From sunlight to electricity: the photovoltaic effect and solar system architecture
I'd put my money on the sun and solar energy. What a source of power! I hope we don't have to wait until oil and coal run out before we tackle that.
This module covers the following five lessons:
In this lesson you will learn where solar energy comes from and how the photovoltaic effect converts light directly into electricity.
It all starts with the sun. A continuous, raging nuclear explosion held together with its own gravity. Emitting huge amounts of energy in the form of light.
Light, in the form of photonsDiscrete packets of electromagnetic energy that make up light. Photons from the sun are the energy source that drives photovoltaic electricity generation., explode out of the sun and rain down on the earth. They are the primary source of energy that is ultimately converted into oil, gas, coal, hydro and wind power.
Photons are also the primary source of energy for solar power. This more direct form of conversion of the sun's energy is known as the photovoltaic effectWhen a photon strikes a silicon semiconductor it knocks an electron loose, producing a flow of direct current. First observed by Edmond Becquerel in 1839..
The photovoltaic (PV) effectCoined from Greek phos (light) and Italian volta (after Alessandro Volta, the father of the battery). It only works in materials called semiconductors — silicon being the workhorse. is the process by which light energy is converted directly into electrical energy. The term combines 'photo' (light) with 'voltaic' (electricity).
This phenomenon was first discovered in 1839 by a 19-year-old French physicist, Alexandre Edmond Becquerel, who observed that certain materials produced small amounts of electric current when exposed to light.
The first practical conversion of sunlight to electricity was created by Bell Labs in 1954 and was soon adopted to power satellites, marking the beginning of solar use in the US space programme.
To date, over 200 billion PV cells have been produced! Solar power has been the fastest-growing source of electricity in the world for the last 20 years.
In this lesson, you will explore the internal structure of a PV cell and understand the three-step process that turns sunlight into electric current.
The PV cell is the fundamental building block of all solar power. It is a thin wafer of silicon, typically around 18 cm square, that has been engineered to convert sunlight into an electrical current.
Click on the play arrow to open the interactive 3D model and explore the cell. Click through the annotations to see the main parts.
This model is extra-thick to show the layers in more detail, but most cells today are around 0.2 mm thick, about the same thickness as a sheet of paper.
A PV cell is made of two thin layers of silicon, each manufactured with slight impurities — a process known as dopingThe added impurity is measured in parts per million — a few grains of phosphorus or boron in a wafer otherwise pure to nine decimal places.. One layer is doped with phosphorus, which has one more electron on its outer shell than silicon (creating the 'n-type' or negative layer). The other is doped with boron, which has one fewer electron on its outer shell (creating the 'p-type' or positive layer). At the boundary between these two layers, known as the P-N Junction, an electric fieldAn invisible push on charged particles, always in the same direction. forms naturally.
Cross-section of a PV cell: photons free electrons in silicon, and the electric field drives current through an external circuit
When a photon enters the PV cell, it may strike a silicon atom with enough energy to knock one of its electrons free. The electric fieldForms automatically where the two doped silicon layers meet. at the boundary immediately pushes that electron toward the n-type (negative) side of the cell. This seems counterintuitive because we know that opposites attract, so why would an electron move toward the negative side? The answer is that the electric field at the junction acts as a one-way gate. Electrons cannot return past the boundary, so they accumulate on one side, building up an electrical voltage across the cell. This is the magicThe electric field which forms at the P-N Junction is the magic ingredient that makes solar power work. that makes a PV cell work.
Metal contacts on each side of the cell collect these electrons. When an external circuit is connected, the electrons flow through it to reach the other side of the cell, producing an electric current. This flow of electrons is the electricity generated by the PV cell.
Those contacts are worth a closer look. The fine lines on a cell are fingersScreen-printed in silver paste, about 30 micrometres wide — and getting narrower every year as silver prices bite. — about a hundred per cell, thinner than a hair — picking up electrons wherever they're freed. The busbarsThe jump from 3 flat busbars to 16 round wires took under a decade. The industry calls it multi-busbar, or MBB. crossing them carry that current on to the next cell.
There's a tension here: metal blocks light, but electrons lose energy travelling far through silicon. We want as much metal as possible and as little as possible, at once. Panels once had three flat busbars; today they use twelve to sixteen round wires, fine as thread, which scatter light onto the cell rather than shading it.
The silicon underneath isn't flat either. Polished silicon reflects about a third of the light hitting it — an awkward start for a device meant to absorb it. So the surface is etched into microscopic pyramidsThe etch exposes the crystal planes of the silicon, so the pyramids form themselves at 54.7° — an angle set by the crystal, not the machine.. A ray bouncing off one lands on the next instead of escaping. Add an anti-reflective coating — the reason cells look blue-black rather than silver — and reflection falls to two or three per cent.
A PV cell works through three steps: (1) a photon enters the cell and knocks an electron free from a silicon atom; (2) the built-in electric fieldAbout 0.6 V across a layer one micrometre thin. at the p-n junction pushes the electron to the negative side; (3) electrons accumulate on one side, creating a voltage, and flow through an external circuit as electric current.
In full sun, a single PV cell produces around 7.5 wattsPower is the instantaneous rate of electrical output, measured in watts (W), kilowatts (kW), or megawatts (MW). of power and a voltage of approximately 0.5 volts — much less than a domestic AAA battery. That 7.5 watts is enough to power a small USB desk fan or a standard LED light bulb. Over the course of an average day, in an averagely sunny location, a single cell produces enough to charge a smartphone.
A single PV cell can charge your phone — a small, flexible, daytime-friendly energy task that aligns well with when solar power is available.
In this lesson you will see how individual cells are assembled into modules — the panels you recognise on rooftops — and understand how voltage scales up.
A single PV cell is useful as a concept, but 7.5 watts is not enough to do much practical work. To generate meaningful amounts of power, cells are assembled into modulesAbout a square metre and 20 kg of glass, silicon and aluminium — the standard unit you order, ship, and bolt to a roof. — the flat, rectangular panels you see on rooftops and in solar farms.
A typical PV module contains around 120 half-cut cellsWatch this space! This number is increasing all the time as modules get larger., laid out side by side to collect as much light as possible. Electrically, the cells are connected in series — stacked on top of each other in terms of voltage — so that the voltage produced by each cell combines across the module. Each cell contributes about 0.5 V, so a 60-cell module produces around 30 V, roughly equivalent to an e-bike battery.
Modern panels are increasingly built with half-cellNow standard since around 2018 — over 90% of panels shipped in 2024 were half-cell. designs — each silicon cell laser-cut in two, then wired as two separate strings (top half and bottom half). This simple internal change makes today's larger formats viable: 144 or even 156 half-cells per panel, pushing voltages to around 40 V and headline outputs past 700 W. A side benefit is shade tolerance. If the bottom half is shaded, the top half keeps producing.
Shade used to be solar's great weakness. Cells are wired in series, and a series circuit runs at the pace of its weakest member, so one leaf or chimney shadow could drag down a whole panel. That has largely been engineered away, in three steps. Tiny automatic switchesThe component is a bypass diode. A shaded cell doesn't just stop producing, it starts consuming — and can overheat. The failure is called hot-spotting. — usually three per module — sense a shaded section and give the current a detour around it. Half-cut cells split the panel into two independent halves, so shading one doesn't stop the other. And on awkward roofs, small electronics on the back of each panelOptimisers and microinverters — module-level power electronics, or MLPE. Common on rooftops, rare on utility-scale sites where nothing shades anything. let every module work at its own best point regardless of its neighbours. Shade still costs you output. It no longer costs you the array.
The cells are sandwiched between protective layers: tempered glass on the front to withstand hail and wind, an encapsulant material to protect the cells from moisture, and a backsheet to seal everything from behind. An aluminium frame provides structural rigidity. Those layers work for a remarkably long time. Solar panels don't fail so much as fade: a modern module loses about 0.4% of its output a yearNREL's long-run median for silicon is nearer 0.5% a year. The n-type cells now dominating production beat it., and after thirty years still produces around 87% of what it did when new. In the 2000s, guaranteeing 80% after 25 years marked you out as a serious manufacturer.
There were once two kinds of silicon panel: multicrystalline, cast in a block and speckled blue, and monocrystallineCzochralski growth: a seed crystal drawn slowly from molten silicon, rotating, so the whole ingot forms as one continuous lattice., grown as a single crystal and sliced from a round ingot. The industry argued for years about which would win. It's settled — monocrystalline silicon is about 98% of all panels made. Diamond-wire sawing closed the cost gap and the better efficiency did the rest. Clipped corners on a panel are the giveaway: it was cut from a cylinder.
Click through the annotations in the 3D model above to explore the layers of a PV module: glass, encapsulant, cells, backsheet, and frame.
In full sun, the module above produces around 450 watts of peak power. That is the standard rooftop format; the larger panels going into solar farms clear 600 W, and modules above 700 W are now reaching the market. Either way, one module is enough to power a television, a few laptops, or a microwave on low power. Over the course of a day, a single module produces enough energy to run a domestic fridge.
A single module can power a fridge for a day. A typical rooftop installation of 10–20 modules can cover a significant portion of a household's daily electricity needs, depending on location and consumption patterns.
To date, the world has manufactured around 4 billion solar PV modules — roughly one module for every two people on Earth. Annual production has more than doubled since 2020: in 2024 alone the industry shipped over a billion new modules.
In this lesson, you will learn how modules combine into arrays and systems, and why solar's scalability is one of its most powerful advantages.
Modules are grouped together into arraysFrom two modules on a shed roof to a million in a desert solar farm — the array is the level at which we start measuring output in kilowatts and megawatts. — collections of modules mounted on a shared structure, usually an aluminium frame oriented to face the midday sun. An array might contain anywhere from a handful of modules (on a residential rooftop) to thousands (in a utility-scale solar farm).
A small commercial-scale array might contain around 65 of the larger 600 W modules, giving a total peak output of around 40 kW. Because it is not always sunny, and there is no output at night, this 40 kW array will produce the equivalent of around 3–6 kW on average. In energy terms, that is approximately 30 to 60 MWh per year — equivalent to the average annual energy use of around 10 houses or 25 people.
The 3D model of an array shows how the modules are fitted together on a simple support structure and orientated to face the midday sun. (Note: it may take a while to load).
ScalabilityAlmost no other technology spans 12 orders of magnitude on the same physics — a 5 W phone charger and a 500 MW solar farm are the same thing, just stacked differently. is one of the most important advantages of solar power. The same basic technology — the PV cell — works at every scale. A single cell charges a phone. A module runs a fridge. An array of 25 modules makes a 10 kW rooftop system for a house. Combine 25 arrays of 100 modules, and you have a 1 MW solar farm, enough to power a small village. Scale further to hundreds of MW, and you are powering a city. This modular, stackable quality is unique among energy sources.
Drag the slider to scale up from a single cell to a whole solar farm.
SAME TECHNOLOGY → ANY SCALE
In this lesson you will explore the four main scales at which solar is deployed, and understand how each one relates to the electricity grid — including the scale that doesn't touch a grid at all.
Solar PV systems are deployed at four main scales, each with distinct characteristics and roles in the energy system.
Residential solarBehind-the-meter generation: power is consumed on site first, with surplus exported. Typical UK install ~£5–7k for 4 kWp. typically involves 10–30 modules mounted on a rooftop, producing 4–12 kW of peak power. These systems are designed to offset a household's electricity consumption, often paired with net metering or feed-in tariff arrangements that allow excess power to be exported to the grid (more on these later).
C&I solarCommercial & Industrial. The biggest growth segment globally — supermarket roofs, warehouses, factories, often with onsite storage to flatten demand peaks. covers a broad range, from 50 kW rooftop installations on warehouses and office buildings to multi-MW ground-mounted systems. The economics of C&I solar are often compelling because commercial electricity tariffs are high and daytime demand aligns well with solar generation. These are the first truly commercial systems designed to export reasonable quantities of power to other consumers through the grid.
Utility-scale solarLarge solar farms selling power to the grid. India's Bhadla Solar Park (~2.2 GW) covers 57 km² — comparable in output to a single nuclear reactor. refers to large, centralised solar farms, typically ranging from 10 MW to over 1 GW. The largest solar farms in the world now exceed 2 GW of capacity — the equivalent of a large nuclear power station in terms of peak output.
Those three scales have something in common: they all plug into a grid that is already there. For around 650 million people, it isn't.
For the roughly 650 million people with no mains electricity — most of them in sub-Saharan Africa — solar isn't a way to shave a bill. It's the first electricity they will ever have. A solar home systemThe industry counts these in "solar energy kits". GOGLA's member companies alone sold over 10 million of them in 2025 — the highest year on record. is a single panel and a small battery, anywhere from about 10 W to a few hundred, running lights, phone charging and a radio, and at the top end a fan, a TV or a fridge. Most are sold on pay-as-you-goPAYGo for short. Unlock codes arrive by SMS and are paid off in small instalments over mobile money; miss one and the kit locks itself. That remote switch is what makes lending to a customer with no credit history possible at all. terms rather than as a lump sum no household could raise at once.
A mini-gridCalled a micro-grid at the smaller end. Most now pair solar with batteries and keep a diesel generator only as backup — the exact reverse of the arrangement they replace. is the step up: typically tens of kilowatts to a few megawatts, with its own poles and wires serving a village or a small town. It is a real grid. It just isn't the grid. That distinction matters, because a mini-grid can run a workshop, a grain mill or a clinic, not only a household.
At the grid level, solar is now one of the largest sources of new electricity generation capacity being added globally each year. Taken together — off-grid, residential, C&I, and utility-scale — solar is simultaneously the most distributed and one of the largest-scale energy technologies in the world.
Understanding these four scales matters because each one meets the electricity grid at a different point. Residential solar feeds into low-voltage distribution networks. Utility-scale solar connects to high-voltage transmission networks. C&I solar sits in between. Off-grid systems and mini-grids don't connect to the national grid at all — they either serve a single household directly or build a small grid of their own. These differences affect grid planning, market design, and the challenges of integrating solar — themes we will return to throughout this course.
A small school consumes approximately 50 MWh of electricity per year. In the UK, a 40 kW array produces about 30–60 MWh per year.
1. How many modules (at 450 W each) for a 40 kW array?
2. Would a 40 kW array cover the school's annual consumption?
3. What scale of deployment is this?
1. 89 modules (40,000 W ÷ 450 W = 88.9, so 89 to clear 40 kW).
2. Likely yes — 30–60 MWh covers the 50 MWh target in most years.
3. Small commercial-scale (C&I) installation.
Drag each solar component on the left onto the device or place on the right that it can reasonably power.
Seven quick questions to see what's stuck. There's no pass mark, and you can retake it as many times as you like.
Solar power works by converting sunlight directly into electricity through the photovoltaic effect. A PV cell, made of two doped layers of silicon, creates an electric field at their junction. When photons strike the cell, they free electrons, which are driven by the electric field to create an electrical current.
Cells are assembled into modules (around 120 half-cut cells, approximately 450 W), and modules into arrays, forming systems that can scale from a few kilowatts on a rooftop to hundreds of megawatts in a utility-scale solar farm.
Solar PV is deployed at four scales — off-grid, residential, C&I, and utility — each meeting the electricity grid at a different point, or in the case of off-grid systems and mini-grids, not meeting it at all. This scalability is one of solar's most distinctive and powerful characteristics.
Next up: In Module 2, we will explore the solar generation profile — how the output of a solar system varies through the day, across seasons, and by geography.
You now understand how solar power works — from the photovoltaic effect in a single cell to gigawatt-scale solar farms.
