores are tanks, caverns and piles of fuel. Members of the IEA must hold oil stocks equal to at least 90 days of their net imports. China holds about 1.4 billion barrels of strategic stocks. India's strategic caverns at Visakhapatnam, Mangaluru and Padur hold 5.33 million tonnes, about 39 million barrels, which is less than ten days of its imports; refiners' commercial stocks add more.

Natural gas is stored in depleted fields, aquifers and salt caverns, filled in summer and drawn down in winter. The European Union's storage holds roughly 1,100 TWh when full, and its filling level each autumn is watched as a signal of winter security. Coal power stations keep stockpiles measured in days of burn. India's Central Electricity Authority tracks them plant by plant every day, and when stocks ran low in late 2021 it quickly became national news.

Now compare these with electricity storage. In March 2026, after the Strait of Hormuz closed, IEA members agreed to release 400 million barrels of oil, about four days of world use. At about 1.7 MWh per barrel, that release held roughly 680 TWh of energy. The world's grid batteries, more than 250 GW at the end of 2025 with typical durations of two to three hours, hold in the order of 0.5 to 0.75 TWh (calculated). One emergency oil release therefore contained around a thousand times more energy than every grid battery on Earth.

The comparison is not entirely fair. Oil is burned in engines and power stations that waste most of its energy, while stored electricity comes back almost ready to use. Batteries are also cycled daily, so over a year they move hundreds of times their own capacity. But the gap explains why replacing the storage that comes built into fossil fuels is one of the hardest parts of the energy transition. The tile "Fuel stocks and strategic reserves" has more on how reserves are held and used.

Data Energy held in different stores (approximate)
StoreSizeEnergy held
EU gas storage when fullabout 100 bcmabout 1,100 TWh
IEA emergency oil release, March 2026400 million barrelsabout 680 TWh
India's strategic petroleum reserve5.33 Mt, about 39 million barrelsabout 66 TWh
World pumped hydro, end 2025just over 200 GW, many plants 6 hours or moreabout 1.2 TWh at 6 hours; up to about 9 TWh on IHA's estimate of reservoir storage
World grid batteries, end 2025over 250 GW, typically 2 to 3 hoursabout 0.5 to 0.75 TWh

Oil at about 1.7 MWh per barrel; EU gas storage held about 1,130 TWh when 99% full in October 2023. Battery energy is calculated from power and typical durations of two to three hours. The pumped hydro range runs from a six-hour average to the International Hydropower Association's bottom-up estimate of reservoir storage; neither is a reported total.

Source: Calculated from IEA (2026), ISPRL, GIE data reported by ANSA (2023), IHA (2021 and 2026) and Rystad Energy (2026), See module sources

1.3 Pumped hydro's long lead meets the battery surge

For most of the past century, storing electricity meant pumped hydro: two reservoirs at different heights, with water pumped up when power is cheap and run down through turbines when it is valuable. Writing a few years ago, Smil noted that pumped hydro made up more than 99% of the world's storage capacity. The International Hydropower Association counted 189 GW of pumped storage at the end of 2024 and just over 200 GW a year later, after a record 11.7 GW was added in 2025. China builds most of it: of about 243 GW under construction worldwide at the end of 2025, 218 GW was in China.

Batteries have changed the picture in only a few years. The IEA reports that 108 GW of battery storage was added worldwide in 2025, 40% more than in 2024. China added just over 63 GW, about 60% of the total, and the United States about 19 GW. Installed battery capacity is now eleven times its 2021 level. Rystad Energy put it above 250 GW at the end of 2025, overtaking pumped hydro when measured in gigawatts for the first time.

Measured in energy, the order is different. Most batteries discharge for two to four hours, while many pumped hydro plants run for six to twelve hours or more, so pumped hydro still holds more stored energy: about 1.2 TWh if its plants averaged six hours, and up to about 9 TWh on the IHA's estimate of what its reservoirs can hold, against roughly 0.5 to 0.75 TWh for batteries (calculated). Batteries are lengthening, though: the average new project lasted about three hours in 2025, up from about two in 2023, and more are being built for four hours or longer.

The IEA's 2024 report on batteries concluded that tripling renewable capacity by 2030, as agreed at the COP28 climate summit, would need about 1,500 GW of energy storage by 2030, roughly six times the 2023 level, with batteries rising fourteenfold to 1,200 GW. India is pursuing both routes, with large tenders for batteries paired with solar and a renewed push for pumped storage. The "Pumped hydro storage" and "Lithium iron phosphate batteries" tiles give the details.

See it in the table

H₂O WaterGas Natural gasLi LithiumNaCl SaltH2 HydrogenLFP Lithium iron phosphate batteriesPHS Pumped hydro storageSPR Fuel stocks and strategic reserves

Check your understanding

A battery is rated at 50 MW and 200 MWh. What is its duration at full output?

Duration is energy divided by power: 200 MWh divided by 50 MW is four hours.

How did the energy in the IEA's 400 million barrel oil release of 2026 compare with all the world's grid batteries?

About 680 TWh of oil against roughly 0.5 to 0.75 TWh of battery storage.

Measured in gigawatts, which technology became the world's largest electricity store in 2025?

Batteries passed about 250 GW against just over 200 GW of pumped hydro, though pumped hydro still holds more energy.
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