📖 Chapter 3 of 11 · Green Energy - Advance 1 · Class 8
Chapter 3: How Much Power Can a Solar Panel Really Make?
In this chapter
- Inside a solar cell: the p–n junction.
- Efficiency: why a panel turns only about a fifth of sunlight into electricity.
- Inverters, rooftop systems, solar pumps and floating solar.
- Meet Sunita from Haryana and Arun from Telangana.
The Big Idea
A solar cell is made of two layers of silicon: an n-type layer with extra electrons and a p-type layer with “holes” (places missing electrons). Where they meet, a junction forms with a built-in electric field. Light frees electrons, the field sweeps them to one side, and they flow through the circuit. Today’s common panels convert roughly 20% of the sunlight that hits them into electricity.
Did You Know?
Under the PM-KUSUM scheme, the Government of India helps farmers replace diesel pumps with solar-powered pumps, and set up small solar plants on their land. A solar pump has no fuel bill and no smoke.
Story Time — Sunita and the Silent Pump
Sunita, age 13 · Hisar, Haryana
For years, Sunita woke to the roar of her family’s diesel pump. It drank expensive fuel, filled the air with black smoke, and broke down just when the wheat needed water most.
Last winter, her father installed a solar pump with help from the PM-KUSUM scheme. A row of panels was mounted on a frame near the tube well. On the first sunny morning, water gushed into the channel — and there was no noise at all.
Sunita, who loves maths, wanted to know how much power the panels made. The label on each panel said 550 W. “There are 10 panels, Papa, so 5,500 watts!” The technician smiled. “Only at noon, on a clear, cool day. The 550 W rating is measured under strong, standard sunlight. In the morning, or when the panels get very hot, they make less.”
He showed her the pump controller. At 8 a.m. it read 2.1 kW; at noon, 4.6 kW. “Why not 5.5?” she asked. “Heat, dust and wiring all take a share,” he said. “That is why we clean the panels and let air flow behind them.”
Sunita made a graph of power through the day. It rose like a hill and fell again by evening. “So the pump works hardest exactly when the Sun is highest,” she said, “which is also when the fields are thirstiest!”
Story Time — Arun and the Floating Panels
Arun, age 12 · Ramagundam, Telangana
Arun’s uncle works at the NTPC power station in Ramagundam. On a family visit, he pointed to a reservoir. Instead of plain water, Arun saw row after row of blue panels, floating on the surface. “It is one of the largest floating solar plants in India,” his uncle said.
“Why put panels on water?” Arun asked. “Three reasons,” said his uncle, counting on his fingers. “One: we do not need to use up farmland. Two: the water keeps the panels cooler, and cooler panels work better. Three: the panels shade the water, so less of it evaporates in the hot summer.”
Arun was surprised that heat could lower a panel’s output. His uncle explained, “Silicon cells lose a little voltage as they get hotter. On a 45°C day, a rooftop panel can be much hotter than the air.”
“And how do the panels not sink or drift away?” Arun asked. “They sit on floats made of tough plastic, tied together and anchored to the bottom,” his uncle replied. “Engineers also check that the plant does not harm the fish and water quality.”
On the way back, Arun counted the reservoirs they passed. “India has so many lakes and dams,” he said. “If even a small part were covered, how much power could we make?” His uncle grinned. “That, Arun, is exactly the question engineers are asking.”
Inside the Cell: the p–n Junction
Pure silicon does not conduct well. Engineers add tiny amounts of other elements — this is called doping. Adding phosphorus gives extra electrons (n-type). Adding boron creates holes (p-type). At the junction between them, an electric field builds up, like a one-way slope for electrons.
- A particle of light is called a photon. A photon with enough energy frees an electron and leaves a hole.
- The junction’s field pushes electrons towards the n-side and holes towards the p-side.
- Connect the two sides with a wire, and electrons flow through it: an electric current.
- One cell gives only about 0.5–0.6 volt, so dozens of cells are joined in a panel.
From Sunlight to Socket
On a clear day at noon, sunlight delivers about 1,000 watts on every square metre facing the Sun. A panel with 20% efficiency turns about 200 W of that into electricity. The rest is reflected or becomes heat.
- Inverter: converts DC from the panels into 230 V, 50 Hz AC.
- Net meter: counts power flowing both ways, so extra solar power can be sent to the grid.
- Capacity factor: because of night, clouds and low morning Sun, a panel in India typically produces only about a fifth of what it would if it ran at full rating all day and night.
🔢 Numbers Corner
Daily solar energy (kWh) ≈ panel rating (kW) × peak sun hours × 0.8 (the 0.8 covers heat, dust and wiring losses). A 3 kW rooftop with 5 peak sun hours gives about 3 × 5 × 0.8 = 12 kWh a day.
Myth or Fact?
Myth: Solar panels work best on the hottest days.
Fact: They need bright light, not heat. Very hot panels actually produce a little less. A bright, cool, clear day is ideal.
🔬 Try it in the Green Energy Lab
Experiment with these ideas in Lab 1: Solar Rooftop Planner.
Solved Examples
Work it out first, then check the answer.
1. What is doping, and why is it needed?
Show answer
Answer: Doping is adding tiny amounts of elements like phosphorus or boron to silicon. It creates n-type and p-type layers, whose junction forms the electric field that drives the current.
2. A panel receives 1,000 W of sunlight and is 20% efficient. What is its output?
Show answer
Answer: 1,000 × 20 ÷ 100 = 200 W.
3. A 4 kW rooftop system gets 5 peak sun hours a day. Estimate its daily output (use a 0.8 loss factor).
Show answer
Answer: 4 × 5 × 0.8 = 16 kWh per day, or about 480 units in a 30-day month.
4. Give three advantages of floating solar plants.
Show answer
Answer: They save land, the water cools the panels so they work better, and the panels reduce evaporation from the reservoir.
5. How many cells of 0.5 V each are needed in series to make about 36 V?
Show answer
Answer: 36 ÷ 0.5 = 72 cells.
Check Your Understanding
A. Fill in the blanks
- A solar cell has an n-type layer and a -type layer.
- A particle of light is called a .
- Common solar panels today are about % efficient.
- An changes DC into AC.
- Farmers get solar pumps under the scheme.
B. True or False
- Panels produce more power when they are extremely hot. ( ___ )
- Floating solar can reduce evaporation from reservoirs. ( ___ )
- A single solar cell gives about 230 volts. ( ___ )
- A net meter can count power going to and from the grid. ( ___ )
C. Choose the correct option
- Adding phosphorus to silicon makes it (a) n-type (b) p-type (c) metallic
- Sunlight at noon delivers about how much power per square metre? (a) 10 W (b) 100 W (c) 1,000 W
- The floating solar plant in Arun’s story is at (a) Ramagundam (b) Bhadla (c) Modhera
D. Answer in detail
- Why did Sunita’s pump show 4.6 kW instead of 5.5 kW at noon?
- Explain how the p–n junction makes electrons flow in one direction.
- Calculate the monthly output of a 2 kW rooftop with 5.5 peak sun hours (use 0.8 losses and 30 days).
Brain Buster
Hold a small calculator’s solar strip in sunlight, then in the shade, then under a lamp. Does it still work? What does this tell you about the light each source gives?
Check your understanding
Tap an answer to see if it is correct. This is just for practice — nothing is saved.
1. What forms the electric field inside a solar cell?
- The glass cover
- The p–n junction
- The aluminium frame
Show answer
B. The p–n junction
2. A 20% efficient panel receives 800 W of light. Output?
- 40 W
- 160 W
- 640 W
Show answer
B. 160 W
3. Why does floating solar often work better?
- Water cools the panels
- Water makes sunlight stronger
- Fish push the panels
Show answer
A. Water cools the panels
4. A 5 kW system, 4 peak sun hours, 0.8 losses → daily output?
- 16 kWh
- 20 kWh
- 25 kWh
Show answer
A. 16 kWh
5. What does an inverter do?
- Stores energy
- Changes DC into AC
- Cleans panels
Show answer
B. Changes DC into AC