Storing solar energy: from phone batteries to grid-scale storage
Electricity is the ultimate perishable commodity. If you don't use it, you have to dissipate it or you have to store it.
This module covers the following six lessons:
In this lesson, you will learn what happens inside a lithium-ion battery during charge and discharge — the building block of every EV, home battery, and grid-scale battery storage in the world.
A modern battery is a sandwich of three layers: an anodeGraphite stores lithium ions between its carbon layers like books slid onto a shelf — releasing them on discharge and accepting them again on charge. (the negative electrode, typically made of graphite), a cathodeCathode chemistry is what's behind most battery brand-names you've heard — each a different mix of nickel, manganese, cobalt, iron or aluminium. (the positive electrode, typically a lithium metal oxide), and an electrolyteMost cells today use a flammable organic liquid; solid-state alternatives — a glass or polymer instead — are the next generation, promising higher energy density and lower fire risk. separating them. The electrolyte conducts lithium ions but blocks electrons. This one-way gate is what makes the battery do useful work.
When you charge the battery, an external power source pushes electrons in. Inside, lithium ions are forced from the cathode through the electrolyte to the anode, where they slot into the graphite structure. The anode "fills up" with lithium and the energy is stored as a chemical potential difference between the two electrodes.
When you connect a load (a phone, a motor, a household), the process reverses. Lithium ions flow back through the electrolyte from anode to cathode, while electrons take the long way round through the external circuit, powering whatever you have connected. The voltage across the cell drives the flow.
The movement of electrons and lithium ions is reversible. Lithium ions don't stay attached to either electrode permanently; they're guests in a host structure, like books in a library. That reversibility is why a single Li-ion cell can cycle thousands of times. It is also why poor charging (overheating, deep discharge, very fast cycling) damages the structure and shortens cycle life.
Some energy is lost as heat during every charge and every discharge. A modern lithium-ion battery delivers around 85–90% of what you put in, meaning 10–15% is lost to inefficiency. That is higher than pumped hydro at around ~75% efficiency (25% lost). The losses matter at scale: a 1 GWh battery cycling daily wastes 100–150 MWh per day, as heat. It is important to remember this when sizing systems.
The interactive cell below lets you see the charge and discharge process in action. Click Charge to push electrons and ions back to the anode and watch the cell fill up. Once full, click Discharge to send them the other way and power a load. Notice that energy doesn't flow forever. Once the source electrode is empty or the receiving electrode is full, the cell stops.
In this lesson, you will learn how battery storage solves the timing mismatch between when electricity is generated and when it is needed.
In Module 3, we identified the central challenge of solar power: the timing mismatch between when electricity is generated (midday) and when it is needed (morning and evening). Battery storage directly addresses this problem. A battery stores excess solar energy during the day and releases it when demand is high, typically in the evening.
This function is called time-shiftingThe economic version is called temporal arbitrage: buy electricity when it's cheap (sunny midday), sell when it's expensive (evening peak).. The battery takes energy produced at one time and makes it available at another. It is conceptually simple, like filling a water tank during the day and using the water in the evening.
If we consider this on the household scale, it is the same process. In the figure below, scrub through the day or press Play to watch charging fill in across midday and discharging fill in through the evening peak. The amount of energy stored in the battery is shown on the right.
You already use batteries every day. Your phone battery stores energy from the charger and releases it as you use the phone throughout the day. A laptop battery does the same. An electric vehicle battery stores energy from a charging point and releases it to drive the motor. In each case, the fundamental function is identical: store now, use later.
Time-shifting is the core function of battery storage in a solar energy system. It means absorbing excess energy when generation exceeds demand (typically midday) and releasing that energy when demand exceeds generation (typically evening). Without time-shifting, solar energy that is not immediately consumed is either exported to the grid at low value or wasted entirely.
In this lesson you will learn to distinguish between three related but different concepts: power, energy, and duration — the essential vocabulary for understanding battery systems.
To understand battery systems, you need to distinguish between three related but different concepts: powerLike the diameter of a water pipe — sets the maximum flow at any one moment., energyLike the size of a water tank — sets the total amount you can hold., and durationTank size divided by pipe diameter — how long the tank lasts at full flow..
Power describes how fast energy can be delivered or absorbed. It is measured in watts, kilowatts (kW)1,000 watts., megawatts (MW)1,000,000 watts.. A battery with a power rating of 10 MW can deliver 10 megawatts of electrical power at any given instant. Think of power like the diameter of a water pipe — it determines how much water can flow through the pipe.
Energy describes how much total electricity the battery can store. It is measured in watt-hours (Wh); an amount of power over a period of time, which is always an hour. Home systems are usually measured in kilowatt-hours (kWh)A thousand watt hours., whereas utility systems are measured in megawatt-hours (MWh)A million watt-hours.. A battery with an energy capacity of 40 MWh can store 40 megawatt-hours of electricity. Think of energy like the size of a water tank — it determines how much water can be held.
Duration is simply energy divided by power. It tells you how long the battery can sustain its maximum power output. A battery rated at 10 MW and 40 MWh has a duration of 4 hours (40 ÷ 10 = 4). Most grid-scale batteries today have durations of 1–4 hours.
Filling a bath of water is a helpful analogy. A small dribble from the tap (equivalent to low power) will take a long time to fill the bath, but opening the tap to increase the flow (equivalent to increasing power) fills it much more quickly.
A large bath can hold more water (equivalent to higher storage capacity) and will take longer to fill than a small bath.
You can use the sliders in the diagram below to change power and storage capacity to help you understand this important concept.
Power (MW) = how fast energy flows. Like the speed of a car.
Energy (MWh) = how much energy is stored. Like the size of the fuel tank.
Duration (hours) = how long it lasts at full power. Energy ÷ Power.
A 10 MW / 40 MWh battery delivers 10 MW for 4 hours (or 5 MW for 8 hours, etc.). Understanding this distinction is critical: a battery with high power but low energy can deliver a burst but runs out quickly. A battery with high energy but low power delivers steadily but cannot meet sharp peaks.
In this lesson, you will explore battery storage at device, household, and grid scales, and see how the same fundamental technology adapts to different applications.
Like solar PV itself, battery storage works at every scale, from the battery in your pocket to grid-scale installations the size of football pitches.
Device scale: Your phone battery is typically 10–15 Wh. It charges in an hour or two and powers your phone for a day. An e-bike battery might be 500–700 Wh. These are tiny by energy system standards but illustrate the core principle: store when connected to power, use when disconnected.
Household scale: Home batteries (like the Tesla Powerwall) are typically 5–13 kWh, with power ratings of 3–5 kW. A 10 kWh home battery can store enough energy to cover a typical household's evening electricity demand for 3–4 hours. Charged from rooftop solar during the day, it can reduce or eliminate grid reliance in the evening, cutting electricity bills and increasing self-consumption of solar energy.
Grid scale: Grid-scale battery energy storage systems (BESS) range from 1 MW to over 1 GW, with energy capacities from a few MWh to several GWhA billion watt-hours — enough to run an average UK household for roughly 350 years.. These systems are connected to the electricity grid and perform a range of functions: time-shifting solar and wind generation, improving grid stability and helping move power around the system. The largest BESS projects in the world now exceed 1 GWh of storage capacity.
One grid-BESS unit holds roughly 33 million phone charges. Same chemistry, same physics, more than seven orders of magnitude apart.
Your household uses 25 kWh per day. Your rooftop solar produces 20 kWh per day, mostly between 9am and 4pm. During that window, your household uses only 4 kWh, meaning 16 kWh of solar is excess.
Questions:
1. How large a battery (in kWh) would you need to store all the excess solar?
2. If your battery has a power rating of 5 kW, how long can it discharge at full power with 16 kWh stored?
3. Your evening demand (4–9pm) is 10 kWh. Can the battery cover this? What about overnight demand (6 kWh)?
1. 16 kWh (to store all excess midday solar).
2. 16 kWh ÷ 5 kW = 3.2 hours at full power.
3. Yes, the battery can cover the 10 kWh evening peak. It would then have 6 kWh remaining, just enough for overnight — leaving you fully solar-powered for the day.
In this lesson you will understand how batteries enable higher solar penetration and why solar and storage growth are fundamentally linked.
At the system level, batteries play a critical role in enabling higher penetration of solar energy. Without storage, there is a practical limit to how much solar a grid can absorb: beyond a certain point, midday generation exceeds demand, and the excess must be curtailed (wasted). Batteries raise this ceiling by absorbing the midday surplus and injecting it back when needed.
The same is true of all forms of variable renewable energy. Windy days cause onshore and offshore wind farms to flood the grid with generation, and it isn't always windy when demand is high.
Without batteries, all forms of variable renewable generation are limited by this timing mismatch. With batteries, the mismatch can be managed. This is why every serious energy transition scenario includes massive growth in storage. The question is no longer whether batteries are needed, but how much and how fast they can be deployed.
Because solar generates on a predictable daily cycle, the longest a battery ever has to hold charge is overnight — at most around 16 hours. That matters financially: a battery sized for the daily solar–evening loop gets used 365 times a year, so its capital cost spreads across thousands of cycles. Wind, by contrast, has multi-day lulls that would need much larger, much less-utilised storage to cover. Solar's daily rhythm is what makes today's 2-to-4-hour batteries a cost-effective match.
Until 2020, grid batteries earned most of their revenue from millisecond frequency response and other ancillary services. That has flipped. BloombergNEF projects roughly 80 % of grid-battery throughput through 2028 will be straight energy shifting: charge cheap, discharge expensive. Time-shifting is now the main job.
The ancillary services haven't gone away though. They earn less, but the grid still needs them. Here are four of the key functions batteries still quietly perform.
Tap each card to see the roles batteries play in the grid.
Flatten demand peaks
The battery discharges during the highest-demand hours, so the grid never has to call on expensive peaking plants. Shorter peaks, lower bills, less carbon emitted.
Smooth variable output
Solar and wind output swings minute-to-minute as clouds pass. A battery absorbs the spikes and fills the dips, so what reaches the grid looks flat and predictable.
Sub-second support
The grid runs at a precise 50 Hz (or 60 Hz in the Americas). Batteries inject or absorb power within milliseconds to hold that number steady when a big generator trips — faster than any thermal plant can react.
Capture midday surplus
When solar output exceeds what the grid can absorb, the surplus is normally wasted. Batteries soak it up, so every kilowatt-hour generated is used — raising the effective value of every new solar panel.
Batteries often stack several of these roles at once — one asset, multiple grid services.
Home systems, EVs plugged in overnight, and even commercial fridges are increasingly aggregated into virtual power plants (VPPs)A pool of small, distributed batteries and flexible loads controlled as a single grid-facing asset, dispatched by software rather than from a control room., software-orchestrated pools that bid into wholesale markets as if they were one large generator. Tesla’s South Australia VPP already dispatches more flexible capacity than several gas peakers combined.
In this lesson, you will see how electric vehicle batteries reshape the household demand curve, and how the same batteries — sitting in driveways — can become the largest distributed storage asset in the world.
EVs have a significant impact on demand flexibilityChanging the timing of when electricity is used. and energy storageTime-shifting energy from when it is generated to when it is used. — the two key responses to the challenge posed by variable renewables such as solar power.
Electric vehicle batteries are huge, and charging them makes a material difference to the domestic demand curve. Workplace charging for those who drive every day, or at-home charging for those who don't, can be timed to occur during the peak of solar generation.
The chart below shows the impact of EV charging on the domestic demand curve, it dwarfs everything else, and with variable charging speeds, it can match the solar curve.
Add appliances to build the household curve, then add an EV. Loads in navy are fixed, yellow are flexible. We've added a slider and buttons so you can set the start time and choose the shape of the EV charge.
EV charging — 50 kWh added, 7 kW home chargerTip: Add all devices, shift flexible loads to midday, switch EV charging to Variable mode and drag the start time to 06:00. The EV bell slots underneath the solar reference, for almost perfect self-consumption.
Car batteries are often underused. They sit fully charged even when you do not plan to drive for 24 hours. This means you could use your car battery to power the evening and nighttime demand of your house.
The technology is already available to plug your car into your house and run the house from the car. This is called vehicle-to-home (V2H)Requires a bidirectional charger and a home energy management system. Already standard on Ford F-150 Lightning and several Hyundai/Kia EVs. when the car powers a single building, or vehicle-to-grid (V2G)Used today for peak shaving, frequency response, and time-shifted energy — fleets of school buses and Nissan Leafs are already doing it commercially in the UK, US and Japan. when it discharges back into the network. Both rely on the car's onboard charger being able to push energy out, not just pull it in. Charging standards already define the protocols, and EVs from BYD, Nissan, Hyundai, Ford and Volvo support some flavour of bidirectional charging today, with most other manufacturers expected to follow over the next two to three years.
An average UK household uses about 8 kWh per day. A 60 kWh EV battery — sitting in the driveway, full — could power that household for almost a full week.
Multiply across a fleet: 30 million UK cars at 60 kWh each is 1.8 TWh of mobile storage. That is larger than every utility-scale battery on the planet combined.
The barriers are no longer technical. They are commercial: who pays for the bidirectional charger, how the energy company compensates the driver for the export, and how battery warranties handle the extra cycles. Each is solvable, and pilot programmes in Australia, the UK, Japan and the Netherlands are working through them now.
The takeaway: every EV is a wheeled grid battery, and a fleet of them is the world's largest distributed storage asset — one that grows with every new car sold.
Test your understanding with these six questions. Your score will be shown at the end.
Battery storage addresses solar's central challenge — the timing mismatch — by time-shifting energy from when it is generated to when it is needed.
Understanding batteries requires distinguishing between power (how fast), energy (how much), and duration (how long). Most domestic, commercial and grid-scale batteries today have 1–4 hour durations.
Batteries work at every scale: from phone batteries (Wh) to home systems (kWh) to grid-scale BESS (MWh to GWh). At the system level, they enable higher solar penetration by absorbing midday surplus and releasing it during evening peaks.
Solar and storage are complementary technologies. Their combined growth is the foundation of the energy transition. In the next module, we examine the costs of both technologies — and why those costs have been falling so rapidly.
Next up: In Module 5, we will explore the costs of solar and batteries, and understand the economics that are driving the global energy transition.
You now understand how battery storage works — from time-shifting and power-energy-duration to grid-scale BESS and system integration.
