Why is electricity so hard to store?
― We turn "flow" into something else and keep that instead
Tap water can sit in a tank. Petrol can sit in a tank too. But electricity is the one thing you can't store as-is. When the power goes out, your appliances stop instantly. Why is "saving up electricity" so hard?
Every kitchen has a tap, and whenever you want water, you turn it on and it flows. Water you don't use can sit in a storage tank.
Electricity doesn't work that way. Electricity made at a power plant vanishes the instant it isn't used. Smartphones and electric cars have a storage device called a "battery," but that isn't storing electricity itself.
So what exactly is being stored inside a battery?
Electricity is really tiny particles called electrons moving through a wire — a flow. You can't hold a flow itself in a container.
Devices that store electricity convert it into chemical energy, potential energy, or some other form, then store that instead.
Let's look at this "convert first, then store" idea through the main methods used to store electricity.
Electricity is a "flow," not a "thing"
Electricity is really the movement of tiny particles called electrons through a wire. Unlike tap water, which is a "thing" you can sit in a container, it is a flow that is always in motion.
When a circuit breaks, the flow of electrons stops, but those stopped electrons aren't sitting somewhere waiting to be collected later. That's exactly why "storing electricity for later" requires converting it into another form of energy first.
Batteries convert electricity into "chemical energy"
Inside a battery, charging uses electrical energy to drive a chemical reaction, changing the state of the materials inside. Given the right conditions, this chemical reaction can run in reverse, and when the battery discharges (is used), the reverse reaction happens, releasing electrical energy again.
In other words, a battery doesn't trap electricity itself — it hands electricity off to a chemical-reaction "relay baton," then converts it back to electricity later.
It means letting electricity change into a different form of energy for a while.
Capacitors store electricity closer to its original form ― but…
A component called a capacitor stores electricity differently from a battery. Two facing metal plates each build up a charge, one positive and one negative, so the electricity is stored directly as charge, without being converted into a chemical reaction. In that sense, a capacitor is the device that stores electricity in the form closest to electricity itself.
Capacitors do have a weakness, though. For the same size and weight, they can store far less energy than a battery. In exchange, they have a strength: they can release and take in stored electricity almost instantly. That makes them useful for things like camera flashes, where a burst of high power is needed for a split second.
At large scale, pumping water uphill is the mainstream method
To store energy on the scale of a power plant, pumped-hydro storage is widely used. When there is surplus electricity, it drives pumps that send water from a lower reservoir up to a higher one. When electricity is needed, that water is released back downhill to spin generators and produce electricity.
This works on exactly the same principle as batteries and capacitors: electrical energy is converted into the water's potential energy (energy from being at a height) and stored that way. It's said to be one of the largest contributors to electricity storage worldwide.
Because electricity can't be stored as-is, the amount generated and the amount used must be kept almost perfectly matched at every instant. As sources with variable output, like solar and wind, make up a bigger share of generation, this balancing act gets harder, which is thought to make storage even more important.
Something you can check yourself
- Find a hand-crank (dynamo) torch or radio if you have one
- Confirm that the light stays on only while you're turning the handle
- Confirm that the light goes out the instant you stop turning the handle (for a simple type with no internal battery or capacitor)
- If your model does have a battery or capacitor inside, confirm that the light stays on for a while after you stop
A model that goes dark the instant you stop turning lets you feel directly that electricity is a "flow," and without some way to store it, it simply ends on the spot.
Summary
Electricity isn't a "thing" you can store like water — it's a "flow" that is always in motion. That's why storing it requires converting it first: into chemical energy for a battery, into charge itself for a capacitor, or into the water's potential energy for pumped-hydro storage. "Storing electricity" really means "letting electricity change into a different form of energy for a while."
Electricity won't wait for you in a reservoir like water does.
Right up until the moment it's used, it's always in disguise, waiting for its cue.
For why the lithium-ion batteries behind this "conversion to chemical energy" occasionally catch fire, see this article.
Want to know more? ― Terms, numbers, and links to the textbookLabels show whether each point is junior-high-level or an open research question
- JHSCovered in junior high school science
- HSCovered in high school "Basic Physics"
- HS+High school "Chemistry"/"Physics," or advanced/sidebar textbook content
- Univ.Not covered in high school — university-level (electrochemistry, energy engineering)
- ResearchNot yet settled even at university level — an active research question
JHSTerms: vocabulary around storing electricity
- Electric current: the flow of electrons moving through a wire.
- Chemical energy: energy that can be released or absorbed through a chemical reaction.
- Potential energy: energy an object has by virtue of being at a height.
- Capacitor: a component that stores electricity directly, as charge.
HSChecking the numbers: how much size is needed to store the same amount of electricity
We calculate and compare how much weight or volume batteries, capacitors, and pumped-hydro storage each need to store the same 1kWh (kilowatt-hour) of energy.
| Energy to be stored | Assume 1kWh (1000Wh) |
| Energy density of lithium-ion batteries | A typical benchmark figure is around 200Wh/kg |
| Energy density of capacitors (electric double-layer type) | A typical benchmark figure is around 5Wh/kg |
| Weight needed for a battery | 1000 ÷ 200 ＝ 5 |
| Weight needed for a battery | about 5 kg |
| Weight needed for a capacitor | 1000 ÷ 5 ＝ 200 |
| Weight needed for a capacitor | about 200 kg |
| How many times heavier is the capacitor? | 200 ÷ 5 ＝ 40 |
| Weight ratio | about 40× |
The math works out so that a capacitor needs roughly 40 times the weight of a battery to store the same 1kWh. This number itself shows why capacitors suit instantaneous uses while batteries are used for large-scale storage.
1kWh is about 3.6×10⁶ J (joules). To store that by lifting water up a height of 100m (a typical dam head figure), we use potential energy = mass × gravitational acceleration × height.
| Gravitational acceleration × height | 9.8 × 100 ＝ 980 |
| Mass of water needed | (3600000) ÷ (980) ≒ 3673 |
| Mass of water needed | about 3673 kg (about 3.7 tonnes) |
The math shows that storing just 1kWh with a 100m head needs roughly 3.7 tonnes of water. This is why pumped-hydro storage is hard to build anywhere except mountainous or river-fed sites with enough land and water available.
※ Energy density and dam head are typical benchmark figures; actual products and facilities vary.
HS+What happens inside a battery
Inside a battery, a chemical reaction called an oxidation-reduction (redox) reaction takes place. During discharge, one electrode releases electrons (oxidation) while the other accepts electrons (reduction), simultaneously, and this electron movement becomes the current flowing through the external circuit. During charging, electricity supplied from outside drives this reaction in reverse, restoring the original state.
Univ.Energy density and power density are different metrics
When comparing storage technologies, two different metrics are used: energy density (how much total energy can be stored) and power density (how quickly energy can be released or absorbed). Batteries have high energy density and capacitors have high power density — a trade-off relationship. More recently, storage technologies with intermediate properties that balance both, called "electric double-layer capacitors" and "lithium-ion capacitors," have also been developed.
ResearchWhat's still unclear
- "All-solid-state batteries," which use no liquid electrolyte, are a next-generation battery technology expected to improve safety and energy density, and research and development toward practical use is underway, but technical challenges remain before mass production.
- Research also continues into batteries using materials other than lithium, such as sodium-ion batteries, which face fewer resource constraints. These could eventually broaden the range of battery materials available, but whether they can match lithium-ion performance remains a research question.
- Technology that converts electricity into hydrogen for storage, then converts it back to electricity when needed, is also being studied as an option for large-scale, long-duration energy storage. Efficiency losses at each conversion step have been noted as a challenge, and research into improving efficiency for practical use continues.
"How to store electricity" is a field of ongoing research whose importance keeps growing alongside the spread of renewable energy.
Links to the textbook (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science ・ current and energy | Basic terms: current, chemical energy, potential energy |
| HS | Basic Physics ・ units of energy | Estimating required weight/water volume from energy density |
| HS+ | Chemistry ・ redox reactions | How the chemical reaction inside a battery works |
| Univ. | Electrochemistry ・ energy engineering | Trade-off between energy density and power density |
| Research | Energy storage tech (ongoing research) | All-solid-state batteries, sodium-ion batteries, hydrogen energy storage |
- Explanatory materials on "storage batteries and pumped-hydro storage" from the Agency for Natural Resources and Energy (資源エネルギー庁).
- Textbook explanations of battery redox reactions in electrochemistry.
- Textbook comparisons of energy density and power density between capacitors and batteries in electrical engineering.
- Research reviews on all-solid-state batteries and next-generation storage batteries in energy engineering.
- Basic physics textbook explanations of potential energy and energy units (joules, kilowatt-hours).
※ Figures such as energy density and dam head are typical benchmark values; actual products and facilities vary.
※This article is a general-audience science explainer. For handling batteries or electrical equipment, follow the instructions provided with the specific product and guidance from your electricity provider or relevant authorities.